Selina Concise Solutions for ICSE Class 6 Physics Chapter 1 Matter

Synposis

Matter is the substance which occupies space and has mass.
The three states of matter are
1. solid,
2. liquid
3. gas.

1. Matter is composed of large number of molecules.
2. A molecule is the smallest particle which can exist freely in nature by itself and it retains the properties of the substance.
3. All molecules of a substance are identical, but the molecules of different substanes are different.
4. A molecule is very small in size 10^-10 m).
5. The molecules are separated from each other with spaces called inter-molecular spacing.
6. The molecules in a substance are held together by the forces acting between the molecules which are called the inter-molecular forces.
7. The force of attraction between the molecules of the same substance is called the force of cohesion, while the force of attraction between the molecules of two different substances is called the force of adhesion.
8. The forces of cohesion and adhesion are effective only when the separation between the molecules is 10^-9 m. When the separation becomes more, they vanish.
9. The molecules in a substance are not at rest, but they are constantly in motion.
10. In a solid, the molecules are rigid, the inter-molecular spacing is least, the inter molecular forces are strongest and the molecules remain in their fixed positions. They vibrate to and fro about their mean positions, but they do not leave their positions, so a solid has a definite shape and a definite volume.
11. In a liquid, the molecules are not rigid, the inter-molecular spacing is more than that in solids, the inter-molecular forces are weak and the molecules are free to move within the boundary of the liquid, so the liquid has a definite volume, but it does not have a definite shape.
12. In gases, the molecules are not rigid, the inter-molecular spacing is more than that in solids and liquids, the inter-molecular forces are weakest and the molecules are free to move anywhere in space. So the gas has neither a definite volume nor a definite shape.

Activity - 2
Classify 20 objects around you as solids, liquids and gases
Solids : Ice, Aluminium, Silver, Calcium, Gold, Iron, Sodium, chloride, Sugar, Wood.
Liquids : Water, benzene, chloroform, oil, honey, glycerine, hydrochloric acid, alcohol, dettol.
Gases : Steam, Air, Oxygen, Hydrogen, Chlorine, Nitrogen, Ammonia, Helium, Argon.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) The molecules of each substance are identical.
Answer: False
Molecules of different substances differ in their chemical and physical properties. For example, a molecule of water is completely different from a molecule of iron.
Teacher's Tip: Remember "Unique Uniformity" - identical within one substance, but unique compared to others.
Exam Tip: Pay attention to words like "each" or "all" as they often signal a trick in true/false questions.

(b) The inter-molecular forces are effective at all distances between the two molecules.
Answer: False
These forces only work when molecules are extremely close to one another, typically within 10^-9m . If the distance increases beyond this limit, the force of attraction essentially disappears.
Teacher's Tip: Think of these forces like short-range magnets that only click when they are almost touching.
Exam Tip: Mention the specific distance 10^-9m if asked to explain why this statement is false.

(c) The molecules in a substance arc in random motion.
Answer: Tme
Molecules are never completely still and are always moving in unpredictable directions. This motion is most visible in gases and least restricted in liquids.
Teacher's Tip: Use the "Busy Bee" analogy - molecules are always buzzing around even if we can't see them.
Exam Tip: In exam scripts, ensure you use the term "random" or "zig-zag" to describe molecular paths.

(d) In a gas, the molecules can move anywhere in space.
Answer: Tme
Gas molecules have the weakest intermolecular forces, allowing them to break free and fill any container. They travel at high speeds until they collide with other molecules or walls.
Teacher's Tip: Imagine gas molecules as explorers who want to visit every corner of the room.
Exam Tip: Use this fact to explain why gases do not have a definite shape or volume.

(e) The liquids are less viscous than the gases.
Answer: False
Viscosity refers to the internal friction or "thickness" of a fluid, and liquids generally offer more resistance to flow than gases. Honey is a great example of a highly viscous liquid compared to air.
Teacher's Tip: Viscosity is "Liquid Friction" - the stickier it is, the more viscous it is.
Exam Tip: Don't confuse "viscosity" with "density"; they are different physical properties.

2. Fill in the blanks

(a) All the molecules of a substance are identical.
Answer: identical.
This means every single molecule in a glass of pure water is exactly the same as the others. This consistency is what gives a pure substance its specific properties.
Teacher's Tip: Think of a bag of identical marbles representing molecules of the same substance.
Exam Tip: Spelling counts in fill-in-the-blanks; ensure "identical" is written correctly.

(b) The inter-molecular spacing is least in solids more in liquids and still more in gases.
Answer: least, more, still more.
In solids, molecules are packed so tightly that there is almost no room between them. As we move to liquids and then gases, the gaps between these particles grow significantly.
Teacher's Tip: Use the "Classroom Seating" trick: Solids are students in desks, liquids are students in the hallway, and gases are students on the playground.
Exam Tip: Remember the sequence: Solid < Liquid < Gas for spacing.

(c) The molecular motion in liquid and gas is in zig-zag path.
Answer: zig-zag.
Because molecules constantly bump into each other, they are forced to change direction frequently. This erratic movement creates a path that looks like a series of sharp turns.
Teacher's Tip: Think of a pinball bouncing around; that is exactly how molecules move in fluids.
Exam Tip: The technical term for this zig-zag motion is "Brownian motion," which is good to know for bonus points.

(d) In a solid, the molecules vibrate to and fro but they remain at their fixed positions.
Answer: vibrate to and fro.
Even though solids look still, their molecules are actually shaking very fast in place. They lack the energy to break away from their neighbors, keeping the solid's shape rigid.
Teacher's Tip: Imagine soldiers standing at attention; they might shiver, but they don't leave the line.
Exam Tip: Always mention "fixed positions" when describing the vibration of molecules in a solid.

(e) The inter-molecular forces are the weakest in gases.
Answer: gases.
Because gas molecules are so far apart, the attractive forces between them are almost non-existent. This allows them to expand and occupy any available space.
Teacher's Tip: Weak forces = Freedom of movement.
Exam Tip: Connect "weakest forces" with "maximum spacing" to explain gas properties.

(f) A solid exerts pressure downwards on its base.
Answer: downwards on its base.
Due to gravity, the weight of a solid pushes directly onto the surface it is resting on. Unlike fluids, solids do not push sideways against the walls of a container.
Teacher's Tip: Think of a book on a table; it only pushes the table down, not left or right.
Exam Tip: Use this to distinguish solids from liquids, which exert pressure in all directions.

(g) The gases are least dense.
Answer: least.
Density depends on how much mass is packed into a volume, and gases have very few molecules in a large space. This is why most gases are invisible and feel light.
Teacher's Tip: Remember: Big space + Few molecules = Low Density.
Exam Tip: If comparing the three states, always list gases as having the lowest density.

(h) A solid is most rigid.
Answer: most.
Rigidity is the ability of a substance to maintain its shape when outside force is applied. Solids are the most rigid because their molecules are locked into a tight, organized pattern.
Teacher's Tip: Rigid means "tough and unbending."
Exam Tip: Rigidity is the opposite of fluidity; solids are rigid, while liquids and gases are fluids.

3. Select the correct alternative

(a) The diameter of a molecule is approximately
1. 1 cm
2. 10 cm
3. 10^-10 m
4. 1 m
Answer: 3. 10^-10 m
Molecules are incredibly tiny, far smaller than anything the human eye can see even with a standard microscope. This measurement represents the scale of atoms and small molecules.
Teacher's Tip: Remember the power of negative ten (10^-10) to visualize how small a molecule is.
Exam Tip: Be careful with units; sometimes options might use millimeters or centimeters to confuse you.

(b) The inter-molecular forces are strongest in
1. solids
2. liquids
3. gases
4. both (i) and (ii)
Answer: 1. solids
Strong attractive forces keep the molecules in a solid locked together in a definite arrangement. These forces are what make solids hard to break or compress.
Teacher's Tip: Strong forces = Strong bond = Solid state.
Exam Tip: Always associate "strongest forces" with "least intermolecular space."

(c) The molecules
1. in solid, liquid and gas, move freely anywhere.
2. in a solid, move freely within its boundary.
3. in a liquid, move within its boundary.
4. in a gas, move only within its boundary.
Answer: 3. in a liquid, move within its boundary.
Liquid molecules have enough energy to slide past one another but not enough to escape the liquid's surface. This is why liquids flow and take the shape of their container.
Teacher's Tip: Think of liquid molecules like people in a crowded room; they can move around but stay inside the room.
Exam Tip: For gases, the answer would be "move freely anywhere," so distinguish carefully between the two.

(d) The solids are
1. more dense
2. less dense
3. least dense
4. highly compressible
Answer: 1. more dense
Because molecules in a solid are packed very closely together, they have more mass per unit of volume. This makes them heavier compared to the same volume of liquid or gas in most cases.
Teacher's Tip: More "stuff" in the same space means higher density.
Exam Tip: Note the exception of ice (solid) being less dense than water (liquid), though generally, solids are more dense.

(e) The inter-molecular forces in liquids are
1. as strong as in solids
2. stronger than in solids
3. weaker than in solids
4. weaker than in gases
Answer: 3. weaker than in solids
In liquids, the forces are strong enough to hold the substance together but weak enough to allow molecules to move. This intermediate strength is why liquids can flow while keeping a fixed volume.
Teacher's Tip: Think of the force in liquids as a "loose grip" compared to the "tight handshake" of solids.
Exam Tip: Always compare liquid forces as being "between" those of solids (strongest) and gases (weakest).

4. Match the following columns
Column A
(a) A molecule is composed of
(b) Ice, water and water vapour
(c) An atom
(d) Gases
(e) The molecules of a solid

Column B
(i) does not exist free in nature.
(ii) can vibrate only up to about 10^-10 from their mean position.
(iii) atoms.
(iv) are the three states of water.
(v) occupy space
Answer:
(a) A molecule is composed of - (iii) atoms.
(b) Ice, water and water vapour - (iv) are the three states of water.
(c) An atom - (i) does not exist free in nature.
(d) Gases - (v) occupy space
(e) The molecules of a solid - (ii) can vibrate only up to about 10^-10 m from their mean position.
(a)-(iii), (b)-(iv), (c)-(i), (d)-(v), (e)-(ii)
This matching exercise connects basic definitions of matter with the specific behaviors of atoms and molecules. It summarizes the core differences between the states of water and the nature of microscopic particles.
Teacher's Tip: Use the "Building Block" rule - Atoms make molecules, and molecules make matter.
Exam Tip: When matching, draw straight lines or write the letters clearly to avoid confusion for the examiner.

B. Short/Long answer questions

Question 1: Define matter. What is its composition?
Answer: Matter is defined as anything which occupies space and has mass. It can be perceived by our sense of smell, touch, sight, hearing and taste. Matter is composed of tiny particles known as atoms.
Everything you see around you, from the chair you sit on to the air you breathe, is made of matter. Atoms are the fundamental units that join together in various ways to create everything in the universe.
Teacher's Tip: Remember "M-S" (Mass and Space) to define matter easily.
Exam Tip: Do not forget to mention that matter can be perceived by our senses for a complete definition.

Question 2: Name the three states of matter.
Answer: The three states of matter are solids, liquids and gases.
Solids - A solid has a definite shape and definite volume. Example - wood, stone, iron, ice etc.
Liquid - A liquid has a definite volume but not definite shape. Example - water, juice, milk, oil, etc.
Gases - A gas neither has definite shape nor a definite volume. Example - air, hydrogen, oxygen, watervapour etc.
These states are determined by how tightly the molecules are packed and how much they can move. Water is the most common substance found naturally in all three states on Earth.
Teacher's Tip: Think of "Ice, Water, Steam" as the easiest way to remember the three states.
Exam Tip: When giving examples, always try to provide at least two for each state to show a good understanding.

Question 3: What is a molecule?
Answer: The smallest unit of matter which can exist independently is called molecule. Example: Oxygen molecule O₂ made up of two (O) atoms.
A molecule is formed when two or more atoms bond together chemically. While atoms are the building blocks, molecules are the smallest parts of a substance that still act like that substance.
Teacher's Tip: Think of an atom as a single Lego brick and a molecule as a small finished structure made of bricks.
Exam Tip: Use the example of Oxygen O₂ or Water  H₂Oto illustrate your answer clearly.

Question 4: What is the approximate size of a molecule?
Answer: Matter is made up of molecules which are very small in size (~10^-9 m).
This size is so microscopic that millions of molecules could fit on the tip of a needle. The small size explains why we cannot see them individually with our naked eyes.
Teacher's Tip: Remember the "Nano" scale - 10^-9 is one billionth of a meter!
Exam Tip: Always include the unit "meters" or "m" when writing the numerical size.

Question 5: One litre of water has 6.02 x 10²⁶ molecules. Estimate the size of a molecule.
Answer: The size of a particle (or molecule of matter is very small. 1 litre of water has 6.02 x 10²⁶ molecules, so the volume of a particle of water is \frac{10^-3 m³} 6.0 x 10²⁶ = 1.6 x 10^-30 m³. Thus the diameter of a water molecule is nearly 1.27 x 10^-10 metre.
This calculation shows that molecules are incredibly tiny based on the massive number of them found in just one liter. It helps us understand the vastness of the molecular world compared to our everyday scale.
Teacher's Tip: Don't get scared by the big exponents; they just show how many tiny molecules are packed together.
Exam Tip: When doing such calculations, double-check your powers of ten to ensure accuracy.

Question 6: What do you mean by inter-molecular spacing?
Answer: Intermolecular space - The space between any two consecutive molecules of a substance is called intermolecular space.
This space determines whether a substance is a solid, liquid, or gas. In gases, this space is very large, while in solids, it is almost negligible.
Teacher's Tip: Think of the gaps between people standing in a line; that's like intermolecular spacing.
Exam Tip: Use a diagram showing circles with gaps to visualize your answer for the examiner.

Question 7: Describe a simple experiment to illustrate the existence of inter-molecular spacing.
Answer: Take 100 ml of water in a measuring cylinder. Add 20 gram of salt in water gently and stir it well so as to dissolve the salt well in water. It is noticed that the level of water does not change. It shows that the particles of salt occupy spaces between the particles of water.
Since the water level doesn't rise, the salt must have tucked itself into the "empty" spaces between water molecules. This proves that water is not a solid mass but is made of particles with gaps between them.
Teacher's Tip: Think of this like pouring sand into a jar full of large rocks; the sand fills the gaps.
Exam Tip: Make sure to mention that the "water level remains the same" as this is the most important observation.

Question 8: What do you mean by inter-molecular forces?
Answer: Intermolecular force of attraction - The force of attraction between the molecules (like molecules or unlike molecules) is called intermolecular force of attraction.
These are the invisible "glues" that hold the particles of matter together. The strength of these forces dictates the physical state and properties of the material.
Teacher's Tip: Remember: Force = Attraction (the pull that holds things together).
Exam Tip: Distinguish between intermolecular (between molecules) and intramolecular (within a molecule) forces if you want to show extra knowledge.

Question 9: What are the forces of cohesion and adhesion?
Answer: The force of attraction between the molecules of similar kind is called force of cohesion. Example: The forces between water molecules. This force of cohesion keep the molecules of the substance bind together. The force of attraction between different types of molecules is called force of adhesion. Example: When a glass filled with water is emptied some water particles remain stuck to the glass due to the adhesion between water molecules and glass.
Cohesion is like family members sticking together, while adhesion is like sticking a post-it note to a wall. These forces explain why water forms drops and why it wets surfaces.
Teacher's Tip: Remember: Co- (together/same) for Cohesion, and Ad- (add/different) for Adhesion.
Exam Tip: Use the water-on-glass example to clearly distinguish between these two forces in your answer.

Question 10: State three characteristics of molecules of matter.
Answer: The particles of matter called molecules, have the following characteristics:
1. They are vety small in size.
2. They have spaces between them.
3. They are in constant random motion.
4. They always attract each other.
These characteristics are the foundation of the Kinetic Theory of Matter. They explain everything from how smells spread to why we can't compress solids.
Teacher's Tip: Use the acronym S-S-M-A (Small, Space, Motion, Attraction) to memorize these.
Exam Tip: If the question asks for three, listing four is safe, but make sure the first three are your strongest points.

Question 11: State the approximate spacing between two molecules of a matter.
Answer: The spacing between particles of a matter is called inter-molecular space.
This space varies wildly; it is around 10^-10 m in solids and much larger in gases. The exact distance depends on the state of matter and temperature.
Teacher's Tip: Space is the "Gap" between molecules.
Exam Tip: If a numerical value is required, specify that it is least in solids and maximum in gases.

Question 12: How do the solids, liquids and gases differ in their following properties (a) Size (b) Shape (c) Density
Answer:
Solids: Size - They have definite size; Shape - They have definite shape; Density - Highly dense
Liquids: Size - Indefinite; Shape - Indefinite; Density - Less denser than solids
Gases: Size - Indefinite; Shape - Indefinite; Density - Less denser than liquids and solids
Solids are rigid and compact, while liquids and gases are fluids that take the shape of their containers. This difference is entirely due to how their molecules are arranged and held together.
Teacher's Tip: Think of a brick (solid), water (liquid), and air (gas) to compare these properties quickly.
Exam Tip: Presenting this answer in a table format is the best way to earn full marks for comparisons.

Question 13: The molecules in a substance are in motion. What type of path do they follow?
Answer: The particles in a substance are not at rest (in motion), and they move randomly in all possible directions in a zig-zag Path
This random movement happens because molecules are constantly colliding with one another. These collisions force them to change direction constantly, creating a jagged, irregular path.
Teacher's Tip: Imagine a person trying to walk through a very crowded market; they have to move in a zig-zag way to avoid people.
Exam Tip: Use the term "Random Zig-Zag" specifically to describe this molecular behavior.

Question 14: Describe a simple experiment to illustrate that molecules are not at rest, but they constantly move.
Answer: Take a beaker. Fill it partly with water. Add some lycopodium powder in the beaker containing water. Stir the contents of the beaker with a glass rod. Take out few drops of this suspension on a glass plate. Place it on the table and illuminate it with a table lamp. Observe the glass plate through a microscope. It is found that the fine particles of lycopodium powder move rapidly in a random manner and their path is zig zag as shown in figure below.
The light powder particles are being knocked around by invisible, fast-moving water molecules. Even though we can't see the water molecules, we see their effect on the powder, proving they are in motion.
Teacher's Tip: This is like watching a balloon bounce around in a stadium; you know people are hitting it even if you can't see the individual hands.
Exam Tip: Identifying this as "Brownian Motion" in your answer will impress your teacher.

Question 15: Write down five general properties of solids, liquids and gases.
Answer:
Solids:
1. The molecules here are very tightly packed having negligible or veiy less intermolecular space.
2. They have the strongest intermolecular force of attraction.
3. The molecules have very small vibration about their mean position i.e. small amplitude.
4. They have a definite shape and volume.
5. They are generally hard and rigid.
6. They are good conductors of heat.
Liquids:
1. Molecules are less tightly packed.
2. The intermolecular force of attraction is less than that of solids.
3. The molecules here can move from one place to another
4. Do not have any particular shape of their own and thus acquire the shape of the vessel.
5. A particular quantity of a liquid has a definite volume at a given temperature.
Gases :
1. The force of attraction between the molecules is the least.
2. The intennolecular space is the largest.
3. Neither have a definite shape nor a defmite volume.
4. The molecules move independently.
5. Worst conductors of heat.
These properties define how we interact with different materials in our daily lives. From the hardness of a diamond to the flow of a river, molecular behavior is the cause.
Teacher's Tip: Focus on three things: Spacing, Force, and Movement to describe any state.
Exam Tip: If the question asks for five properties, make sure to list five distinct bullet points for each state.

Question 16: Give the molecular model for a solid and use it to explain why a solid has a definite volume and a definite shape.
Answer: Here the molecules are very tighty packed that there is no or very less intermolecular space and there is high intermolecular force of attraction (force of cohesion). The molecules do not move about their mean position and thus solids have a definite shape and volume.
Because the molecules are "locked" in place by strong forces, they cannot slide past each other to change the object's shape. This internal stability ensures that a solid keeps its form regardless of the container it is in.
Teacher's Tip: Visualize a solid as a box full of tightly packed oranges that cannot move at all.
Exam Tip: Mention "strong intermolecular force of attraction" as the main reason for the definite shape.

Question 17: Describe the molecular modcl for a liquid. I-low does it explain that a liquid has no definite shape, but has a definite Volume ?
Answer: Here the molecules are less tightly packed as compared to solids and also there is lesser force of intermolecular attraction. The intermolecular distance is greater than that in the solids. Thus, they donot have a definite shape but acquire the shape of the vessel in which they are contained but have a definite volume at a given temperature.
Liquid molecules are close enough to keep a constant volume, but free enough to slide over each other to take a new shape. This balance between attraction and movement is what makes liquids flow.
Teacher's Tip: Liquids are like "Social Molecules" - they stay close together but keep moving around.
Exam Tip: Be clear that "fixed volume" comes from the attraction, while "no fixed shape" comes from the ability to move.

Question 18: A gas has neither a definite volume nor a definite shape. Describe the molecular model to explain it.
Answer: Here the molecules are far apart from each other i.e. have the greatest intermolecular distance which result into the weakest intermolecular forces of attraction. The molecules as are not bound by any strong force move about freely and thus gases do not have a definite shape and also do not have any definite volume.
Gas molecules are essentially independent of each other, flying in straight lines until they hit something. Because there is so much empty space between them, they can be squeezed together or allowed to spread out forever.
Teacher's Tip: Think of gas molecules as "Lone Wolves" that don't want to stick to anyone else.
Exam Tip: Use the term "Weakest intermolecular forces" to explain why gases expand to fill any volume.

Question 19: Distinguish between the three states of matter—solid, liquid and gas on the basis of their molecular models.
Answer: Solids: Here the molecules are very tighty packed that there is no or very less intermolecular space and there is high intermolecular force of attraction (force of cohesion). The molecules do not move about their mean position and thus solids have a definite shape and volume. Liquids: Here the molecules are less tightly packed as compared to solids and also there is lesser force of intermolecular attraction. The intermolecular distance is greater than that in the solids. Thus, they donot have a definite shape but acquire the shape of the vessel in which they are contained but have a definite volume at a given temperature. Gases: Here the molecules are far apart from each other i.e. have the greatest intermolecular distance which result into the weakest intermolecular forces of attraction. The molecules as are not bound by any strong force move about freely and thus gases do not have a definite shape and also do not have any definite volume.
This comparison shows a spectrum of molecular freedom, from the "locked" solid to the "free" gas. As we add energy (heat), molecules move from being tightly bound to being completely independent.
Teacher's Tip: Use the "Packing" trick: Solids are tight, Liquids are loose, and Gases are far apart.
Exam Tip: A comparative table is the most effective way to present this answer in an exam.

Question 20: Distinguish between solids, liquids and gases on the basis of their following properties: (a) compressibility (b) fluidity (c) rigidity (d) expansion on heating
Answer:
(a) Compressibility: Solids - Not compressible; Liquids - Negligibly compressible; Gases - Highly compressible.
(b) Fluidity: Solids - Not possible; Liquids - Can flow; Gases - Can flow.
(c) Rigidity: Solids - Highly rigid; Liquids - Less rigid; Gases - Not rigid.
(d) Expansion on heating: Solids - Low; Liquids - More than solids; Gases - More than liquids.
These properties are direct consequences of the intermolecular spaces between particles. For example, gases compress easily because they have huge gaps that can be closed under pressure.
Teacher's Tip: Remember: Space = Compressibility. If there's no space, you can't squeeze it!
Exam Tip: Ensure you use terms like "Negligible" or "Highly" to accurately describe the degree of the property.

Question 21: What do you mean by the change of state of matter? Explain: (a) the change of a solid into a liquid at a constant temperature, and (b) the change of a liquid into a gas at a constant temperature.
Answer: The change in state of matter of a substance from solid to liquid or from liquid to gas is brought by imparting heat energy to it at a constant temperature.
(a) The process of change of a substance from solid state into its liquid state on absorption of heat at a particular temperature, called the melting point, is called melting or fusion i. e. Solid -(Melting/Heat absorbed)-> Liquid.
(b) The process of change of a substance from a liquid state to its gaseous state at a particular temperature, called the boiling point, is called boiling or vaporisation, i.e. Liquid -(Boiling/Heat absorbed)-> Gas.
When we add heat, molecules move faster and eventually overcome the forces holding them in their current state. This transformation happens at specific temperatures that are unique to each substance.
Teacher's Tip: Think of heat as "Energy for Movement" - the more you add, the more molecules break free.
Exam Tip: Always mention that these changes occur at "constant temperature" to show you understand phase transitions.

ADDITIONAL QUESTIONS

Question 1: Define matter.
Answer: Anything that has mass and occupies space is called matter.
This is the simplest definition of matter used in science. It covers everything from the smallest dust particle to the largest planet in the galaxy.
Teacher's Tip: If you can weigh it and it takes up room, it's matter.
Exam Tip: This is a very common 1-mark question; memorize it exactly as written.

Question 2: What is volume?
Answer: The amount of space occupied by a matter is called its volume.
Volume tells us how much "room" an object takes up in three-dimensional space. It is usually measured in liters or cubic centimeters.
Teacher's Tip: Volume is just "Occupied Space."
Exam Tip: Don't confuse volume with mass; mass is the amount of stuff, and volume is the space it fills.

Question 3: What is mass?
Answer: Mass is the quantity of matter contained in the body.
Mass remains constant no matter where you go in the universe because the number of atoms in your body doesn't change. It is typically measured in grams or kilograms.
Teacher's Tip: Think of mass as the "Total Count" of particles in an object.
Exam Tip: Remember that mass is different from weight; weight changes with gravity, but mass does not.

Question 4: If an object weighs 6 kg on earth. What will be its weight on moon?
Answer: Weight of body on moon = 1 / 6 th of its weight on earth. ∴ Body will weigh 1 / 6 × 6 = 1 kg on moon
Because the moon is smaller than the earth, it has much weaker gravity. This results in everything being significantly lighter there, though the amount of matter stays the same.
Teacher's Tip: Just divide the Earth weight by 6 to find the Moon weight.
Exam Tip: Always show the calculation (1/6 x 6) to get full marks for the logic.

Question 5: If an object is taken to the moon from the earth what will be its mass?
Answer: Mass of a body does not change with change in gravity. So mass of a body will remain the same on moon.
Since mass is just the amount of "stuff" inside you, moving to another planet doesn't add or remove any atoms. Only your weight (the pull of gravity) would change.
Teacher's Tip: Mass is "Stubborn" - it never changes regardless of location.
Exam Tip: This is a favorite trick question in exams; remember: Weight changes, Mass stays the same.

Question 6: Name the smallest particle from which matter is made up.
Answer: The smallest particle from which matter is made up is atom.
Atoms are the tiny, indivisible units that combine to form everything we know. Each chemical element is made of one specific type of atom.
Teacher's Tip: Think of atoms as the "Basic Lego Bricks" of the universe.
Exam Tip: While molecules are small, the "atom" is the absolute fundamental unit mentioned in this context.

Question 7: What are molecules?
Answer: Molecules are made of atoms. Molecules exhibit the properties of that kind of matter and has independent existance.
A molecule is what you get when atoms decide to "hold hands" or bond together. It is the smallest part of a substance that can still be recognized as that substance.
Teacher's Tip: Atoms are the alphabet; molecules are the words.
Exam Tip: Mention "independent existence" as it is a key scientific distinction for molecules.

Question 8: Give one difference between atoms and molecules.
Answer: Atoms may or may not have independent existance. Molecules have independent existance.
Most atoms are too reactive to stay alone and prefer to bond with others. Molecules, however, are stable enough to exist on their own in nature.
Teacher's Tip: Atoms are "Social" (they need others), Molecules are "Independent."
Exam Tip: This is the most important scientific difference between the two; learn it by heart.

Question 9: Define : (a) Intermodular force of attraction. (b) Intermodular space.
Answer: (a) The molecules of matter are always in motion and attract each other with a force called intermodular force of attraction due to which they are held together.
(b) The molecules can move only when there are gaps or space between them, this space is called intermolecular space.
These two factors are in a constant tug-of-war: the force pulls molecules together, while the space allows them to move. Their balance determines if something is solid, liquid, or gas.
Teacher's Tip: Forces are the "Glue," and Space is the "Gap."
Exam Tip: Use the correct spelling "intermolecular" (with an 'e') as found in standard textbooks.

Question 10: Classify the following into solids, liquids and gases. Oxygen, milk, common salt, wax, stone, water vapour, carbon-dioxide, sugar, mercury, coal, blood, butter, copper, coconut oil, kerosene.
Answer: Solids : Common salt, Wax, Stone, Sugar, Coal, Butter, Copper.
Liquids : Milk, Mercury, Blood, Coconut, oil, Kerosene.
Gases : Oxygen, Water, vapour, Carbondioxide.
This classification is based on the physical state of these materials at room temperature. Note that mercury is a rare example of a metal that is liquid at room temperature.
Teacher's Tip: If you can hold it, it's solid; if you can pour it, it's liquid; if it floats away, it's gas.
Exam Tip: Double-check substances like "mercury" and "wax" as students often misclassify them.

Question 11: Why do solids, liquids and gases differ in their physical states?
Answer: 1. Intermolecular force of attraction. 2. Intermolecular spaces are two important properties of matter that account for the different states of matter.
The difference in physical state is simply a matter of how strong the internal pull is and how much room there is between particles. Change these two variables, and you change the state of the substance.
Teacher's Tip: It's all about the "Forces and the Spaces."
Exam Tip: Mentioning both "force" and "space" is essential for a full-mark answer.

Question 12: What are fluids? Give two examples.
Answer: Substances that can flow are called fluids. e.g. gases (oxygen, hydrogen), liquids (water, petrol, sulphuric acid).
Fluids are materials that do not have a fixed shape and can be poured or flow through pipes. Both liquids and gases are classified as fluids because their molecules can move past each other.
Teacher's Tip: "Fluid" doesn't just mean liquid; it includes gases too!
Exam Tip: Always provide one liquid and one gas example to show you know fluids include both states.

Question 13: Define interconversion of states of matter.
Answer: The process by which matter changes from one state to another and back to original state, without any change in its chemical composition.
An example is ice melting into water and then freezing back into ice; the material stays as H₂O throughout. This is a physical change, not a chemical one.
Teacher's Tip: Interconversion is like a "Reversible Costume Change" for matter.
Exam Tip: The key phrase to include is "without any change in chemical composition."

Question 14: What are the two conditions for the interconversion of states of matter?
Answer: Two conditions are : change in 1. Temperature 2. Pressure
By heating/cooling or by squeezing/expanding matter, we can force it to change its state. For instance, high pressure can turn a gas into a liquid, like in an LPG cylinder.
Teacher's Tip: Think of "P & T" (Pressure and Temperature) as the two control knobs for matter.
Exam Tip: Be sure to list both conditions; temperature is common knowledge, but pressure is equally important.

Question 15: How a liquid changes into its gaseous state ? Explain ?
Answer: As a liquid is heated, its particles starts gaining energy and move more vigorously which increases the gaps between the particles and decreasing the force of attraction. Ultimately a liquid changes into gaseous state.
Heat gives molecules the speed they need to break the "handshakes" of intermolecular forces. Once they are moving fast enough, they fly apart and become a gas.
Teacher's Tip: Heat is like "Caffeine" for molecules; it makes them move much faster!
Exam Tip: Use words like "energy," "vigorously," and "gaps" to describe the molecular process clearly.

Question 16: Water cycle is an example of inter conversion of states of water. Explain.
Answer: Water from oceans, rivers lakes from leaves of trees (transperation) changes into vapours when temperature increases or evaporates and enters the atomsphere as clouds when temperature falls the vapours change into water and some of it in the form of snow fall on mountains and earth in the form of water and hales and this continues. Thus water cycle is example of inter convertion of states of water.
The Earth naturally recycles water by constantly changing it from liquid to gas and back to liquid or solid. This giant natural system ensures we always have a supply of fresh water.
Teacher's Tip: The water cycle is just nature's way of melting, evaporating, and freezing water on a huge scale.
Exam Tip: Mention specific processes like "evaporation" and "condensation" to make your explanation more scientific.

Question 17: State the general properties of a solid.
Answer: General Properties Of A Solid : 1. Solids are hard that is, they have a definite shape and volume. 2. Solids are generally incompressible. 3. Solids are rigid, that is, they do not flow.
These properties exist because the molecules in a solid are packed as tightly as possible. Because there is no room to move, solids remain tough and keep their form.
Teacher's Tip: Remember the three "D"s: Definite shape, Definite volume, and Durable (hard).
Exam Tip: If asked to explain *why* solids are incompressible, mention the "negligible intermolecular space."

Question 18: What is the relation between intermolecular space and intermolecular force?
Answer: The force of attraction between the molecules of a given substance is called intermolecular force and the space between these molecules is called intermolecular space. The basic relation between the two is that they are inversely proportional to each other. More is the intermolecular force lesser is the intermolecular space and vice-versa.
This means that when the "pull" between molecules is very strong, they are dragged closer together, leaving less space. Conversely, if they are far apart, the pull between them becomes very weak.
Teacher's Tip: Think of it like a hug: the stronger the hug (force), the less space there is between two people.
Exam Tip: Use the term "inversely proportional" to get a high score on this question.

Question 19: Why liquids do not have a definite shape?
Answer: Molecules of a liquid are held by weak intermolecular forces. This force is sirong enough to hold the molecules together but not strong enough to hold them at fixed positions. As a result liquids have a fixed volume but not shape.
Because liquid molecules can slide around each other, they will always move until they hit the walls of whatever container they are in. This fluidity is why you can pour water into any shaped glass.
Teacher's Tip: Liquid molecules are like a group of friends walking together; they stay together but keep changing their order.
Exam Tip: Contrast this with solids, where forces are strong enough to keep molecules in "fixed positions."

Question 20: What happens when a solid is heated ?
Answer: When a solid is heated, its molecules gain energy and vibrate faster. A stage comes ,when they overcome intermolecular force of attraction and start moving from each other. This results in melting of solid.
Heating provides the kinetic energy needed to break the rigid bonds of the solid state. Once these bonds weaken, the solid transforms into a more flexible liquid state.
Teacher's Tip: Heat acts like a "Key" that unlocks molecules from their fixed spots.
Exam Tip: Be sure to mention that vibrations increase *before* the substance actually melts.

Question 21: Give reasons for the following.
Answer:
1. Gases can be compressed easily. Ans. The reason for this property of gases is that there is very large intermolecular space between gas molecules. On mere applying pressure, they are easily compressed.
2. Liquids can flow easily. Ans. In liquids intermolecular force is weaker than that of solids. So molecules in a liquids can slip over one another and liquids can flow uniike solids.
3. We need to classify things. Ans. We need to classify things in order to distinguish them. In this way, things can be categorized and can be easily studied.
4. Pure substances have fixed melting or boiling points. Ans. Pure substances consists of only one kind of matter. All the particles of a pure substance are alike. It has a definite composition and similar properties. This is the reason that pure substances have fixed melting or boiling points.
5. Electricity is not considered matter. Ans. Electricity neither has mass nor it occupies space. Beside it can not be seen by our eyes. This is why electricity is not considered mattet
These logical explanations help us understand why different substances behave the way they do in our environment. Science isn't just about facts, but about the "why" behind the physical world.
Teacher's Tip: For "Give Reason" questions, always start your thought process with "Because..."
Exam Tip: For the electricity question, specifically state that it lacks mass and volume to prove it isn't matter.

Question 22: Define the following terms. 1. Matter 2. Intermolecular force 3. Element 4. Atom 5. Molecule
Answer:
1. Matter - Anything that has mass and occupies space is called matter.
2. Intermolecular force - The force of attraction between the molecules of a given substance is called intermolecular force.
3. Element - It is defined as that pure substance which contains only one type of atoms e.g. hydrogen, chlorine.
4. Atom - An atom is the smallest part of an element that takes place in a chemical reaction.
5. Molecule - A molecule is a smallest part of a compound that exists independently.
These definitions are the vocabulary of chemistry. Mastering these terms is the first step toward understanding more complex scientific concepts.
Teacher's Tip: Use a "Russian Doll" analogy: Atoms go inside Molecules, and Molecules go inside Matter.
Exam Tip: Learn these definitions verbatim; they are frequently asked in the "Define the following" section of exams.

Question 23: Write your observation and conclusion for the following: (1) When a small stone is gently dipped into a glass filled with water. (2) When one of the balloons suspended to the metre scale is punctured while other remains inflated ?
Answer: (1) You will see that some water flows out of the tumbler and collects in the bowl. Remove the stone from the tumble. The level of water in the tumbler comes down. Now, pour the water collected in the bowl back into the tumbler. The glass tumbler is filled again. This is because the stone occupied space and therefore drives the water out of the tumbler. This proves that not only solids but liquids also occupy space.
(2) Take two similar balloons and inflate them equally. Suspend one balloon to the left of a metre scale and the other one to the right of it. Balance the scale in the middle with the help of a peg. When one is punctured, the scale tilts toward the inflated one, proving air has mass.
These simple experiments prove the two core properties of matter: mass and volume. Even things we can't see, like air, follow these rules.
Teacher's Tip: The "Stone in Water" experiment is a classic way to see volume in action.
Exam Tip: When writing observations, describe exactly what you *see* (e.g., "water flows out"). When writing conclusions, state the *scientific law* it proves.

Question 24: Give reasons : (a) Why do liquids and gases flow but solids do not ? (b) A gas fills up the space available to it. (c) The odour of scent spreads in a room. (d) We can walk through air. (e) Liquids have definite volume but no difinite shape.
Answer:
(a) The molecules of liquids and gases are far apart i.e. have more gaps, intermolecular attraction force is very less as compared to solids, hence liquids and gases can flow but solids do not as gaps in solid molecules is less and molecular force of attraction very strong.
(b) Intermolecular force of attraction is least and intermolecular spaces are very large, hence gases can fill up the space available to them.
(c) Scent fumes (molecules) being gases fill the spaces between air molecules and the molecules of air fill the spaces between scent molecules due to diffusion, fumes spread into a room. OR Due to inter-mixing of scent molecules and air molecules, scent fumes spread into the room.
(d) The molecules of air are far apart i. e. large gaps and we can walk through air easily.
(e) The molecules of liquid are loosely packed and intermolecular force of attraction is small but number of molecules in it remain the same. Hence liquids have definite volume but no definite shape.
These phenomena are everyday examples of the kinetic theory of matter. Understanding the microscopic gaps and forces explains why we can move through air but not through a brick wall.
Teacher's Tip: Use the "Gap Theory" - more gaps mean more movement and flow.
Exam Tip: For the scent question, make sure to use the word "diffusion" as it is the scientific term for particles spreading out.

Question 25: Give reasons : (a) When a teaspoon of sugar is added to half a glass of water and stirred, the water level in the glass does not rise. (b) When an empty gas jar is inverted over a gas jar containing a coloured gas, the gas also spreads into the empty jar. (c) A red ink drop added to small amount of water in a glass turns water red in some time.
Answer:
(a) Add one teaspoon of sugar to it and stir. The sugar disappear but the level of water in the glass does not rise that means the volume of water has not increased. Because the sugar particles are adjusted between the water molecules. The shows that there are intermolecular gaps in water.
(b) This shows that gases can fill up all the space that they get, and they have neither a fixed shape nor a fixed volume. They have no free surfaces, either.
(c) If we put a drop of red ink in a glass of water, its particles diffuse with particles of water slowly but continuously and the water turns red.
These observations prove that matter is made of tiny particles with space between them. They also show that these particles are in constant motion, allowing them to mix and spread naturally.
Teacher's Tip: Think of sugar dissolving as "Parking a car" in the empty spots between water molecules.
Exam Tip: For question (c), the key word is "diffusion" – the spontaneous mixing of particles.

Question 26: Differentiate between the following. (a) Liquids and gases (b) Atoms and molecules
Answer:
(a) Liquids: Have definite volume but no definite shape; Intermolecular force of attraction is weaker than that in solids; Molecules are not as tightly packed as in solids; Molecules have no fixed position; Can be compressed slightly; Are capable of flowing.
Gases: Have no definite shape or volume; Intermolecular force of attraction is the weakest (almost negligible); Intermolecular space is the maximum and the molecules are far apart; Molecules move around freely; Can be easily compressed; Can flow in all directions.
(b) Atoms: 1. it is the smallest part of an element. 2. It does not have independent existence.
Molecules: 1. It is the smallest part of a compound. 2. It has an independent existence.
Understanding these differences helps scientists predict how different substances will react under various conditions. Solids, liquids, and gases are just different ways for molecules to hang out together.
Teacher's Tip: Use a table to compare these; it makes the differences "Pop" and easier to study.
Exam Tip: In the atom vs. molecule comparison, "independent existence" is the point that scores the most marks.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks
(a) Water is a matter because it has mas and occupies space.
(b) Any matter which has a definite volume but no definite shape is called a liquid.
(c) Fluid can flow.
(d) The molecules are at a greater distance in gases compared to liquids.
(e) Water boils at 100 °C.
(f) The physical state of a substance, which has a fixed volume but no fixed shape is liquid.
(g) All matter is made up of tiny particles called atoms.
(h) Liquids have a definite Volume.
(i) The temperature at which a liquid boils is called the boiling point of that liquid.
(j) Molecules in a solid are packed very closely together.
(k) Liquids have no definite shape.
(l) When a gas is cooled, its molecules loose energy.
(m) Matter is anything that has mass and occupies space.
Answer: (a) mass, space; (b) liquid; (c) Fluid; (d) gases; (e) 100 °C; (f) liquid; (g) atoms; (h) Volume; (i) boiling; (j) closely; (k) shape; (l) loose; (m) mass, space.
These blanks cover the core definitions and physical properties discussed in the chapter. From boiling points to molecular packing, these facts are the building blocks of Class 6 science.
Teacher's Tip: For the "loose energy" part, remember that cooling is the opposite of heating; it slows things down.
Exam Tip: When filling in units like "100 °C", always include the degree symbol and the 'C' for Celsius.

1. Write whether the following statements are true or false.

(a) Only water can exist in three different states.
Answer: True (Note: In standard earth conditions, this is often taught, though other substances can under extreme lab conditions).

(b) If the container in which a gas is collected has an opening, the gas will flow out and spread itself indefinitely.
Answer: True

(c) Solids have the largest inter-molecular space.
Answer: False

(d) There is no difference between evaporation and boiling.
Answer: False

(e) All solids, on heating, first change to the liquid and then to the gaseous state always.
Answer: False

(f) The intermolecular force of attraction is the weakest in gases.
Answer: True

(g) A gas has no free surface.
Answer: True

(h) Intermolecular force of attraction is greater in gases than in liquids.
Answer: False
These statements test common misconceptions about the states of matter. Correcting them helps reinforce the actual scientific behavior of molecules.
Teacher's Tip: Remember "S-L-G" (Solid-Liquid-Gas) for increasing space and decreasing force.
Exam Tip: Watch out for "Only" or "Always" in statements (a) and (e); they are often clues that the statement might be false (except in specific taught examples).

2. Write true or false for each statement. Rewrite the false statements correctly.

(a) Matter cannot exist in different states.
Answer: False. Matter can exist in different states.

(b) If the intermolecular space is more than the intermolecular force will be weaker.
Answer: True

(c) Solids and liquids can flow.
Answer: False. Gases and liquids can flow.

(d) Solids can be compressed easily.
Answer: False. Solids cannot be compressed easily.

(e) The smallest part of an element capable of independent existence is called an atom.
Answer: True

(f) The intermolecular space in a gas is almost negligible.
Answer: False. The intermolecular space in a gas is very large.
Rewriting statements is a great way to ensure you actually understand the logic rather than just guessing. It forces you to construct the correct scientific fact in your mind.
Teacher's Tip: When rewriting, just change the key word (e.g., "cannot" to "can") to make it right.
Exam Tip: In exams, you usually get half marks for saying "False" and the other half for the correct rewrite. Don't skip the second part!

3. For each of the following statements, say whether it describes a solid, a liquid or a gas.

(a) Particles move about very quickly.
Answer: Liquid (Note: Usually Gas, but textbook says Liquid here).

(b) Particles are quite close together.
Answer: Solid

(c) Particles are far apart and move in all directions.
Answer: Gas
Identifying states based on particle behavior is a fundamental skill in Class 6 science. It helps us visualize what is happening at a level we cannot see.
Teacher's Tip: Speed = Energy = Gas (mostly). Closeness = Solid.
Exam Tip: If a statement mentions "all directions," it is almost always referring to a gas.

4. Match the following
Column A
(a) Solids
(b) Sublimation
(c) Boiling point
(d) Gases
(e) Intermolecular space

Column B
(i) Can flow in all directions.
(ii) The temperature at which a liquid changes into its gaseous state.
(iii) Any number of free surfaces.
(iv) Gaps between particles.
(v) Change of state from solid to gas.
Answer:
(a) Solids - (iii) Any number of free surfaces.
(b) Sublimation - (v) Change of state from solid to gas.
(c) Boiling point - (ii) The temperature at which a liquid changes into its gaseous state.
(d) Gases - (i) Can flow in all directions.
(e) Intermolecular space - (iv) Gaps between particles.
This question helps you understand the unique physical characteristics of different states of matter and the specific terms used for phase transitions. It clearly differentiates how solids maintain fixed boundaries while gases are free to expand into any available space.
Teacher's Tip: Remember "S.S.S" - Solids have multiple Surfaces, while gases have none!
Exam Tip: When matching definitions, read the full sentence back to yourself to ensure the scientific logic is sound before finalizing your choice.

2. Match the columns.
1. iron and gold
2. melting
3. intermolecular space
4. compound
5. gases

a. molecules are very far apart
b. pure substance
c. non-metals
d. the space between the molecules
e. change from solid to liquid
f. metals
Answer:
1. iron and gold - f. metals
2. melting - e. change from solid to liquid
3. intermolecular space - d. the space between the molecules
4. compound - b. pure substance
5. gases - a. molecules are very far apart
This matching exercise connects physical changes and chemical substances to their fundamental molecular properties. It helps reinforce how macroscopic behaviors, such as state transitions, are directly caused by the arrangement and movement of tiny particles.
Teacher's Tip: Use "M-S-G" to remember the gas property: Molecules Stay Gapped (far apart).
Exam Tip: In "Match the columns" questions, always write the number and letter pairs clearly and reread the matched statements to ensure they make scientific sense.

5. Name the phenomenon which causes the following changes

(a) Formation of water vapour from water.
Answer: vaporation.

(b) Disappearance of camphor.
Answer: sublimation.

(c) Conversion of ice into water.
Answer: melting.

(d) Conversion of water into steam.
Answer: boiling.
Naming the specific phenomenon shows that you understand the different ways matter can change state. Each term describes a unique process triggered by temperature changes.
Teacher's Tip: Camphor "disappearing" without getting wet is the perfect example of sublimation.
Exam Tip: Use "Vaporization" as the broad term, and "Boiling" or "Evaporation" if you need to be more specific.

6. Give two examples for each of the following

(a) Substances which sublime.
Answer: Naphthalene, camphor, dry ice.

(b) Substances which do not change their states.
Answer: Paper, sugar.

(c) Substances which are rigid and not compressible.
Answer: Glass, stone, pen.
Providing examples helps ground scientific theories in reality. It's much easier to remember that solids are rigid when you think of a hard stone or glass.
Teacher's Tip: Keep a list of "Sublimation Superstars" like Naphthalene and Dry Ice.
Exam Tip: Always provide two examples when asked, even if you know three or four.

MULTIPLE CHOICE QUESTIONS

1. Which one is a kind of matter:
1. light 2. petroleum 3. sound 4. heat
Answer: 2. petroleum
Petroleum has mass and takes up space, whereas light, sound, and heat are forms of energy. Matter and energy are the two fundamental "things" that make up our world.
Teacher's Tip: If you can put it in a bottle, it's matter. You can't bottle sound!
Exam Tip: Energy forms like light and heat are the most common wrong answers; ignore them when looking for matter.

2. The state of matter which has no definite shape or volume is called
1. solid 2. liquid 3. gas 4. water
Answer: 3. gas
Gases have so much energy and such weak forces that they refuse to stay in one shape or size. They will expand to fill a giant room or compress into a small tank.
Teacher's Tip: Gases are the "Space Fillers" of the universe.
Exam Tip: "No shape AND no volume" = Gas. "No shape but FIXED volume" = Liquid.

3. There are large intermolecular gaps in
1. water 2. iron ball 3. common salt 4. air .
Answer: 4. air .
Air is a mixture of gases, and as we know, gas molecules are very far apart. This is why you can walk through air without feeling much resistance.
Teacher's Tip: Gaps = Airiness. Think about how easy it is to move your hand through air compared to water.
Exam Tip: Look for the gaseous option when asked about "large gaps" or "maximum space."

4. All kinds of matter
1. occupy space and have definite mass 2. have mass and definite shape 3. can change their states 4. have definite volume
Answer: 1. occupy space and have definite mass
These are the two non-negotiable requirements for something to be called matter. While shape and state can change, mass and space occupancy are universal.
Teacher's Tip: Remember: Matter = Mass + Space.
Exam Tip: Option 1 is the most scientifically accurate definition; the others only apply to some types of matter.

5. A kind of matter which can sublime is
1. water 2. plastic 3. milk 4. iodine
Answer: 4. iodine
Iodine crystals turn directly into a purple vapor when heated, skipping the liquid phase. This makes iodine a classic example used in science labs to demonstrate sublimation.
Teacher's Tip: Iodine, Camphor, and Naphthalene are the "Sublimation Trio."
Exam Tip: Learn the color of iodine vapor (purple) as it's often asked in related questions.

6. A substance which can change its state
1. wood 2. oxygen 3. paper 4. cloth
Answer: 2. oxygen
While we usually see oxygen as a gas, it can be turned into a liquid or even a solid at extremely low temperatures. Wood and paper generally burn or decompose before they can melt or evaporate.
Teacher's Tip: Most pure elements and simple molecules like oxygen can change state with enough cooling or heating.
Exam Tip: Pure substances (like oxygen) change state, while complex mixtures (like wood) usually do not.

7. The process by which a solid changes into a liquid is called
1. freezing 2. melting 3. condensation 4. evaporation
Answer: 2. melting
Melting happens when a solid absorbs enough heat to overcome the intermolecular forces holding its molecules in place. It is a phase change that everyone sees when they use ice cubes.
Teacher's Tip: Melting is "Solid to Liquid." Freezing is the reverse.
Exam Tip: Ensure you know the "partner" processes: Melting/Freezing and Evaporation/Condensation.

8. A solid is a state of matter that has
1. no definite shape. 2. large intermolecular space. 3. high intermolecular force of attraction. 4. no definite volume.
Answer: 3. high intermolecular force of attraction.
This strong internal pull is what keeps the solid rigid and gives it a fixed shape and volume. Without this force, the molecules would just wander off like they do in a gas.
Teacher's Tip: Strong attraction = Solid foundation.
Exam Tip: High attraction is the *reason* for definite shape and volume; look for the "cause" in multiple-choice questions.

9. Which of the following is a property of the liquids ?
1. they can flow 2. they are malleable 3. they have a definite shape 4. they are rigid
Answer: 1. they can flow
Because liquid molecules can slide past each other, the substance as a whole is fluid. This allows it to move through pipes and take the shape of any container.
Teacher's Tip: If it flows, it's a fluid (Liquid or Gas).
Exam Tip: Malleability and rigidity are properties of solids, not liquids; use elimination to find the right answer.

10. Gases
1. cannot be compressed easily. 2. occupy the entire space of the container. 3. have definite shapes. 4. cannot flow.
Answer: 2. occupy the entire space of the container.
Gases will always expand to fill every available cubic inch of whatever container they are placed in. This is because their molecules move independently and rapidly in all directions.
Teacher's Tip: Gas molecules are like "Adventurers" - they explore every corner of their world.
Exam Tip: This is the most defining physical characteristic of a gas; memorize it for your MCQ sections.