Selina Concise Solutions for ICSE Class 8 Chemistry Chapter 1 Matter

Points to Remember:

• Matter has mass and occupies space.
• Matter is made up of atoms and molecules.
• Atoms are the smallest particles of matter which may or may not have independent existence.
• Molecules are capable of independent existence. They are made up of atoms of same kind or different kinds.
• The atoms and molecules are in random motion.
• There are gaps between the molecules of matter called as intermolecular space.
• There exists a force of attraction between the molecules known as intermolecular force of attraction.
• Matter exists in three states: solids, liquid and gas.
• Matter can change from one state to another on changing temperature and pressure.
• The change of state of a matter from one form into another is called interconversion of states of matter.
• Matter can neither be created nor be destroyed. Only it can be changed from one form to another during a chemical reaction. This is known as law of conservation of mass.

Exercise

Question 1: Define:
(a) matter
(b) intermolecular force of attraction.

Answer:
(a) Matter is anything which has mass, occupies space and can be percieved by our senses.
Example: Air, Book.
(b) The molecules of matter are always in motion and attract each other with a force called intermolecular force of attraction due to which they are held together.
Everything you see, touch, or smell is made of matter because it takes up room and has weight. This invisible force acts like a tiny magnet that pulls molecules toward each other to keep them from flying apart.
Teacher's Tip: Think of "Intermolecular Force" as the "Invisible Glue" that holds particles together.
Exam Tip: Always include "occupies space" and "has mass" to get full marks for the definition of matter.

Question 2: What are the three states of matter ? Define each of them with two examples.

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.
Matter takes different forms based on how close its particles are and how much they move. While a rock keeps its shape, water takes the shape of the glass you pour it into.
Teacher's Tip: Remember: Solids stay, Liquids flow, and Gases grow (to fill any space).
Exam Tip: Mention both "shape" and "volume" for each state to provide a complete scientific definition.

Question 3: Define interconversion of states of matter. What are the two factors responsible for the change 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 is called interconversion state of matter.
Two factors responsible for change of state of matter are: change in
(i) Temperature (ii) Pressure
This means a substance like ice can turn into water and then back to ice without becoming a new chemical. By either heating it up or squashing it with pressure, we force the molecules to change their behavior.
Teacher's Tip: Think of "Interconversion" as a "Two-Way Street" where matter can travel back and forth between states.
Exam Tip: Be sure to state that the "chemical composition" remains the same during these physical changes.

Question 4: State the main postulates of kinetic theory of matter.

Answer:
The main postulates of the theory are:
1. Matter is composed of very small particles called atoms and molecules.
2. The constituent particles of a kind of matter are identical in all respects.
3. These particles have space or gaps between them which is known as interparticular or intermolecular space.
4. There exists a force of attraction between the particles of matter which holds them together. This force of attraction is known as interparticular or intermolecular force of attraction.
5. Particles of matter are always in a state of random motion and possess kinetic energy, which increases with increase in temperature and vice-versa.
This theory explains that everything is made of tiny moving dots that never stop vibrating. When we heat things up, these dots dance much faster and push each other further away.
Teacher's Tip: Kinetic means "Motion," so this theory is all about "Matter in Motion."
Exam Tip: If asked for postulates, try to list at least four to ensure you cover all the fundamental points.

Question 5: What happens to water if
(a) it is kept in a deep freezer
(b) it is heated
Explain the phenomenon of change of state of water.

Answer:
(a) When water is kept in a deep freezer, it gets cooled and change into ice at 0°C ice.
(b) Water on heating changes into steam at 100°C
Phenomenon of change of state of water:
Water is a liquid under ordinary conditions but, when it is kept in a deep freezer, it changes into ice at 0°C and when ice is kept at room temperature again changes back into liquid water.
Similarly, water on heating change into steam at 100°C, which on cooling changes back into liquid water. But there is no change in the chemical composition of water. When its state changes from liquid to solid or liquid to gaseous state.
Ice \xleftarrow{Cool} \xrightarrow{Heat} Water \xleftarrow{Cool} \xrightarrow{Heat} Steam
(solid) (liquid) (gas)
Water is a unique substance because we can easily see it in all three states in our daily lives. Freezing and boiling are just physical movements of molecules getting closer or further apart.
Teacher's Tip: 0°C is the "Magic Number" for freezing and 100°C is the "Magic Number" for boiling.
Exam Tip: Use the diagram showing arrows for heat and cool to make your answer more visual and professional.

Question 6: (a) State the law of conversation of mass.
(b) What do you observe when, barium chloride solution is mixed with sodium sulphate solution ?

Answer:
(a) “Matter can neither be created nor be destroyed in a chemical reaction”. However, it may change from one form to another in the process.
It can also be stated as, “In a chemical reaction, the total mass of the reactants is equal to the total mass of the products”.
(b)
BaCl₂aq. + Na₂SO₄(aq.) \rightarrow BaSO₄(ppt.) + 2NaCl(aq.)
We will observe that a white insoluble solid (precipitate) of barium sulphate is formed along with a solution of sodium chloride. Wait for ten minutes to complete the reaction and the solid formed to settle down.
Weigh the content again and note the reading.
We will observe that,
total mass of the apparatus + reactants = total mass of apparatus + products
Hence the law of conservation of mass is verified.
This law proves that even when chemicals react and change colors or forms, no weight is lost or gained. The atoms simply trade partners, like kids switching teams in a game, but the total number of kids stays the same.
Teacher's Tip: Mass is like LEGO bricks; you can build a car or a house, but you still have the same number of bricks.
Exam Tip: In your explanation, always use the terms "Reactants" (the start) and "Products" (the end).

Question 7: Give reasons:
(a) A gas can fill the whole vessel in which it is enclosed.
(b) Solids cannot be compressed.
(c) Liquids can flow.
(d) When magnesium is burnt in air, there is an increase in mass after the reaction.

Answer:
(a) Because, in gases, the molecules are free to move. They are not stuck to each other and the intermolecular force of attraction is least in the gases. So the gas almost filled the whole vessel in which it is enclosed.
(b) In solids, particles are closely packed. There is a strong force of attraction and the intermolecular space is almost zero. Therefore the molecules are not free to move, which makes them hard and rigid. So solids can not be compressed.
(c) In liquids intermolecular force is weaker because the particles are not closely packed and hence there is large intermolecular space. So molecules in a liquids can move randomely and hence liquids can flow easily.
(d) When magnesium ribbon is burnt in air, a white solid, magnesium oxide is formed. The mass of magnesium oxide is more than the mass of magnesium. This is because mass of oxygen used is not taken. If that is considered, the total mass of the reactants and the products is found to be almost equal.
2Mg + O₂ x
{Heat} 2MgO
Gases are like energetic puppies running everywhere, while solids are like people sitting in a crowded movie theater with no room to move. In the magnesium experiment, the extra weight comes from the "invisible" oxygen gas that joined with the metal.
Teacher's Tip: For compression questions, always talk about the "Intermolecular Space."
Exam Tip: For the magnesium question, clarify that the law of conservation of mass still holds true if you count the gas.

Question 8: Fill in the blanks:
(a) The change of a solid into a liquid is called melting or fusion.
(b) The process in which a solid directly changes into a gas is called sublimation.
(c) The change of water vapour into water is called condensation.
(d) The temperature at which a liquid starts changing into its vapour state is evaporation or vaporisation.

Answer:
(a) melting or fusion
(b) sublimation
(c) condensation
(d) evaporation or vaporisation.
These terms describe the specific names we give to matter when it changes its physical appearance. Sublimation is the rarest one, where a solid "skips" the liquid stage entirely.
Teacher's Tip: Fusion is just a scientific "fancy name" for melting.
Exam Tip: Learn the difference between evaporation (surface only) and boiling (entire liquid) if your teacher asks for details.

Question 9: Give two examples for each of the following:
(a) The substances which sublime.
(b) The substances which do not change their state on heating.

Answer:
(a) Camphor, iodine, naphthalene, ammonium chloride, dry ice (solid carbon dioxide), etc.
(b) Gases do not change their state on heating. Example: O₂.
Substances like naphthalene (mothballs) disappear over time because they turn directly into gas at room temperature. On the other hand, a gas like oxygen just gets hotter when heated but stays a gas.
Teacher's Tip: Naphthalene and Camphor are the most common examples of sublimation found in homes.
Exam Tip: Be careful! Most solids melt before they turn to gas; only the specific list above "sublimes."

Question 10: Define:
(a) Diffusion.
(b) Brownian motion.

Answer:
(a) Diffusion: The intermixing of two or more substances due to the motion of their particles in order to get a uniform mixture is called ‘diffusion’.
(b) Brownian motion: The haphazard, random motion of suspended particles on the surface of a liquid or in air is called ‘Brownian motion’.
Diffusion is why you can smell perfume from across the room even if nobody is blowing it toward you. Brownian motion is the "zig-zag" dance that small particles do because they are being constantly hit by moving molecules.
Teacher's Tip: Diffusion = Spreading out. Brownian Motion = Random dancing.
Exam Tip: Use the word "haphazard" or "zig-zag" to describe Brownian motion for better marks.

Question 11: When sodium chloride is added to a definite volume of water and stirred well, a solution is formed, but there is no increase in the level of water. Why?

Answer:
This is because there is some space between the particles of water in which the salt particles get accomodated when dissolved.
Imagine filling a jar with golf balls; there is still plenty of room between the balls for sand to fit. Similarly, salt crystals break apart and hide in the tiny gaps between the water molecules.
Teacher's Tip: This experiment proves that water molecules have "gaps" or "Intermolecular spaces."
Exam Tip: Use the keyword "accommodated" to describe how the salt fits into the water spaces.

Question 12: What do you observe when a gas jar which appears empty is inverted over a gas jar containing Bromine vapours? Name the phenomenon.

Answer:
When a gas jar full of bromine vapours (reddish brown) is inverted over a gas jar containing air over it. It is observed that after sometime, the reddish brown vapours of bromine also spread out into the upper jar. This mixing is called diffusion. The rate of diffusion is the fastest in gases and the slowest in solids. It increases with an increase in temperature.
Even though the bromine is heavier than air, the constant motion of its molecules carries it upward. This proves that gas particles are always moving and will spread out to fill any available space.
Teacher's Tip: Bromine is used in this experiment because its "Reddish Brown" color is easy to see.
Exam Tip: Mention that diffusion is "fastest in gases" as a supporting fact in your answer.

Question 13: Why can a piece of chalk be broken easily into smaller pieces while a coal piece cannot be broken easily?

Answer:
The particles of matter have force acting between them. This force keeps the particles together. The strength of this force of attraction is lesser in chalk, hence it could be broken easily into smaller pieces.
But the strength of inter-molecular force of attraction is very strong in coal, therefore it is not possible to break them into small pieces.
Not all solids are equally strong because the "glue" holding their molecules together varies. Chalk has very weak intermolecular forces, while coal has much tougher ones.
Teacher's Tip: Different substances have different "strengths" of attraction force.
Exam Tip: Always compare the "force of attraction" in both substances when answering this question.

ADDITIONAL QUESTIONS

Question 1: Define matter.
Answer: Anything that has mass and occupies space is called matter.
Basically, if you can weigh it and it takes up room, it is matter. This includes things you can't see, like the air in a balloon.
Teacher's Tip: Matter is the "stuff" that the universe is made of.
Exam Tip: Mass and Space are the two essential components of this definition.

Question 2: What is volume ?
Answer: The amount of space occupied by a matter is called its volume.
Volume tells us how big an object is in terms of the room it takes up. For example, a basketball has more volume than a marble.
Teacher's Tip: Think of volume as the "size" or "capacity" of an object.
Exam Tip: Don't confuse volume (space) with mass (weight/quantity).

Question 3: What is mass ?
Answer: Mass is the quantity of matter contained in the body.
Mass doesn't change even if you go to space; it is the actual amount of "stuff" inside you. It is usually measured in kilograms or grams.
Teacher's Tip: Mass stays the same everywhere, but weight can change with gravity.
Exam Tip: Use the word "quantity" to distinguish mass from volume.

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
therefore Body will weigh 1/6 × 6 = 1 { kg} on moon.
Gravity on the moon is much weaker than on Earth, which makes you feel lighter. However, you are still the same "size" and have the same number of atoms.
Teacher's Tip: To find Moon weight, just "Divide by 6."
Exam Tip: Show the calculation step (1/6 \×{Earth weight}) to get full marks for the math.

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 the actual count of particles inside an object, it doesn't care about gravity. If you have 5 million atoms on Earth, you still have 5 million atoms on the Moon.
Teacher's Tip: Mass is "Stable," Weight is "Variable."
Exam Tip: This is a "trick" question; remember that mass *never* changes based on location.

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 like the individual LEGO bricks of the universe. They are so tiny that billions of them can fit on the head of a pin.
Teacher's Tip: Atoms are the "Building Blocks" of everything.
Exam Tip: The word "atom" comes from a Greek word meaning "indivisible."

Question 7: What are molecules ?

Answer: Molecules are made of atoms. Molecules exhibit the properties of that kind of matter and has independent existance.
When two or more atoms join together, they form a molecule. While an atom might be unstable alone, a molecule is like a complete "team" that can exist on its own.
Teacher's Tip: Atoms are like letters, and molecules are like the "Words" they form.
Exam Tip: Emphasize "independent existence" when defining a molecule.

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 are the finished, stable groups that we actually interact with in substances like water or air.
Teacher's Tip: Think of a molecule as a "Stable Group" and an atom as a "Single Unit."
Exam Tip: Use a simple T-chart or bullet points to show the contrast clearly.

Question 9: Define:
(a) Intermolecular force of attraction.
(b) Intermolecular space.

Answer:
(a) The molecules of matter are always in motion and attract each other with a force called intermolecular 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.
Force is the "pull" that keeps things together, while space is the "gap" between them. In a solid, the pull is huge and the gap is tiny, which is why solids are so hard.
Teacher's Tip: Force is the "Magnetism," Space is the "Distance."
Exam Tip: Use these two terms to explain why solids, liquids, and gases look different.

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.
Classification helps us group things with similar behaviors, like flowing or keeping a shape. Mercury is a special liquid because it is actually a metal, even though it flows like water.
Teacher's Tip: Butter and Wax are solids that melt easily, so don't let them confuse you.
Exam Tip: Double-check "mercury"; it is a common exam favorite because it is a liquid metal.

Question 11: Why do solids, liquids and gases differ in their physical states ?

Answer: Intermolecular force of attraction and Intermolecular spaces are two important properties of matter that account for the different states of matter.
The balance between the "pull" and the "gap" determines if a substance is hard, runny, or invisible. When the pull is stronger than the motion of the particles, matter becomes a solid.
Teacher's Tip: It's a tug-of-war between "Force" and "Motion."
Exam Tip: Mention both "force" and "space" to provide a complete scientific reason.

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 substances where molecules can slide past each other because their forces aren't strong enough to lock them in place. Both liquids and gases are considered fluids because they can be poured or blown.
Teacher's Tip: Liquid is a fluid, but not every fluid is a liquid (it could be a gas!).
Exam Tip: Don't forget that gases are also "fluids"—students often forget this!

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.
This is a physical change, not a chemical one, like an actor changing costumes between scenes. The substance stays the same (like H₂O), it just changes its look (ice to water).
Teacher's Tip: Use water (H₂O) as your mental model for this definition.
Exam Tip: Emphasize "without any change in chemical composition" for full marks.

Question 14: What are the two conditions for the interconversion of states of matter ?

Answer: Two conditions are: change in (i) Temperature (ii) Pressure.
Heating things up or cooling them down is the most common way we change states. We can also squash gases into liquids by using extreme pressure, like in a gas cylinder.
Teacher's Tip: Temperature is about "Heat," Pressure is about "Squeezing."
Exam Tip: Be sure to list *both* factors; just mentioning temperature is only half the answer.

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.
The added heat gives the molecules enough "kick" to break free from the liquid's surface. Once they break free, they zoom off into the air as independent gas particles.
Teacher's Tip: Heat is like "Energy Fuel" that makes molecules move faster.
Exam Tip: Use the phrase "move more vigorously" to describe the molecular motion when heated.

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 acts like a giant laboratory where water is constantly switching between liquid, gas, and solid (ice/snow). This constant recycling ensures that we always have a supply of fresh water to drink.
Teacher's Tip: Clouds are "Condensation," and Rain is "Return."
Exam Tip: Mention evaporation, condensation, and freezing to show all three states are involved.

Question 17: State the general properties of a solid.

Answer:
General Properties of a Solid:
Solids are hard that is, they have a definite shape and volume.
Solids are generally incompressible.
Solids are rigid, that is, they do not flow.
Solid molecules are like people standing in a very tight line—they can vibrate a little but can't leave their spot. This is why a wooden block doesn't change shape when you move it from a box to a table.
Teacher's Tip: Rigid = "Cannot be bent or changed easily."
Exam Tip: List "Definite shape" and "Definite volume" as the two most important points.

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.
Think of it like an elastic band: the more it pulls things together (force), the closer those things get (less space). If you let the pull weaken, things can drift further apart.
Teacher's Tip: Strong Pull = Tiny Gaps. Weak Pull = Big Gaps.
Exam Tip: Use the scientific term "inversely proportional" to get the highest marks.

Question 19: Why liquids do not have a definite shape ?

Answer: Molecules of a liquid are held by weak intermolecular forces. This force is strong 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 the "glue" is weak, liquid particles can slide and tumble over each other like a pile of marbles. They stay close to each other, but they can't stop themselves from shifting to fill the bottom of a container.
Teacher's Tip: Liquids "Slide" but don't "Fly away."
Exam Tip: Explain that molecules are "not at fixed positions" to justify the lack of shape.

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.
Heat acts like a hammer that breaks the rigid structure of the solid. Eventually, the vibration becomes so violent that the molecules "break out" of their locked positions and turn into a liquid.
Teacher's Tip: Melting is the "jailbreak" of molecules from their solid positions.
Exam Tip: Mention that molecules "vibrate faster" as the first step in the melting process.

Question 21: Give reasons for the following.
1. Gases can be compressed easily.
2. Liquids can flow easily.
3. We need to classify things.
4. Pure substances have fixed melting or boiling points.
5. Electricity is not considered matter.

Answer:
1. 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. In liquids intermolecular force is weaker than that of solids. So molecules in a liquids can slip over one another and liquids can flow unlike solids.
3. 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 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 neither has mass nor it occupies space. Beside it can not be seen by our eyes. This is why electricity is not considered matter.
Gases are mostly empty space, so you can easily squeeze the molecules closer together. Electricity is a form of energy (moving electrons), not a physical "stuff" that you can weigh on a scale.
Teacher's Tip: Matter has "Stuff" (Mass), Energy has "Power."
Exam Tip: For the compression question, emphasize "large intermolecular space."

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 terms are the alphabet of chemistry. An element is like a pure color of paint (only one type of atom), while a molecule is a specific mixture (like water).
Teacher's Tip: Elements are "Pure," Compounds are "Mixed."
Exam Tip: Practice these five definitions together as they often appear in a single section of the exam.

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. (Conclusion: The scale will tilt toward the inflated balloon, proving that air has mass).
These experiments show that even "invisible" things like air have weight and take up room. When the stone goes into the water, it "kicks out" the water to make room for itself.
Teacher's Tip: Displacement = "Moving something out of its place."
Exam Tip: Use the word "occupies space" to describe why the water level rises or overflows.

Question 24: How will you prove by an experiment that solids occupy space ?

Answer:
Experiment to show that solids occupy space: Take a glass bowl. Place a glass tumbler full of water in it. Now tie a stone with thread and lower into the water. Some water flows out of tumbler into bowl. When we remove stone from tumbler the level of water in tumbler comes down. This shows that solids occupy space.
The stone is solid matter, and since matter must have volume, it pushes the liquid aside to find a spot. This simple test is the best way to visualize how solids interact with their surroundings.
Teacher's Tip: This is called the "Displacement Method."
Exam Tip: Mention that the "overflowed water" is equal to the space the stone took up.

Question 25: Name the three states of matter and define them.

Answer:
There are three states of matter (i) Solid (ii) Liquid (iii) Gas
1. Solid State: The molecules are very close to each other hence intermolecular spaces are small and intermolecular force is strong. Hence solids have definite volume, rigid, retain definite shape and are incompressible.
2. Liquids: The molecules are less closely packed have more intermolecular spaces than solid, less stronger forces than solids. Hence liquids have definite volume but no definite shape. They take the shape of container in which they are put.
3. Gases: The molecules in the gases are for apart with weakest force of attraction. Hence gases have neither definite volume nor definite shape and compressible.
The way matter looks depends on how its tiny "molecular family" behaves. In a solid, they stay hugged tight; in a liquid, they hold hands and move; in a gas, they run away from each other.
Teacher's Tip: Focus on "Force" and "Space" as the main defining features.
Exam Tip: Providing a small sketch of particle arrangement for each state will earn you extra marks.

Question 26: 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.
Flowing happens when molecules can roll over each other, which is impossible in a rigid solid. Walking through air is easy because we are basically pushing through lots of empty space between tiny gas dots.
Teacher's Tip: Scent spreading is the most famous real-life example of "Diffusion."
Exam Tip: Use the term "random motion" to explain why gases fill their containers.

Question 27: 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.
The sugar and the ink don't "create" new volume; they just fill the "holes" that were already there in the water. Mixing gases is even faster because gas molecules are moving at hundreds of miles per hour!
Teacher's Tip: Think of the sugar in water as "Parking cars in empty spots."
Exam Tip: Be sure to name "Diffusion" as the reason for the ink and the colored gas spreading.

Question 28: Give an experiment to explain that there are intermolecular spaces between water molecules.

Answer: Take a completely filled glass of water. Add a spoon full of sugar. Stir it well. The volume of water hence the level of water in glass remains the same where has gone the volume of sugar added? Actually the sugar molecules took the spaces (gaps) between the molecules of water and level of water in glass remains the same. This shows that there are intermolecular spaces between the water molecules.
If water were a solid block with no holes, the sugar would have to sit on top and make the level rise. Because it stays the same, we know water is like a "sponge" with tiny microscopic gaps.
Teacher's Tip: You can do this at home with salt or sugar to see it yourself!
Exam Tip: Mention "Stirring" as it helps the sugar particles find the spaces faster.

Question 29: Differentiate between the following.
(a) Liquids and gases
(b) Atoms and molecules

Answer:
(a) Liquids and Gases
Liquids
1. Have definite volume but no definite shape
2. Intermolecular force of attraction is weaker than that in solids
3. Molecules are not as tightly packed as in solids and intermolecular space is more than that in solids.
4. Molecules have no fixed position
5. Can be compressed slightly
6. Are capable of flowing
Gases
1. Have no definite shape or volume
2. Intermolecular force of attraction is the weakest (almost negligible)
3. Intermolecular space is the maximum and the molecules are far apart.
4. Molecules move around freely
5. Can be easily compressed
6. Can flow in all directions.
(b) Atoms and molecules
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.
Liquids stay in a puddle, but gases fly out into the whole sky. While an atom is like a single lego brick, a molecule is a finished spaceship built from several bricks.
Teacher's Tip: Use the term "Negligible" for gas forces to sound like a pro.
Exam Tip: For differentiation, use a table format to align points side-by-side.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks:
(a) Water is a matter because it has mass 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.
(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: Follow the bold terms above.
These blanks cover the core definitions of the chapter, like boiling points and molecular packing. Remembering these "key words" is essential for quick recall in class.
Teacher's Tip: Review these blanks as a "Quick Fire" quiz before a test.
Exam Tip: Be careful with the spelling of "vaporisation" and "intermolecular."

2. I. Write whether the following statements are true or false.
(a) Only water can exist in three different states. True (Note: Textbook answer says True, but scientifically others can too, though water is the most common example).
(b) If the container in which a gas is collected has an opening, the gas will flow out and spread itself indefinitely. True
(c) Solids have the largest inter-molecular space. False
(d) There is no difference between evaporation and boiling. False
(e) All solids, on heating, first change to the liquid and then to the gaseous state always. False
(f) The intermolecular force of attraction is the weakest in gases. True
(g) A gas has no free surface. True
(h) Intermolecular force of attraction is greater in gases than in liquids. False

Answer: (a) True (b) True (c) False (d) False (e) False (f) True (g) True (h) False.
True/False questions test your understanding of exceptions, like how some solids (like camphor) skip the liquid state and go straight to gas. It's also important to remember that gases are the "loosest" state of matter.
Teacher's Tip: "False" statements are often the opposite of a scientific law.
Exam Tip: If a statement says "Always" or "Only," look closer as there is usually an exception.

II. Write true or false for each statement. Rewrite the false statements correctly.
(a) Matter cannot exist in different states. False. Matter can exist in different states.
(b) If the intermolecular space is more than the intermolecular force will be weaker. True
(c) Solids and liquids can flow. False. Gases and liquids can flow.
(d) Solids can be compressed easily. False. Solids cannot be compressed easily.
(e) The smallest part of an element capable of independent existence is called an atom. True (Note: OCR says '4s', it is 'is').
(f) The intermolecular space in a gas is almost negligible. False. The intermolecular space in a gas is very large.

Answer: Follow the corrections in bold provided above.
Correcting these statements helps you focus on *why* something is scientifically wrong. For example, knowing that gases have huge spaces helps you understand why the textbook says "negligible space" is false.
Teacher's Tip: "Negligible" means so small it doesn't count; gases are the opposite of that!
Exam Tip: When rewriting, change only the key word to keep the sentence structure simple and correct.

3. For each of the following statements, say whether it describes a solid, a liquid or a gas.
(a) Particles move about very quickly.
(b) Particles are quite close together.
(c) Particles are far apart and move in all directions.

Answer: (a) Liquid (Textbook Ans); (b) Solid (c) Gas
Description (a) usually refers to liquids because they have enough energy to slide, while (c) refers to gases where they fly everywhere. Solids are the only state where everyone stays "close together" in their seats.
Teacher's Tip: Quick = Liquid/Gas, Packed = Solid, Scattered = Gas.
Exam Tip: Look for the "motion" keywords (sliding vs flying vs vibrating) to identify the state.

4. I. 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.

Matching helps connect a concept to its specific definition or behavior. For instance, the "boiling point" is a specific temperature marker that belongs to the liquid-to-gas transition.
Teacher's Tip: Gases have "zero" free surfaces, and liquids have "one" (the top).
Exam Tip: Match the easiest ones first (like Gaps = Space) to narrow down the harder choices.

II. 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 exercise groups substances and processes. Iron and gold are metals, which means they are solid elements with very strong intermolecular forces.
Teacher's Tip: Iron and Gold are the "Strong Duo" (Metals).
Exam Tip: Be precise; melting specifically refers to the "Solid to Liquid" change.

5. Name the phenomenon which causes the following changes:
(a) Formation of water vapour from water.
(b) Disappearance of camphor.
(c) Conversion of ice into water.
(d) Conversion of water into steam.

Answer:
(a) Formation of water vapour from water is vaporation.
(b) Disappearance of camphor is sublimation.
(c) Conversion of ice into water is melting.
(d) Conversion of water into steam is boiling.
Phenomena are the natural names for these physical transformations. Evaporation can happen slowly at the surface, while boiling happens aggressively throughout the liquid.
Teacher's Tip: Camphor "Sublimes" - it's the most common example of skipping liquid state.
Exam Tip: Use "Vaporisation" as a general term and "Boiling" for change at a fixed temperature.

6. Give two examples for each of the following:
(a) Substances which sublime
(b) Substances which do not change their states.
(c) Substances which are rigid and not compressible.

Answer:
(a) Naphthalene, camphor, dry ice.
(b) Paper, sugar.
(c) Glass, stone, pen.
Rigid things like glass don't squish because their atoms are already touching with no gaps. Subliming things are the "houdinis" of chemistry because they disappear into thin air.
Teacher's Tip: "Rigid" things are usually the hardest objects you find in the classroom.
Exam Tip: Don't use "water" for things that don't change state, as water changes state all the time!

MULTIPLE CHOICE QUESTIONS

1. Which one is a kind of matter:
1. light
2. petroleum
3. sound
4. heat

Answer: 2. petroleum
Petroleum is a liquid matter because it has mass and volume. Light, sound, and heat are all forms of energy, which have no mass and do not take up physical space.
Teacher's Tip: If you can put it in a bottle, it's matter!
Exam Tip: Energy forms like light and heat are the most common "distractors" in matter questions.

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 are the "rebels" of matter because they expand to fill any size or shape of container. Their molecules have almost zero force keeping them together.
Teacher's Tip: Gas = "No Limits" (No fixed shape/volume).
Exam Tip: Remember: Solids (Both fixed), Liquids (Volume fixed), Gases (Neither fixed).

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 all gases have huge gaps between their molecules. This is why you can move your hand through air so easily but not through an iron ball.
Teacher's Tip: "Gaps" are the reason why we can compress gases like air.
Exam Tip: Always look for the gas option when "large gaps" are mentioned.

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
This is the "Golden Rule" for matter. While some matter lacks shape or volume (like gas), *everything* that is matter must have mass and take up room.
Teacher's Tip: Mass + Space = Matter.
Exam Tip: Don't pick "definite shape" as a general rule, because liquids and gases don't have it.

5. A kind of matter which can sublinje (sublime) is
1. water
2. plastic
3. milk
4. iodine

Answer: 4. iodine
Iodine is a solid that, when heated, turns into a beautiful purple gas without becoming a liquid. This rare property makes it a favorite for science experiments.
Teacher's Tip: Iodine, Camphor, and Dry Ice are the "Big Three" of sublimation.
Exam Tip: Sublimation only happens to specific substances; learn the list by heart.

6. A substance which can change its state
1. wood
2. oxygen
3. paper
4. cloth

Answer: 2. oxygen
Oxygen is usually a gas, but if we cool it down to extremely low temperatures, it becomes a liquid. Things like wood or paper burn into new chemicals (ash) instead of just changing state.
Teacher's Tip: Most gases can be liquified under high pressure or cold temperature.
Exam Tip: Avoid choosing solids that burn (like wood) when asked about state changes.

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 occurs when heat gives solid particles enough energy to start sliding past each other. It is the exact opposite of freezing.
Teacher's Tip: Think of an "Ice Cream" cone on a hot day.
Exam Tip: Another name for melting is "fusion"—keep that in mind!

8. A solid is a state of matter that has
1. no definite shape.
2. large intermolecuiar space.
3. high intermolecuiar force of attraction.
4. no definite volume.

Answer: 3. high intermolecuiar force of attraction.
This strong "molecular hug" is what keeps a solid rigid and hard. Because the pull is so tight, the molecules cannot move far enough to change shape.
Teacher's Tip: Strong Pull = Strong Shape.
Exam Tip: Force is the *reason* why solids have a definite shape and volume.

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
Liquids are "fluid," meaning they pour and take the shape of their container. This is because their molecules are not stuck in one single spot like those in a solid.
Teacher's Tip: Flowing is the "Superpower" of both liquids and gases.
Exam Tip: If you see the word "Malleable," remember that it's a property of metals, not liquids.

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.
Unlike liquids, which only fill the bottom, gas molecules fly out in every direction until they hit a wall. This is why a tiny bit of smoke can eventually fill a whole room.
Teacher's Tip: Gases are "space-fillers."
Exam Tip: Gases are the *only* state of matter that can be compressed easily.