Selina Concise Solutions for ICSE Class 8 Physics Chapter 1 Matter

1. Matter

1. Matter Every substance living and non-living that we see is made up of matter and MATTER “is something which has mass, occupies space and can be perceived by our senses.” e.g. Hydrogen, milk, oxygen, pen, table, water, iron, air, oil, sugar etc.
Matter is anything that you can touch, taste, or smell in the physical world. Since it has mass, it means it weighs something and takes up a certain amount of room.
Teacher's Tip: Use the acronym "MOS" - Mass, Occupies space, Senses - to remember what matter is.
Exam Tip: When giving examples of matter, try to include one solid, one liquid, and one gas to show complete understanding.

2. Matter is composed of tiny particles called molecules, which are in constant motion, have spaces between them and have inter-molecular attraction.
Even though matter looks solid or continuous, it is actually made of billions of tiny pieces. These pieces never stay still and are always pulling on each other.
Teacher's Tip: Imagine molecules like a crowd of people constantly moving and jostling in a room.
Exam Tip: Always mention "constant motion" when describing the molecular nature of matter.

3. Every molecule can exist freely in nature and has all the properties of matter.
A single molecule of water still behaves exactly like water and has the same chemical traits. It is the smallest part of a substance that can stay on its own.
Teacher's Tip: Think of a molecule as the smallest "representative" of a substance.
Exam Tip: Use the word "freely" specifically when defining a molecule.

4. A molecule is composed of ATOMS, but atom cannot exist free in nature.
Atoms are like the individual LEGO bricks that snap together to build a molecule. While the molecule is stable alone, the individual atoms are usually too reactive to stay separate.
Teacher's Tip: Atoms are the "building blocks," and molecules are the "buildings."
Exam Tip: Remember that the main difference between an atom and a molecule in this context is the ability to exist independently.

5. INTER-MOLECULAR FORCE ‘The molecules of a matter exert a force of attraction on each other - The force of attraction is called INTER-MOLECULAR FORCE
This is the invisible "glue" that holds matter together and determines if it is a solid, liquid, or gas. The stronger this force, the closer the molecules stay to one another.
Teacher's Tip: Think of "Inter" as "between," so this is the force "between molecules."
Exam Tip: Don't confuse this with intra-molecular forces; focus on the attraction between separate molecules.

6. MOLECULAR FORCE This force in solid is very strong and we cannot break a solid easily. In liquids this force is less strong and in molecules of gas it is very less. -
In solids, the "glue" is so strong that the molecules are locked in place, which is why solids have a fixed shape. In gases, the force is so weak that molecules fly away from each other.
Teacher's Tip: Solid = Strong, Liquid = Loose, Gas = Ghostly weak.
Exam Tip: Use this concept to explain why solids have a definite shape but gases do not.

7. FORCE OF COHESION “The inter-molecular force of attraction between the molecules of same substance is called FORCE OF COHESION.” i.e. between water and water.
Cohesion is what allows water to form droplets because the water molecules want to stick to their own kind. It is the internal attraction within a single type of matter.
Teacher's Tip: "Co" means together with its own kind - like "co-workers" of the same company.
Exam Tip: Mention "same substance" to get full marks for the definition of cohesion.

8. FORCE OF ADHESION “The force of attraction between the molecules of two different substances is called FORCE OF ADHESION” i.e. between glue and paper.
Adhesion is why water sticks to a glass or why paint sticks to a wall. It involves the attraction between two completely different types of molecules.
Teacher's Tip: Think of "Adhesive tape" which is designed to stick to different things.
Exam Tip: Use the example of "glue and paper" to illustrate adhesion clearly in your answer.

9. Matter is composed of tiny particles and molecules of matter have spaces between them can be proved by experiment. Take 50 c.c. of water in a measuring cylinder. Add a small quantity of salt in it. Salt gets dissolved in water and still level remains at 50 c.c. Where has salt gone? The salt molecules enter into spaces of water and water molecules into spaces of salt molecules. This experiment show that particles of matter are very minute and cannot be seen by naked eye and there are spaces between molecules.
This experiment proves that matter is not "solid" all the way through but has tiny gaps. Because the salt fits into the gaps of the water, the total volume doesn't increase.
Teacher's Tip: Think of this like pouring sand into a jar full of large marbles; the sand fills the gaps.
Exam Tip: When describing this experiment, emphasize that the water level "remains the same."

10. The molecules of matter are in constant motion can be seen by opening a bottle of perfume in a comer of room, it reaches the other parts of the room.
Perfume molecules move through the air by bumping into air molecules and spreading out. This natural spreading of particles from one place to another is known as diffusion.
Teacher's Tip: Smelling food from the kitchen in your bedroom is the perfect real-life example of molecular motion.
Exam Tip: This example is often used to prove that molecules in gases move very fast and freely.

11. SUBLIMATION Change of solid directly into vapours on absorbing heat.
Some substances skip the liquid phase entirely when they get hot enough. Examples include camphor, naphthalene balls, and dry ice.
Teacher's Tip: Sublimation is like taking a "shortcut" from solid to gas.
Exam Tip: Always mention that it happens "directly" without turning into a liquid first.

12. DEPOSITION “The change of vapours directly into solid on cooling.”
Deposition is the exact opposite of sublimation, where a gas turns into a solid instantly when it hits a cold surface. This is how frost forms on windows during a very cold night.
Teacher's Tip: Think of "depositing" or "leaving behind" a solid layer from the air.
Exam Tip: Note that deposition is also known as "desublimation."

13. MELTING “Change of solid in liquid at fixed temperature on heating.”
When you heat a solid, the molecules start vibrating so fast that they break free from their fixed positions. This temperature stays the same until every bit of the solid has turned into liquid.
Teacher's Tip: Melting happens when molecules get enough energy to "break out" of their solid cage.
Exam Tip: Use the term "fixed temperature" to describe the melting point accurately.

14. FUSION or FREEZING “Change of liquid to solid state on cooling at a fixed temperature.”
Freezing involves removing heat energy so that molecules slow down and get trapped by their attractive forces again. It is the reverse process of melting.
Teacher's Tip: "Fusion" in this context is just another word for the molecules "fusing" together into a solid.
Exam Tip: Remember that for water, the melting point and freezing point are the same (0°C).

15. EVAPORATION Surface phenomenon i.e. only takes place at surface “Is change of liquid to vapours.” Evaporation has cooling effect. Takes place at all temperatures.
Molecules at the top of a liquid escape into the air, even if the liquid isn't boiling. Because the hottest molecules escape first, the liquid that is left behind feels colder.
Teacher's Tip: Sweating is our body's way of using the "cooling effect" of evaporation.
Exam Tip: Distinguish evaporation from boiling by noting it happens at "all temperatures."

16. VAPORIZATION “Change of liquid to vapour state on heating at constant temperature.”
Vaporization (or boiling) happens throughout the entire liquid once it reaches a specific temperature. It is a rapid process compared to the slow, quiet process of evaporation.
Teacher's Tip: Think of boiling water in a kettle as "forced" vaporization using a stove.
Exam Tip: Specify that vaporization occurs at a "constant temperature" called the boiling point.

Test Yourself

A. Objective Questions
 

1. Write true or false for each statement
 

(a) The temperature of a substance remains unaffected during its change of state.
Answer: True.
All the heat energy being added is used to break the bonds between molecules rather than raising the temperature. This is why a pot of boiling water stays at 100°C until it's all gone.
Teacher's Tip: During a phase change, the thermometer "takes a break" and stays still.
Exam Tip: This "hidden heat" is called Latent Heat; remember this for higher classes.

(b) Ice melts at 100°C.
Answer: False. The ice melts at 0° by absorption of heat.
Ice is the solid form of water, and it begins to turn into liquid as soon as it goes above the freezing point. 100°C is actually the temperature where liquid water turns into steam.
Teacher's Tip: 0 is for ice, 100 is for steam.
Exam Tip: Always double-check your units and specific temperatures for water.

(c) Water at 100°C has more heat than the steam at 100°C.
Answer: False.
Steam actually contains extra energy (latent heat) that was used to turn the liquid into gas. This is why steam burns are often much more severe than hot water burns.
Teacher's Tip: Steam is water "plus" the extra energy it needed to escape the liquid.
Exam Tip: Remember that steam at 100°C always has MORE heat energy than water at the same temperature.

(d) Evaporation of a liquid causes cooling.
Answer: True.
As fast-moving (hot) molecules leave the surface, the average energy of the remaining liquid drops. This drop in energy results in a lower temperature for the liquid.
Teacher's Tip: This is why you feel a chill when you step out of a swimming pool on a windy day.
Exam Tip: Explain this by stating that heat is "absorbed" from the surroundings or the remaining liquid.

(e) Water evaporates only at 100°C.
Answer: False.
Evaporation happens at all temperatures, even on a cold day. Boiling is the process that specifically requires water to reach 100°C.
Teacher's Tip: Puddles dry up after rain even if the sun isn't boiling the water!
Exam Tip: Use the phrase "at all temperatures" to describe evaporation in your exams.

(f) Boiling takes place at all temperatures.
Answer: False.
Boiling only happens when a liquid reaches its specific boiling point temperature. At this point, the pressure inside the bubbles equals the atmospheric pressure outside.
Teacher's Tip: Boiling is a "picky" process that waits for the exact right temperature.
Exam Tip: Contrast boiling with evaporation, which *does* happen at all temperatures.

(g) Evaporation takes place over the entire mass of the liquid.
Answer: False.
Evaporation is a surface phenomenon, meaning it only happens to the molecules at the very top. Boiling is the process that occurs throughout the entire bulk of the liquid.
Teacher's Tip: Evaporation is like a "top-down" exit, while boiling is "everywhere at once."
Exam Tip: Use the term "surface phenomenon" specifically when talking about evaporation.

(h) The process of a gas converting directly into solid is called vaporization.
Answer: False.
The process of a gas converting directly into a solid is called deposition. Vaporization is actually when a liquid turns into a gas.
Teacher's Tip: Vaporization creates "vapour" (gas); Deposition "deposits" a solid.
Exam Tip: Learn the cycle of change of state carefully to avoid mixing up these names.

The process of a liquid converting directly into gas is called vaporization.
This term covers both boiling and evaporation as they both result in a gas. It describes the general transition from the liquid phase to the gaseous phase.
Teacher's Tip: Think of the "vapour" being created from the "liquid."
Exam Tip: Vaporization is the broad category for liquid-to-gas changes.

(i) At high altitudes water boils above 100°C.
Answer: False.
At high altitudes, the air pressure is lower, so water molecules find it easier to escape into the air. This causes water to boil at a lower temperature, often well below 100°C.
Teacher's Tip: Less air "pushing down" on the water means it can boil with less heat.
Exam Tip: Remember: Higher altitude = Lower boiling point.

(j) The melting point of ice is 0°C.
Answer: True.
This is the standard temperature where solid water (ice) begins to turn into liquid water. It is a fundamental constant used in science across the world.
Teacher's Tip: This is the same temperature as the freezing point of water.
Exam Tip: Use 0°C as your reference point for all water-related phase questions.

2. Fill in the blanks
 

(a) Evaporation takes place at all temperature.
This slow process happens constantly whenever a liquid surface is exposed to air. Heat from the environment provides enough energy for some surface molecules to fly away.
Teacher's Tip: If you leave a glass of water out, it will eventually disappear because of this.
Exam Tip: The keyword here is "all" temperatures.

(b) Freezing process is just reverse of melting.
In melting, you add heat to turn solid to liquid; in freezing, you remove heat to go from liquid to solid. They both happen at the same temperature for a pure substance.
Teacher's Tip: Think of melting and freezing as a two-way street.
Exam Tip: Use the word "reverse" to show the relationship between these two processes.

(c) Sublimation is a process that involves direct conversion of a solid into its vapour on heating.
Sublimation is unique because it skips the messy liquid stage completely. This usually happens in substances where the attractive forces are easily overcome by heat.
Teacher's Tip: Dry ice is the most famous example of sublimation you might see in science shows.
Exam Tip: "Direct conversion" is the most important part of this definition.

(d) The temperature at which a solid converts into a liquid is called its melting point.
Each substance has its own unique melting point, which helps scientists identify what a material is. For instance, iron melts at much higher temperatures than chocolate.
Teacher's Tip: The "Point" refers to a specific number on the thermometer.
Exam Tip: Always include the word "point" after "melting" when referring to temperature.

(e) The smallest unit of matter that exists freely in nature is called molecule.
While atoms are smaller, they usually can't survive on their own. The molecule is the smallest stable piece that retains the chemical properties of the whole substance.
Teacher's Tip: A molecule is like a complete sentence, while atoms are just individual letters.
Exam Tip: If the question mentions "exists freely," the answer is almost always "molecule."

(f) Molecules of a substance are always in a state of motion and so they possess kinetic energy.
Kinetic energy is the energy of movement. Even in a solid, molecules are vibrating in place, meaning they always have some level of energy.
Teacher's Tip: More heat = More motion = More kinetic energy.
Exam Tip: Connect "motion" directly to "kinetic energy" in your answers.

(g) Intermolecular space is maximum in gases less in liquids and the least in solids.
In a solid, molecules are packed tight like sardines in a tin. In a gas, they are far apart like people in a giant open field.
Teacher's Tip: Space and Force are opposites: More space = Weaker force.
Exam Tip: Use the words "maximum," "less," and "least" to compare the three states correctly.

(h) Intermolecular force of attraction is maxiumum in solids, less in liquids and the least in gases.
The strong force in solids keeps everything rigid. In gases, the force is so tiny that the molecules don't care about staying near each other at all.
Teacher's Tip: This force is the "magnetism" that tries to pull molecules together.
Exam Tip: This is the most common reason given for why solids have a definite shape.

3. Match the following
 

Column A
(a) Molecules
(b) 100°C
(c) 0°C
(d) At all temperatures
(e) Camphor
Column B
(i) water boils
(ii) evaporation
(iii) changes from solid to gas
(iv) matter
(v) water freezes
Answer:
Column A - Column B
(a) Molecules - (iv) matter
(b) 100°C - (i) water boils
(c) 0°C - (v) water freezes
(d) At all temperatures - (ii) evaporation
(e) Camphor - (iii) changes from solid to gas
These pairings help summarize the most important facts about phase changes and the composition of matter. Camphor is a great example of sublimation, skipping the liquid phase entirely.
Teacher's Tip: Match the process to its specific temperature or property.
Exam Tip: Be careful with (b) and (c); don't swap boiling and freezing temperatures.

4. Select the correct alternative
 

(a) The inter-molecular force is maximum in
1. solids
2. gases
3. liquids
4. none of the above
Answer: 1. solids
Solids have the strongest forces, which is why they don't change shape when you put them in a cup. These forces lock the molecules into a rigid structure.
Teacher's Tip: If the force wasn't maximum, the solid would melt or flow away!
Exam Tip: Solids are the only state with a truly "maximum" attractive force.

(b) The inter-molecular space is maximum in
1. liquids
2. solids
3. gases
4. none of the above
Answer: 3. gases
In gases, molecules are very far apart, occupying only a tiny fraction of the total volume they fill. This massive amount of empty space is why gases are so easy to compress.
Teacher's Tip: Think of how much space there is in the air around you compared to a block of wood.
Exam Tip: Space and Force are always inverse; because gas force is minimum, the space is maximum.

(c) The molecules can move freely anywhere in
1. gases
2. liquids
3. solids
4. none of the above
Answer: 1. gases
Gas molecules are not bound by strong forces, so they can travel in straight lines until they hit something. This "free movement" allows gases to fill any container completely.
Teacher's Tip: Molecules in gases are like free-roaming birds, while solids are like people in a crowded bus.
Exam Tip: Focus on the word "anywhere" - liquids can move but are still stuck within the liquid's volume.

(d) The molecules move only within the boundary of
1. liquids
2. gases
3. solids
4. none of the above
Answer: 1. liquids
Liquid molecules are close enough to be attracted to each other but loose enough to slide around. They can move, but they can't fly away into the air easily like gas molecules.
Teacher's Tip: Liquid molecules are like people dancing in a ballroom; they move around but stay in the room.
Exam Tip: Use "boundary" as a clue for liquids, as they have a fixed volume.

(e) The temperature at which a liquid gets converted into its vapour state is called its
1. melting point
2. boiling point
3. dewpoint
4. freezing point.
Answer: 2. boiling point
The boiling point is the specific temperature where a liquid rapidly turns into gas throughout its entire volume. For pure water, this happens at exactly 100°C at sea level.
Teacher's Tip: Boiling is the "loud" version of vaporization that needs a specific temperature.
Exam Tip: "Liquid to Vapour" at a fixed temperature always points to the boiling point.

(f) Rapid conversion of water into steam is an example of
1. evaporation
2. freezing
3. melting
4. vapourization.
Answer: 4. vapourization.
Rapid conversion specifically describes boiling or the general process of vaporization. Unlike evaporation, which is slow and quiet, vaporization is fast and energetic.
Teacher's Tip: Steam is the result of adding lots of heat quickly.
Exam Tip: The word "Rapid" is your biggest clue to choose vaporization over evaporation.

(g) Evaporation takes place from the
1. surface of liquid
2. throughout the liquid
3. mid-portion of the liquid
4. bottom of liquid.
Answer: 1. surface of liquid
Only the molecules at the top have the chance to escape into the air. This is why a wide, shallow dish of water dries up faster than a tall, narrow glass.
Teacher's Tip: Evaporation is a "top-layer" job.
Exam Tip: Always associate "surface" with "evaporation" in descriptive questions.

(h) Boiling takes place from the
1. the surface of the liquid
2. throughout the liquid
3. mid-portion of liquid
4. none of the above.
Answer: 2. throughout the liquid
In boiling, energy is high enough that bubbles of gas can form even at the bottom of the pot. These bubbles then rise to the top and escape as steam.
Teacher's Tip: You can see the bubbles forming "throughout" the water in a boiling pot.
Exam Tip: Use "bulk phenomenon" or "throughout the liquid" to define boiling.

Short/Long Answer Questions
 

Question 1: Define the term matter. What is it composed of ?
Answer: Anything which occupies space and has mass is called matter. Matter is composed of tiny particles called MOLECULES.
Matter includes everything from the air we breathe to the ground we walk on. These tiny molecules are the fundamental units that make up all physical things.
Teacher's Tip: If you can weigh it and it takes up room, it's matter.
Exam Tip: Include both "mass" and "space" for a perfect definition.

Question 2: State three properties of molecules of a matter.
Answer:
1. They are very small in size.
2. They have spaces between them.
3. They are in constant motion and they posses kinetic energy.
Molecules are so tiny that they cannot be seen even with ordinary microscopes. The fact that they move constantly is why smells spread through a house.
Teacher's Tip: Remember the 3 S's: Small, Spaces, Spinning (motion).
Exam Tip: Mentioning "kinetic energy" shows you understand the physics behind molecular motion.

Question 3: What do you mean by the inter-molecular spaces ? How do they vary in different states of matter ?
Answer: INTER-MOLECULAR SPACES “The spacing between the molecules of matter is called Inter-molecular spaces.” The inter-molecular spaces is less in solids more in liquids and still more in gases. Explanation of inter-molecular space : Take water in a measuring cylinder say upto 80 ml. mark. Add 10 gm of salt to it. The volume in cylinder should increase. On dissolving salt we find volume remains same i.e. upto 80 ml mark. This is because there are spaces in water molecules and salt molecules occupy these spaces and volume remains the same.
Inter-molecular spaces are the "empty gaps" between the particles that make up a substance. The variation in these gaps is what gives solids, liquids, and gases their different physical properties.
Teacher's Tip: Think of molecules in a solid like people on a crowded train, and in a gas like people in a park.
Exam Tip: Use the salt experiment as a perfect example to prove that these spaces exist.

Question 4: What is meant by the inter-molecular forces of attraction ? How do they vary in solids, liquids and gases ?
Answer: INTER-MOLECULAR FORCES OF ATTRACTION : “The forces of attraction between the molecules of matter is called the inter-molecular force of attraction.” This inter molecular force is maximum in solids, less in liquids and least in gases.
This force is the invisible bond that tries to pull molecules together. It is strongest in solids, holding them in a fixed shape, and weakest in gases, allowing them to spread out.
Teacher's Tip: Think of this force as "molecular magnets" pulling on each other.
Exam Tip: Use the words "maximum," "less," and "least" for a clear comparison of the states.

Question 5: Which of the following are correct ?
(a) Solids have definite shape and definite volume.
Answer: True. Reason As the molecules here have negligible intermolecular distance between them and have maximum intermolecular force of attraction.
Because the molecules are packed so tightly and held so strongly, they cannot move around each other. This is why a solid brick doesn't change shape on its own.
Teacher's Tip: Definite = Fixed/Unchanging.
Exam Tip: Always provide the "Reason" involving molecular forces for descriptive marks.

(b) Liquids have definite volume but do not have definite shape.
Answer: True.
Liquids take the shape of whatever container you pour them into. However, 1 liter of water is still 1 liter, whether it's in a bottle or a flat tray.
Teacher's Tip: Liquids "flow" because their molecules can slide past each other.
Exam Tip: Remember: Shape = No, Volume = Yes for liquids.

(c) Gases have definite volume but no definite shape.
Answer: False. Correct Gases have neither definite volume nor a definite shape.
Gases will expand to fill any size container and will take whatever shape that container has. Their molecules are far apart and move independently in every direction.
Teacher's Tip: Gases are "rule-breakers" - they don't have a fixed shape OR volume.
Exam Tip: Use the word "neither" to make your correction strong and clear.

(d) Liquids have definite shape and definite volume.
Answer: False. Correct Liquids have a definite volume but not definite shape.
While liquids hold their volume together because of molecular attraction, those same forces aren't strong enough to stop the molecules from sliding. This sliding is what prevents a fixed shape.
Teacher's Tip: Think of water in a glass; it's shaped like the glass, not like a cube.
Exam Tip: Don't get confused between solids and liquids; only solids have a definite shape.

Question 6: Discuss the three states of matter solid, liquids and gas on the basis of molecular model.
Answer:
Solids: Here the molecules are very tightly 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 do not 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.
The molecular model explains how the "internal life" of molecules determines the physical behavior of materials. By looking at the distance and the "pull" between molecules, we can predict if something will be hard, flowy, or airy.
Teacher's Tip: Draw a simple box with dots for each state: packed for solid, loose for liquid, and scattered for gas.
Exam Tip: Use terms like "mean position" for solids to show that they only vibrate and don't travel.

Question 7: What do you mean by the change of state ? Write the flow chart showing the complete cycle of change of state.
Answer: CHANGE OF STATE: “The process of change from one state(form) to another state (form) either by absorption or rejection of heat at a constant temperature is called the CHANGE OF STATE.” COMPLETE CYCLE OF CHANGE OF STATE : On heating a solid changes to liquid and liquid on heating changes to vapours. On cooling vapours condense to LIQUID, LIQUIDS on cooling freeze to SOLIDS. Some SOLIDS on heating change to vapours. On rejection of heat vapours solidify. This cycle can be shown by diagram: SOLID (Heat absorbed -> Melting) -> LIQUID (Heat absorbed -> Vaporisation) -> GAS. GAS (Heat rejected -> Condensation) -> LIQUID (Heat rejected -> Freezing) -> SOLID. SOLID (Heat absorbed -> Sublimation) -> GAS. GAS (Heat rejected -> Deposition) -> SOLID.
Matter can change its physical form just by gaining or losing heat energy. The temperature stays the same during these actual changes because the energy is busy rearranging the molecules.
Teacher's Tip: Absorption of heat makes things "looser" (solid to gas), while rejection makes them "tighter" (gas to solid).
Exam Tip: Practice drawing the cycle with arrows to visualize how heat moving in or out drives the change.

Question 8: Differentiate between melting point and boiling point, giving atleast one example of each.
Answer: MELTING POINT: The temperature at which a solid starts changing into LIQUID without further increase in temperature is called MELTING POINT.” Or The constant temperature at which a solid changes into liquid.” Example : Ice (solid) melts at 0°C into water (liquid) when heated. BOILING POINT : “The temperature at which a LIQUID start changing in vapour without further rise in temperature. Or ‘The constant temperature at which a LIQUID starts changing into GAS (vapours) Example : Boiling point of water (liquid) is 100°C.
The melting point is where a substance transitions from solid to liquid, while the boiling point is the transition from liquid to gas. Both occur at specific, constant temperatures for any pure substance.
Teacher's Tip: Melting point = Solid's "breaking point"; Boiling point = Liquid's "escape point."
Exam Tip: Always mention that the temperature remains "constant" during the change.

Question 9: Describe the process of condensation and sublimation with examples.
Answer: CONDENSATION : “The change of vapours on cooling at fixed temperature to liquid is called condensation.” Example: When water vapours at 100°C are cooled they change into water (liquid). SUBLIMATION : “The process of change of solid directly into vapours on heating is called sublimation.”
Condensation is what creates the "sweat" on a cold soda can as water vapour in the air cools down. Sublimation is a rarer process where the liquid phase is skipped entirely, like with dry ice.
Teacher's Tip: Condensation is "gas to liquid," while Sublimation is "solid to gas."
Exam Tip: Mention "cooling" for condensation and "heating" for sublimation.

Question 10: Explain the term melting and melting point.
Answer: Melting - The change from the solid state to the liquid state on heating at a fixed temperature is called melting. Melting Point - It can be defined as the fixed temperature at which a solid starts changing to its liquid state.
Melting is the action or process of turning into a liquid, while the melting point is the exact number on the thermometer where it happens. For ice, this number is 0°C.
Teacher's Tip: One is the "event" (melting) and the other is the "measurement" (melting point).
Exam Tip: Be clear about the difference between a process and a specific temperature.

Question 11: Describe an experiment to demonstrate that a substance absorbs heat during melting without change in its temperature.
Answer: MELTING POINT OF SOLID (WAX): Put some wax in a test tube. Insert a thermometer in solid wax, so that bulb of thermometer remains in wax and does not touch the sides. Clamp the test tube along with thermometer in hot bath i.e. in water contained in the beaker and set up the apparatus as shown. Note the temperature Heat the beaker over the flame of burner and record the temperature after every minute. First temperature rises and then reaches 55\ °C and wax shines in the test tube. Temperature remains constant for nearly 5 minutes i.e. at 55 °C. This means Wax is melting and temperature remains constant till whole of wax is melted. Then temperature rises again every minute till it reaches Conclusion : The temperature remains constant at 55°C while changing from solid to liquid. This means 55°C is the melting point and heat is absorbed without change in temperature. This heat is absorbed at constant temperature till whole of wax is melted.
This experiment shows that the heat energy being added isn't "lost"; it's being used to do the work of rearranging molecules from a solid into a liquid. The thermometer stays still at 55°C because all the heat is busy melting the wax.
Teacher's Tip: The "plateau" (flat part) on a temperature graph represents the phase change.
Exam Tip: Mention that the thermometer must not touch the sides of the tube to get an accurate reading of the wax itself.

Question 12: Explain the terms vaporization and boiling point.
Answer: VAPORIZATION: “Change of liquid to vapours (gas) on heating at constant temperature is called VAPORIZATION.” When we heat a liquid temperature starts rising till it starts changing into vapours and then temperature remains constant for sometime, through we are supplying heat. This heat supplied is being used to change every molecule of liquid into vapours and temperature does not rise till the whole of liquid is changed into vapours. BOILING POINT : “The temperature at which a liquid starts changing into vapours or gas at constant temperature is called its BOILING POINT.”
Vaporization is the general transformation where liquid molecules gain enough energy to become gas. The boiling point is the specific "threshold" temperature where this process happens rapidly.
Teacher's Tip: Boiling is like a "traffic jam" on the thermometer - it can't go higher until the liquid is gone.
Exam Tip: Explain that heat energy is used to "break molecular bonds" during vaporization.

Question 13: A liquid can change into vapour state (a) at a fixed temperature, and (b) at all temperatures. Name the processes involved in two cases.
Answer: (a) is Boiling point (b) is Evaporation. The process involved in two cases is vaporization or boiling.
Boiling is "forced" at a specific high temperature, whereas evaporation is a "natural" and quiet process that happens slowly at the surface. Both processes ultimately result in the liquid turning into a gas.
Teacher's Tip: Boiling = Bubbles everywhere; Evaporation = Surface only.
Exam Tip: Use "fixed temperature" as your primary way to identify boiling in questions.

Question 14: Some ice is taken in a beaker and its temperature is recorded after each one minute. The observations are listed below: Time (0-5 min): 0°C; Time (6 min): 3.8°C; Time (7 min): 7.6°C; Time (8 min): 11.4°C. From the above observations what conclusion do you draw about the melting point of ice ?
Answer: From the above observations we conclude that ice melts at 0°C during which heat is supplied but temperature does not rise shows that heat supplied is used to change every molecule of ice into water and when whole of ice is melted, temperature starts rising.
The data shows that the temperature was "stuck" at 0°C for the first 5 minutes even though heat was being absorbed. This clearly identifies 0°C as the melting point of the ice.
Teacher's Tip: Whenever the temperature stays the same while heating, a phase change is happening.
Exam Tip: Point out the specific time interval (0 to 5 minutes) where the temperature remained constant.

Question 15: Describe an experiment to demonstrate that water absorbs heat during boiling at a constant temperature.
Answer: BOILING POINT OF WATER AT CONSTANT TEMPERATURE: Take some water in a beaker. Suspend and clamp a thermometer in beaker in water so that bulb of thermometer remains in water without touching bottom and sides of beaker. Supply heat by burner and note the temperature at room temperature (20°C nearly). Record the temperature after every minute. Temperature rises and as it reaches 100°C water starts boiling. Though heat is being supplied temperature does not rise i.e. it remains constant at 100°C and bubbles formed are seen. Thus, boiling point of water is 100°C and at boiling point heat supplied is absorbed by water at constant temperature. Because this heat is being used to change every molecule of water into vapours.
This experiment proves that 100°C is a hard limit for liquid water at normal pressure. All the energy from the burner goes into turning molecules into steam once this "speed limit" is reached.
Teacher's Tip: The "constant temperature" is the most important proof of the boiling point.
Exam Tip: Describe the formation of bubbles to show that boiling is occurring "throughout" the liquid.

Question 16: State (a) the melting point of ice, and (b) the boiling point of water.
Answer: (a) MELTING POINT OF ICE: “Is the constant temperature at which it starts (melting) changing from ice to water.” It is 0°C for ice. (b) BOILING POINT OF WATER : “Is that constant temperature at which water starts (BOILING) changing from water to steam (vapours)”. It is 100°C for water.
These two numbers (0 and 100) are the landmarks of the Celsius temperature scale. They define the solid-to-liquid and liquid-to-gas transitions for our most common substance.
Teacher's Tip: Memorize 0 and 100 as the "magic numbers" for water.
Exam Tip: Don't forget the unit °C when writing these temperatures.

Question 17: What is evaporation ?
Answer: EVAPORATION : “The change of state of a liquid to vapour at all temperatures from the surface of liquid is called evaporation.”
Evaporation is a quiet, natural process where individual molecules at the surface get enough "kick" to jump out into the air. It happens all the time, even when you don't see steam rising.
Teacher's Tip: Evaporation is like a slow "leak" of molecules from the liquid's surface.
Exam Tip: Use the phrase "from the surface of liquid" to distinguish it from boiling.

Question 18: State three factors which affect the rate of evaporation of a liquid.
Answer: Three factors on which affect the rate of evaporation of a liquid: (i) AREA OF EXPOSED SURFACE. (ii) TEMPERATURE OF LIQUID. (iii) NATURE OF THE LIQUID. (iv) PRESENCE OF HUMIDITY.
Evaporation goes faster if you spread the liquid out, heat it up, or if the air around it is dry. Some liquids, like rubbing alcohol, also evaporate much faster than water due to their chemical nature.
Teacher's Tip: Think of "Hot, Dry, and Wide" for fast evaporation.
Exam Tip: Mentioning "Area of exposed surface" is a very common point in scoring full marks.

Question 19: Wet clothes dry more quickly on a warm dry day than on a cold humid day. Explain.
Answer: Rate of evaporation is directly proportional to temperature. Thus, rate of evaporation is higher on warm day i.e. hot day than cold day having low temperature and clothes dry soon on warm day.
Warm air can hold more moisture, and dry air "pulls" water molecules away from the clothes faster. On a humid day, the air is already "full" of water, so there is no room for more molecules to escape.
Teacher's Tip: Warm air acts like a "hungry sponge" for water molecules.
Exam Tip: Mention "directly proportional to temperature" to give a scientific reason.

Question 20: Water in a dish evaporates faster than in a bottle. Give reason.
Answer: Rate of evaporation is more when the area of exposed surface is more. As area exposed in a dish is more, evaporation is also more.
Because evaporation only happens at the surface, a wider surface means more molecules have the chance to escape at the same time. In a bottle, only the small circle of water at the neck is exposed.
Teacher's Tip: If you want water to disappear fast, spread it out!
Exam Tip: Use the term "surface area" to explain this difference clearly.

Question 21: Why are volatile liquids such as alcohol and spirit stored in tightly closed bottles ?
Answer: Rate of evaporation depends on NATURE OF LIQUID i.e. more volatile liquids like ALCOHOL and SPIRIT evaporate easily, hence these are stored in tightly closed bottles to avoid their evaporation.
Volatile liquids have very weak attractive forces between their molecules, so they turn into gas almost instantly when exposed to air. Closing the bottle prevents these molecules from wandering away.
Teacher's Tip: "Volatile" means it "wants" to be a gas.
Exam Tip: Define volatile liquids as those that "evaporate easily at room temperature."

Question 22: A certain quantity of water is heated from 20°C to 100°C. Its temperature is recorded after each 1 minute. Time (0 min): 20°C; Time (1 min): 30°C; ... Time (8-12 min): 100°C. What conclusion do you draw from the above table about the boiling point of water ? Explain.
Answer: From the table given above we note that as thermometer shows 100°C, it becomes constant and through heat is being supplied. This means boiling point of water is 100°C and heat supplied is being used to convert every molecule of water into vapours (steam) till whole of the water gets boiled off.
The fact that the temperature stopped rising exactly at 100°C despite constant heating is the signature of boiling. The "missing" heat is being used as energy for phase change rather than temperature increase.
Teacher's Tip: The flat line on a time-temperature graph is where the "real work" of phase change happens.
Exam Tip: State that 100°C is the boiling point because the temperature remains "constant" at that value.

Question 23: Why is cooling produced on evaporation of a liquid ?
Answer: For evaporation of a liquid it requires HEAT. This heat is taken from the surroundings like body or palm or fore-head or finger and its temperature falls and we feel cool.
To escape the liquid, a molecule must steal some energy from its neighbors. When those neighbors lose energy, they get colder, which is why your skin feels cool when water evaporates from it.
Teacher's Tip: Evaporation is like a "heat thief" that takes warmth away with it.
Exam Tip: Mention that heat is "absorbed from the surroundings" to explain the cooling effect.

Question 24: Explain with an example to demonstrates that when a liquid evaporates, it takes heat from its surroundings.
Answer: If some spirit is poured on cotton wrapped around the bulb of a thermometer, the reading of the thermometer falls. This shows that cooling is produced when a liquid evaporates taking heat from surroundings.
The spirit evaporates very quickly and pulls the necessary heat directly from the thermometer bulb. As the bulb loses heat, the mercury or alcohol inside contracts, showing a lower temperature.
Teacher's Tip: This is the same reason why hand sanitizer feels icy on your hands.
Exam Tip: Mention the "fall in thermometer reading" as the direct proof of cooling.

Question 25: Give two applications of evaporation.
Answer: Two APPLICATIONS OF EVAPORATION: (i) When we sprinkle water on the roads in summer evening, water evaporates by taking heat from the road and produces coolness in the surroundings and it becomes pleasant. (ii) After taking a bath in summer when we come out of water, water evaporates taking heat from our body. The temperature of body falls and we feel refreshed.
We use evaporation to regulate temperature in many ways, from cooling our cities to cooling our own skin. It's nature's built-in air conditioning system.
Teacher's Tip: Think of evaporation as a "natural cooler."
Exam Tip: Use everyday examples like "sprinkling water" or "sweating" for application questions.

Question 26: Explain why in hot summer days water remains cool in earthen pots.
Answer: Water seeps out through the pores in the earthen pot and it evaporates. The latent heat required for evaporation is taken, from water inside the pot which gets cooled.
An earthen pot acts like a "sweating" skin for the water inside. The constant evaporation from the outside surface keeps the temperature of the internal water much lower than the outside air.
Teacher's Tip: Earthen pots are "living" coolers because of their tiny pores.
Exam Tip: Mention "pores" and "latent heat" to provide a complete scientific explanation.

Question 27: A patient suffering from high fever is advised to put wet cloth strips on his forehead. Why ?
Answer: Water in wet’ strips evaporates taking latent heat required for evaporation from the forehead. The temperature of forehead (body of the patient) falls and he feels relieved.
As the water turns to vapour, it effectively "pulls" the excess fever heat out of the patient's head. This is a simple and effective way to lower body temperature without medicine.
Teacher's Tip: The wet cloth acts like a "heat magnet" that carries the fever away as it dries.
Exam Tip: Focus on the "transfer of heat" from the body to the evaporating water.

Question 28: What do you mean by sublimation ? Explain with an example.
Answer: SUBLIMATION : “Change of solid state of matter directly on heating to vapour state (without becoming liquid) and on cooling vapours to solid is called sublimation. Substances are Dry ice, Napthalene balls (Moth balls), Iodine, Ammonium chloride (NH₄Cl) etc.
Sublimation is a "shortcut" phase change where the material never melts into a puddle. This happens because the substance's molecules are much more stable as a gas than a liquid at normal pressures.
Teacher's Tip: Sublimation is like a magic trick where the solid disappears into thin air!
Exam Tip: List "Dry ice" or "Naphthalene balls" as your primary examples for this process.

Question 29: Why does the size of naphthalene balls decrease when left open ?
Answer: When naphthalene balls are left open, due to sublimation they change to vapours and their size decreases.
Because they are constantly turning directly into gas, the solid part of the ball literally "vanishes" into the air over time. This is why you can smell them even though they look like solid white marbles.
Teacher's Tip: The smell of mothballs is actually the "lost" solid molecules floating in the air.
Exam Tip: Use the word "sublimation" as the central reason for the size change.

Question 30: Describe an experiment to demonstrate the process of sublimation.
Answer: Experiment: Take some Ammonium chloride powder in a china dish. Cover the china dish with inverted funnel and put a cotton plug in end of funnel so that vapours do not escape. Set up the apparatus as shown. Heat the dish with burner. Solid ammonium chloride changes into vapours. Which when come in contact of walls of funnel get cooled and change to solid and get deposited there.
This experiment shows both sublimation (solid to gas) and deposition (gas to solid) in one go. You start with a powder at the bottom and end up with a solid "crust" on the cool walls of the funnel.
Teacher's Tip: The "cotton plug" is essential so you don't lose your experiment into the room air.
Exam Tip: Draw the funnel with the "cotton plug" clearly labeled to show you understand how the vapours are trapped.