Changes of state

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Lesson notes

Conservation of Mass

During a state change, the **number of particles** stays the same.
The **mass** before a state change equals the mass after – this is **conservation of mass**.
No particles are lost or gained; only their arrangement and energy change.

Particle Model of Solids

Particles are held together in a **regular pattern**.
They **vibrate** around fixed positions.
Heating increases vibrations until the **melting point** is reached.
Example: melting point of water is **0°C**.

Melting (Solid → Liquid)

At the melting point, temperature **stays constant** while melting occurs.
Particles break free from fixed positions but remain **in contact**.
Once fully melted, particles can **move past each other** (liquid state).

Boiling (Liquid → Gas)

Heating a liquid makes particles move faster until the **boiling point**.
At boiling point, particles gain enough energy to **separate** from each other.
Gas particles are **far apart** and move **quickly in random directions**.
Example: boiling point of water is **100°C**.

Condensation (Gas → Liquid)

Cooling a gas to its **boiling point** causes it to **condense** into a liquid.
Particles lose energy and come closer together.

Freezing (Liquid → Solid)

Cooling a liquid to its **melting point** causes it to **freeze** into a solid.
Particles become fixed in a regular pattern.

Sublimation and Deposition

**Sublimation**: solid changes directly into a gas (e.g., dry ice).
**Deposition**: gas changes directly into a solid (e.g., frost formation).

Summary of State Changes

Solid → Liquid: **melting**
Liquid → Gas: **boiling/evaporation**
Gas → Liquid: **condensation**
Liquid → Solid: **freezing**
Solid → Gas: **sublimation**
Gas → Solid: **deposition**

Particle arrangement in solid, liquid, and gas.

Slides

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Practice questions

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1.What is meant by the conservation of mass during a change of state?
Easy
- AThe total number of particles remains the same, so mass does not change.
- BMass is lost when a solid melts.
- CMass is gained when a gas condenses.
- DThe mass changes but the number of particles stays the same.
2.What is the name of the change of state from solid to liquid?
Easy
- AMelting
- BFreezing
- CCondensation
- DSublimation
3.What is the name of the change of state from gas to liquid?
Easy
- ACondensation
- BEvaporation
- CDeposition
- DFreezing
4.What is the name of the change of state from liquid to gas?
Easy
- ABoiling
- BMelting
- CCondensation
- DSublimation
5.What is the name of the change of state from gas to solid?
Easy
- ADeposition
- BSublimation
- CCondensation
- DFreezing
6.What is the name of the change of state from solid to gas?
Easy
- ASublimation
- BDeposition
- CMelting
- DEvaporation
7.What is the name of the change of state from liquid to solid?
Easy
- AFreezing
- BMelting
- CCondensation
- DDeposition
8.What is the melting point of water?
Easy
- A0°C
- B100°C
- C-10°C
- D50°C

More questions

9.What is the boiling point of water?
Easy
- A100°C
- B0°C
- C50°C
- D200°C
10.Which statement about particles in a solid is correct?
Medium
- AParticles vibrate around fixed positions in a regular pattern.
- BParticles are far apart and move quickly in random directions.
- CParticles are free to move past each other but remain in contact.
- DParticles do not move at all.
11.Which statement about particles in a gas is correct?
Medium
- AParticles are far apart and move around quickly in random directions.
- BParticles vibrate around fixed positions.
- CParticles are in contact but can move past each other.
- DParticles are arranged in a regular pattern.
12.Which of the following statements is true?
Medium
- AAs a solid melts, the temperature remains constant until melting is complete.
- BAs a solid melts, the temperature increases steadily.
- CAs a gas is heated, particles move more slowly.
- DIf particles in a liquid evaporate into a gas, the gas has less mass than the liquid.
13.Water at -10°C is in which state of matter?
Hard
- ASolid
- BLiquid
- CGas
- DPlasma
14.Water at 200°C is in which state of matter?
Hard
- AGas
- BLiquid
- CSolid
- DPlasma
15.Water at 50°C is in which state of matter?
Medium
- ALiquid
- BSolid
- CGas
- DPlasma
16.Which statement about particles in a liquid is correct?
Medium
- AParticles are in contact but can move past each other.
- BParticles vibrate around fixed positions.
- CParticles are far apart and move quickly in random directions.
- DParticles are arranged in a regular pattern.
17.What happens to the movement of particles in a liquid as it is heated towards its boiling point?
Medium
- AThe particles move more quickly.
- BThe particles move more slowly.
- CThe particles stop moving.
- DThe particles vibrate more but do not change speed.
18.A sample of substance X has a melting point of -10°C and a boiling point of 60°C. At 70°C, what state is substance X in?
Hard
- AGas
- BLiquid
- CSolid
- DCannot be determined
19.A student measures the temperature of a substance as it changes from solid to liquid. Which graph best shows the temperature change over time during melting?
Hard
- ATemperature stays constant during melting, then rises after.
- BTemperature rises steadily throughout.
- CTemperature decreases during melting, then rises.
- DTemperature stays constant before and after melting.
20.What is the name of the process when a gas turns directly into a solid without becoming a liquid first?
Medium
- ADeposition
- BSublimation
- CCondensation
- DFreezing
21.What is the name of the process when a solid turns directly into a gas without becoming a liquid first?
Medium
- ASublimation
- BDeposition
- CMelting
- DEvaporation
22.Which of the following is NOT a change of state?
Easy
- ADissolving
- BMelting
- CBoiling
- DCondensation
23.A student places a beaker of water at 20°C in a freezer. The water eventually turns to ice. Which statement correctly describes the particle arrangement during this process?
Hard
- AParticles become fixed in a regular pattern as they lose energy.
- BParticles move faster and become more spread out.
- CParticles remain in the same arrangement throughout.
- DParticles first become more ordered, then less ordered.
24.A student places a beaker of water at 20°C in a freezer. The water eventually turns to ice. Which statement correctly describes the particle arrangement during this change?
Medium
- AThe particles become more ordered and vibrate about fixed positions.
- BThe particles become more disordered and move freely.
- CThe particles remain in the same arrangement but move faster.
- DThe particles separate and move randomly.
25.A sealed container holds a liquid. When the container is heated, the liquid turns into a gas. What happens to the mass inside the container?
Medium
- AThe mass increases because gas particles are lighter.
- BThe mass decreases because gas particles escape.
- CThe mass stays the same because no particles leave the container.
- DThe mass decreases because the gas takes up more space.
26.The table shows the melting and boiling points of four substances. Which substance is a liquid at 100°C?
Hard
- ASubstance A: melting point -50°C, boiling point 50°C
- BSubstance B: melting point 20°C, boiling point 150°C
- CSubstance C: melting point 0°C, boiling point 100°C
- DSubstance D: melting point 120°C, boiling point 200°C
27.A student heats a solid and records its temperature every minute. The data shows the temperature rising, then constant at 80°C for 5 minutes, then rising again. What is happening during the constant temperature period?
Medium
- AThe solid is boiling.
- BThe solid is melting.
- CThe solid is condensing.
- DThe solid is freezing.
28.A gas at 200°C is cooled. It becomes a liquid at 100°C and then a solid at 0°C. Which statement is correct?
Hard
- AThe melting point is 0°C and the boiling point is 100°C.
- BThe melting point is 100°C and the boiling point is 200°C.
- CThe melting point is 200°C and the boiling point is 100°C.
- DThe melting point is 0°C and the boiling point is 200°C.
29.A beaker of water is heated from 20°C to 80°C. Which statement describes the movement of water particles?
Medium
- AThe particles vibrate about fixed positions.
- BThe particles move faster and further apart.
- CThe particles stop moving.
- DThe particles move slower and closer together.
30.A student places a wet towel outside on a cold day. The water in the towel turns into ice. Which change of state has occurred?
Hard
- AMelting
- BEvaporation
- CFreezing
- DCondensation
31.Which statement about changes of state is correct?
Medium
- AMelting, boiling, and sublimation require energy input.
- BFreezing, condensation, and deposition require energy input.
- CMelting, freezing, and boiling require energy input.
- DSublimation, deposition, and condensation require energy input.
32.The diagram shows the arrangement of particles in three states of matter. Which state has particles that are far apart and move randomly in all directions?
Medium
- ASolid
- BLiquid
- CGas
- DPlasma
33.A student measures the temperature of a pure solid as it is heated. The temperature rises steadily, then remains constant at 80°C for several minutes, then rises again. What is happening during the constant temperature period?
Medium
- AThe solid is melting
- BThe liquid is boiling
- CThe solid is subliming
- DThe gas is condensing
34.A student places a beaker of water at 20°C on a balance and heats it until it boils. The mass reading decreases slightly. Which statement best explains this observation?
Hard
- AMass is not conserved during boiling
- BSome water particles escape as gas and are not contained
- CThe balance is affected by the heat
- DThe water expands and becomes less dense
35.A substance has a melting point of -15°C and a boiling point of 78°C. At room temperature (20°C), in which state is the substance?
Medium
- ASolid
- BLiquid
- CGas
- DPlasma
36.Which change of state involves a substance changing directly from a solid to a gas?
Easy
- AMelting
- BBoiling
- CSublimation
- DEvaporation
37.A student heats a solid and records its temperature every minute. The data shows: 0 min: 20°C, 1 min: 30°C, 2 min: 40°C, 3 min: 50°C, 4 min: 50°C, 5 min: 50°C, 6 min: 60°C, 7 min: 70°C. What is the melting point of the substance?
Hard
- A40°C
- B50°C
- C60°C
- D70°C
38.A sealed container holds a liquid and its vapour. The container is heated. What happens to the mass inside the container?
Medium
- AIncreases
- BDecreases
- CStays the same
- DIncreases then decreases
39.A puddle of water disappears on a warm day. Which change of state has occurred?
Medium
- AMelting
- BBoiling
- CEvaporation
- DSublimation
40.A student investigates the cooling of a liquid. The temperature decreases steadily, then remains constant at -10°C for several minutes, then decreases again. Which statement is correct?
Hard
- AThe liquid is boiling at -10°C
- BThe liquid is freezing at -10°C
- CThe gas is condensing at -10°C
- DThe solid is melting at -10°C

Revision notes

## Changes of State - **Conservation of mass**: During a state change, the number of particles stays the same, so mass before = mass after. - **Solid → Liquid**: Melting (at melting point). Particles vibrate more, break free from fixed positions but stay in contact. - **Liquid → Gas**: Boiling (at boiling point). Particles move fast enough to separate and move randomly. - **Gas → Liquid**: Condensation (at boiling point). - **Liquid → Solid**: Freezing (at melting point). - **Solid → Gas**: Sublimation (directly). - **Gas → Solid**: Deposition (directly). - **Melting point of water**: 0°C. - **Boiling point of water**: 100°C. - **Particle arrangement**: - Solid: Regular pattern, vibrate in fixed positions. - Liquid: Free to move past each other, still in contact. - Gas: Far apart, move quickly in random directions. - **During melting/boiling, temperature stays constant** until state change is complete. - **Heating increases particle movement**; cooling decreases it. - **Mass is conserved** in all state changes.

Conservation of mass: The mass before a state change is the same as the mass after a state change.
Melting point: The temperature at which a solid turns into a liquid (e.g., water at 0°C).
Boiling point: The temperature at which a liquid turns into a gas (e.g., water at 100°C).
Melting: Change of state from solid to liquid.
Boiling: Change of state from liquid to gas.
Condensation: Change of state from gas to liquid.
Freezing: Change of state from liquid to solid.
Sublimation: Change of state from solid directly to gas.
Deposition: Change of state from gas directly to solid.

Flashcards

What is conservation of mass?

Mass before a state change is the same as mass after a state change.

Name the change from solid to liquid.

Melting.

Name the change from gas to liquid.

Condensation.

Name the change from liquid to gas.

Boiling (or evaporation).

Name the change from gas to solid.

Deposition.

Name the change from solid to gas.

Sublimation.

Name the change from liquid to solid.

Freezing.

What is the melting point of water?

0°C.

What is the boiling point of water?

100°C.

Describe particle arrangement in a solid.

Particles are held together in a regular pattern and vibrate around fixed positions.

Describe particle arrangement in a gas.

Particles are far apart and move around quickly in random directions.

True or false: As a solid melts, the temperature increases.

False. While melting, temperature stays constant.

True or false: As a gas is heated, the particles move more quickly.

True.

True or false: Regardless of temperature, particles in a solid vibrate the same amount.

False. Vibrations increase with temperature.

True or false: If particles in a liquid evaporate into a gas, the gas has less mass than the liquid.

False. Mass is conserved; the gas has the same mass.

Water at -10°C: state and reason.

Solid (ice). Because -10°C is below the melting point of 0°C.

Water at 200°C: state and reason.

Gas (steam). Because 200°C is above the boiling point of 100°C.

Water at 50°C: state and reason.

Liquid. Because 50°C is between the melting point (0°C) and boiling point (100°C).

What happens to particles when a solid is heated to its melting point?

Particles vibrate more until they break free from fixed positions, becoming a liquid.

What happens to particles when a liquid boils?

Particles move quickly enough to separate from one another, becoming a gas.

What happens to particles when a gas condenses?

Particles slow down and come together to form a liquid.

What happens to particles when a liquid freezes?

Particles slow down and lock into fixed positions, forming a solid.

What is the overall number of particles during a state change?

The number of particles stays the same.

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