States of matter

1. Overview

Matter on Earth exists in three primary physical forms: solids, liquids, and gases. Understanding the arrangement and behavior of particles in these states is fundamental to explaining how substances react to changes in temperature and pressure.

Key Definitions

• Matter: Anything that has mass and occupies space.
• Melting: The process where a solid turns into a liquid.
• Boiling: The process where a liquid turns into a gas at a specific temperature (throughout the liquid).
• Condensing: The process where a gas turns into a liquid.
• Freezing/Solidifying: The process where a liquid turns into a solid.
• Evaporation: The process where a liquid turns into a gas at the surface (can happen at any temperature below boiling).

Core Content

Properties of Solids, Liquids, and Gases

The behavior of matter is determined by the arrangement and motion of its particles (atoms or molecules).

Property	Solid	Liquid	Gas
Shape	Fixed shape	Takes the shape of the container bottom	Fills the entire container
Volume	Fixed volume	Fixed volume	No fixed volume (expands)
Density	High	High (slightly lower than solids*)	Low
Compressibility	Almost impossible to compress	Very difficult to compress	Easy to compress
Arrangement	Regular lattice, particles touching	Random arrangement, particles touching	Random arrangement, far apart
Motion	Vibrate about fixed positions	Slide over one another	Move rapidly and randomly

Changes of State

When energy (heat) is added to or removed from a substance, it can change state.

• To go from Solid → Liquid → Gas: Energy must be supplied. This energy is used to break or overcome the attractive forces between particles.
• To go from Gas → Liquid → Solid: Energy must be released. Particles lose kinetic energy and move closer together as attractive forces pull them into a more ordered structure.

Extended Content (Extended Only)

There is no additional Supplement content for this specific sub-topic (2.1.1).

Key Equations

While this topic is descriptive, it relates closely to the density formula:

$$\text{Density} (\rho) = \frac{\text{Mass} (m)}{\text{Volume} (V)}$$

• $\rho$ (Rho): Density (measured in $kg/m^3$ or $g/cm^3$)
• $m$: Mass (measured in $kg$ or $g$)
• $V$: Volume (measured in $m^3$ or $cm^3$)

Common Mistakes to Avoid

• ❌ Wrong: Thinking that $0\ ^\circ\text{C}$ is only the temperature of ice.
  • ✅ Right: $0\ ^\circ\text{C}$ is the temperature of the melting point of ice and the freezing point of water. Both solid ice and liquid water can exist at $0\ ^\circ\text{C}$ simultaneously during the state change.
• ❌ Wrong: Thinking particles in a liquid are far apart because they can flow.
  • ✅ Right: Particles in a liquid are still in constant contact (touching). They flow because they have enough energy to slide past one another, unlike solids.
• ❌ Wrong: Suggesting that molecules gain energy to become "more ordered."
  • ✅ Right: Molecules lose energy to become more ordered (e.g., as a gas cools to a liquid). Energy is required to break order and create disorder.
• ❌ Wrong: Believing you can see individual molecules under a standard school microscope.
  • ✅ Right: Molecules are far too small to be seen. We can only observe the effects of their motion (like Brownian Motion) on larger, visible particles like smoke or pollen.

Exam Tips

1. Use the word "Touching": When describing solids and liquids, always state that the particles are touching. Many students lose marks by drawing liquid particles with gaps between them.
2. Describe the Motion: If asked about a gas, use the phrase "rapid, random motion." For solids, use "vibrate about fixed positions."
3. State Change Temperature: Remember that during a change of state (melting or boiling), the temperature of the substance stays constant, even though heat is still being added.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0625 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A student places a block of ice at -10°C on a table at room temperature (25°C).

(a) State the three states of matter. [3]

(b) Describe the changes that will happen to the ice block as it warms up to room temperature. [2]

Worked Solution:

(a)

1. Solid, Liquid, Gas [Listing the three states of matter]

How to earn full marks:

• Correctly state all three states of matter (1 mark each).

(b)

1. The ice will absorb thermal energy from the surroundings. [Ice absorbs energy from the warmer room]
2. The temperature of the ice will increase until it reaches 0°C, at which point it will start to melt, changing from solid to liquid. The liquid water will then warm up to room temperature. [Explanation of melting process and subsequent warming]

How to earn full marks:

• Mention absorption of thermal energy or heat transfer (1 mark)
• Mention melting/change of state from solid to liquid at 0°C and the water warming up (1 mark)

Common Pitfall: Many students forget that the ice doesn't instantly become 25°C water. It first melts at 0°C, and then the resulting liquid water warms up to room temperature. Also, remember that 0°C is the temperature at which ice melts, not just "the temperature of ice."

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A sealed container contains a fixed mass of gas.

(a) Define the term "matter". [2]

(b) Describe the motion of the gas particles inside the container. [2]

(c) Explain why gases can be easily compressed, but solids cannot. [2]

Worked Solution:

(a)

1. Matter is anything that has mass and occupies space (volume). [Definition of matter]

How to earn full marks:

• Correctly state that matter has mass (1 mark).
• Correctly state that matter occupies space/volume (1 mark).

(b)

1. The gas particles move randomly. [Gas particle motion]
2. They move at high speeds in all directions. [Description of the speed and direction of gas particle motion]

How to earn full marks:

• Mention random motion (1 mark).
• Mention high speed or motion in all directions (1 mark).

(c)

1. Gas particles are far apart with large spaces between them. [Spacing between gas molecules]
2. When compressed, the particles move closer together, reducing the space between them. Solid particles are already closely packed, with very little space between them, so they cannot be easily compressed. [Explanation of compression in gases vs. solids]

How to earn full marks:

• Mention large spaces between gas particles (1 mark).
• Mention close packing of solid particles (1 mark).

Common Pitfall: When describing gases, avoid saying the particles "move around" without specifying the randomness and high speed. Also, remember that liquids, like solids, have particles close together, so don't describe liquid particles as being far apart.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A student is investigating the melting point of a wax. They heat a sample of solid wax in a test tube, recording the temperature every minute. The results are shown below:

Time (minutes)	Temperature (°C)
0	20
1	30
2	40
3	50
4	55
5	55
6	55
7	55
8	60
9	65

(a) State the melting point of the wax. [1]

(b) Describe what is happening to the wax between minutes 4 and 7. [3]

(c) Explain, in terms of the kinetic theory of matter, why the temperature remains constant during melting. [4]

Worked Solution:

(a)

1. $\boxed{55 \text{ °C}}$ [Reading the melting point from the table]

How to earn full marks:

• Correctly state the melting point with the correct units (1 mark).

(b)

1. The wax is melting. [Identifying the process]
2. The wax is changing from a solid to a liquid. [Change of state]
3. Energy is still being supplied to the wax, even though the temperature is constant. [Observation about constant temperature and energy input]

How to earn full marks:

• Mention melting or change of state (1 mark).
• Mention solid to liquid (1 mark).
• Mention constant temperature AND that energy is still being supplied (1 mark).

(c)

1. During melting, energy is supplied to the wax. [Energy input during melting]
2. This energy is used to overcome the forces of attraction between the wax molecules/particles. [Energy is used to break bonds]
3. This increases the potential energy of the particles, allowing them to move more freely. [Increase in potential energy]
4. Since the energy is increasing the potential energy and not the kinetic energy of the particles, the temperature remains constant. [Constant kinetic energy means constant temperature]

How to earn full marks:

• Mention energy is supplied during melting (1 mark).
• Mention overcoming forces of attraction/breaking bonds (1 mark).
• Mention increase in potential energy (1 mark).
• Mention energy increases potential energy and not kinetic energy, hence constant temperature (1 mark).

Common Pitfall: Many students simply state the melting point without units. Also, be sure to explain that the energy supplied during melting is used to break intermolecular bonds, increasing the potential energy of the particles, not their kinetic energy (which would increase the temperature).

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A student is asked to investigate the rate of evaporation of different liquids. They place equal volumes of propanol, water, and a viscous silicone oil in separate identical beakers and leave them in a well-ventilated room.

(a) State three factors that affect the rate of evaporation of a liquid. [3]

(b) Describe how the student could ensure a fair test in this experiment. [3]

(c) After 24 hours, the student observes that the propanol has evaporated completely, the water has partially evaporated, and the silicone oil has barely evaporated. Explain these observations in terms of the intermolecular forces between the molecules of each liquid. [3]

Worked Solution:

(a)

1. Temperature [Temperature as a factor]
2. Surface area [Surface area as a factor]
3. Airflow/ventilation/humidity [Airflow/ventilation/humidity as a factor]

How to earn full marks:

• Correctly state any three factors affecting evaporation (1 mark each).

(b)

1. Use equal volumes of each liquid. [Control variable: volume]
2. Use identical beakers. [Control variable: beaker type]
3. Place all beakers in the same location with the same temperature, airflow, and humidity. [Control variable: location]

How to earn full marks:

• Mention equal volumes (1 mark).
• Mention identical beakers (1 mark).
• Mention same location/temperature/airflow/humidity (1 mark).

(c)

1. Propanol has the weakest intermolecular forces. [Weak forces in propanol]
2. Water has stronger intermolecular forces than propanol. [Stronger forces in water]
3. Silicone oil has the strongest intermolecular forces of the three liquids. The stronger the intermolecular forces, the more energy is required for the molecules to overcome these forces and escape into the gaseous phase (evaporate). [Strongest forces in silicone oil, and link to energy required for evaporation]

How to earn full marks:

• Mention propanol has the weakest forces (1 mark).
• Mention water has stronger forces than propanol (1 mark).
• Mention silicone oil has the strongest forces AND links this to the energy required for evaporation (1 mark).

Common Pitfall: Students often forget to explicitly link the strength of intermolecular forces to the amount of energy needed for evaporation. Make sure to state that stronger forces require more energy for molecules to escape the liquid phase. Also, be specific about which liquid has the strongest or weakest forces.