Ch 1 · 2.1.1 States of Matter · O Level Physics 5054
Topic 1 of 9 in Thermal Physics — O Level Physics 5054 — 39 lessons.
Introduction
Ice in a glass, the water it melts into, and the steam above a kettle are the same substance in three different states: solid, liquid and gas. Each state behaves in its own way, and the change from one state to another has its own name. (Syllabus section 2.1.1.)
Key ideas — The three states
Two everyday checks:
🧑🏫 Teacher's explanation
Imagine a crowded dance floor. In a solid, everyone is packed tightly together, holding hands in a fixed pattern; they can wiggle (vibrate), but they can't move past each other. This is why solids keep their shape. In a liquid, the dancers are still close together, but they are moving around each other, sliding through the crowd. This allows liquids to flow and take the shape of their container, but they still occupy a fixed amount of space.
📖 Definitions
🔍 Worked example — Easy
A block of iron has a volume of 0.05 m^3. If the iron is melted into a liquid state, what happens to its shape and its volume?
🔍 Worked example — Standard
A syringe is filled with 50 cm^3 of air. When the plunger is pushed, the volume decreases to 10 cm^3. Why was the air easy to compress while water would not be?
🔍 Worked example — Exam-style
Compare the density of a solid block of ice to a gas of water vapor. Which is higher and why?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Changes of state
Melting and freezing happen at the same temperature (the melting point); boiling and condensation happen at the same temperature (the boiling point). Changes straight from gas to solid, or solid to gas, are not needed at O Level.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor. In a solid, everyone is packed tight, standing in a fixed grid, only vibrating slightly in place. As you add heat, you are essentially giving the dancers more energy to move. Eventually, they move so much they break out of their fixed positions and start sliding past each other—this is a liquid. If you keep adding energy, they break free entirely and fly around the room—this is a gas.
📖 Definitions
🔍 Worked example — Easy
A substance has a melting point of 50 ^circC. If the substance is currently at 30 ^circC, what state of matter is it likely to be in?
🔍 Worked example — Standard
A liquid is heated until it reaches its boiling point of 100 ^circC. During the boiling process, the temperature remains at 100 ^circC even though heat is being added. What is the name of this process?
🔍 Worked example — Exam-style
A sample of water is cooled from 25 ^circC to -5 ^circC. Identify the change of state that occurs and the temperature at which it occurs, assuming the melting point of water is 0 ^circC.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the process occurring. [1] (b) State the name of the process that occurs when the steam (gas) hits a cold window and turns back into liquid water. [1] [2 marks]
Key ideas — Everyday examples
🧑🏫 Teacher's explanation
Imagine a crowded dance floor. In a solid, everyone is standing in a strict grid, shoulder-to-shoulder, only vibrating slightly where they stand. This is why solids keep their shape. In a liquid, the dancers are still close together, but they are sliding past one another, allowing the substance to flow and take the shape of the container. In a gas, the dancers have broken free, sprinting around the room at high speeds, colliding with the walls; they take up as much space as possible.
📖 Definitions
🔍 Worked example — Easy
A block of ice at 0 °C is placed in a room. It undergoes melting. Describe the change in the arrangement of particles.
🔍 Worked example — Standard
A puddle of water on a sidewalk disappears on a sunny day. Identify the process and state whether it occurs at a fixed temperature.
🔍 Worked example — Exam-style
Compare boiling and evaporation in terms of where the change occurs in the liquid.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the arrangement and motion of the water particles when the water is boiling. [2] (b) State the name of the process that occurs when water vapor in the air hits a cold window and turns into liquid droplets. [1] [3 marks]
Worked example
Which state? A substance keeps the same volume when it is poured from a beaker into a flask, but it takes the shape of the flask. It cannot be compressed. Name its state and give the change of state if it is cooled until it becomes rigid.
🔍 Worked example — Easy
A substance has a fixed volume of 50 cm^3. When poured into a different shaped container, it takes the shape of that container. Identify the state of matter.
🔍 Worked example — Standard
A substance is currently in a liquid state. If the temperature is lowered significantly until the substance becomes a solid, name the process occurring.
🔍 Worked example — Exam-style
A gas is placed in a sealed, rigid container. The volume of the gas is 2.0 m^3. If the gas is cooled until it turns into a liquid, what happens to the volume of the substance?
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Any two: no fixed shape; no fixed volume (fills its container); can flow; easily compressed. (2) (a) evaporation; (b) condensation; (c) freezing (solidification). (3) Liquids can hardly be compressed: the water keeps its volume, so the plunger barely moves.