States of Matter: Solids, Liquids & Gases — Complete PSLE Guide

All matter around us exists in one of three states: solid, liquid, or gas. In Singapore Primary Science, you need to be able to describe the properties of each state, explain how matter changes from one state to another, and apply this knowledge to everyday situations. This topic is tested from P3 all the way through to PSLE, with increasing depth at each level.

Properties of Solids, Liquids, and Gases

Each state of matter has distinct properties related to its shape, volume, and ability to flow. These must be memorised precisely — PSLE questions often ask you to state or compare specific properties.

Changes of State

Matter can change from one state to another when heated or cooled. For PSLE, you need to know the name of each change and what causes it.

Evaporation vs Boiling

Both evaporation and boiling change liquid into gas, but they are different. Evaporation happens at the surface of a liquid and can occur at any temperature — it happens slowly over time. Boiling happens throughout the entire liquid and only occurs at the boiling point (100°C for water). For PSLE, always distinguish between the two when writing answers.

Factors That Affect the Rate of Evaporation

Evaporation is a key concept in Primary Science because it connects to the water cycle and everyday life. Several factors can speed up or slow down evaporation:

• Temperature — higher temperature speeds up evaporation (more heat energy)
• Surface area — larger surface area speeds up evaporation (more liquid exposed to air)
• Wind (air movement) — wind removes water vapour from above the surface, speeding up evaporation
• Humidity — dry air speeds up evaporation; humid air slows it down

Condensation in Daily Life

Condensation occurs when water vapour in the air cools and changes into liquid water. You see this in many everyday situations in Singapore:

• Water droplets forming on the outside of a cold drink can on a humid day
• "Fog" or "steam" rising from hot food — water vapour condenses in cooler air
• Bathroom mirrors fogging up after a hot shower
• Water droplets on grass in the early morning (dew)

Water and Its Three States

Water is the perfect example for states of matter because it commonly exists as all three: ice (solid), water (liquid), and water vapour (gas). Understanding how water moves between these states is the basis of the water cycle, which is another major PSLE topic.

Water freezes at 0°C and boils at 100°C. Between these temperatures, it exists as liquid. Below 0°C, it is a solid (ice). Above 100°C (or given enough time at lower temperatures through evaporation), it becomes water vapour (gas).

Model Exam Answers

Question: State two properties of a gas that are different from a liquid.

Model Answer: (1) A gas has no fixed volume — it expands to fill its container completely, whereas a liquid has a fixed volume. (2) A gas can be compressed easily, whereas a liquid is very difficult to compress.

Question: Priya noticed water droplets forming on the outside of a cold glass of water. Explain why this happened.

Model Answer: The air around the cold glass contains water vapour. When this water vapour comes into contact with the cold surface of the glass, it is cooled below its condensation point and changes state from gas to liquid. This process is called condensation, and the water droplets that form on the outside of the glass are liquid water.