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Physical and chemical changes is taught in the note for Matter. Read it below and look for the section on this topic.

Matter

10 min read

Matter and its states

Matter is anything that has mass and takes up space. All matter is made of tiny moving particles. The way the particles are arranged and how strongly they attract each other decide whether a substance is a solid, a liquid or a gas.

Particles in a solid, a liquid and a gasThree boxes of particles. In a solid they are packed in a regular pattern and only vibrate. In a liquid they touch but slide past each other. In a gas they are far apart and move quickly in all directions.Solidfixed pattern, vibrateLiquidclose, slide past each otherGasfar apart, move fast
SolidLiquidGas
ArrangementPacked in a regular patternClose together, no patternFar apart
MovementVibrate on the spotSlide past each otherMove quickly and freely
ShapeFixedTakes the shape of the containerFills the whole container
VolumeFixedFixedChanges easily (can be squashed)

Changes of state

A change of state is a physical change. Heat is added or taken away, but no new substance is made. At the melting point a solid becomes a liquid, and at the boiling point a liquid becomes a gas. While a substance is changing state, its temperature stays the same: the heat energy is used to separate the particles, not to make them move faster.

Changes of stateThree boxes for solid, liquid and gas. Melting takes a solid to a liquid and freezing reverses it. Boiling or evaporation takes a liquid to a gas and condensation reverses it. Sublimation takes a solid straight to a gas and the reverse is called deposition.SolidLiquidGasmeltingfreezingboilingcondensingsublimationdeposition
Heating curve of waterA graph of temperature against time as ice is heated. The line rises while ice warms, stays flat at 0 degrees while it melts, rises again as liquid water warms, stays flat at 100 degrees while it boils, then rises as steam warms.time (heat added)temperature (°C)0100ice warmsmeltingwater warmsboilingsteamtemperature stays at 0 °Cstays at 100 °C

Physical and chemical changes

Physical changeChemical change
New substance made?NoYes
Easy to reverse?Usually yesUsually no
Energy changeSmallOften large (heat, light, sound)
ExamplesMelting ice, dissolving sugar, breaking glassBurning wood, rusting iron, cooking an egg, souring milk
Worked exampleIs it physical or chemical?

Say whether (a) frying an egg and (b) evaporating salt water to get salt back are physical or chemical changes.

  1. (a) The egg white turns solid and white and cannot go back: a new substance is made.
  2. (b) The water becomes steam and the salt stays. Condensing the steam gives the water back: nothing new is made.

Answer: (a) is a chemical change. (b) is a physical change.

Try itHeat needed to warm water

Choose a mass of water and how much hotter you want it. The heat needed is Q = m × 4.2 × ΔT, in kilojoules. A 1 kW heater gives 1 kJ every second.

Check yourself At the melting point of ice, what happens to the temperature while the ice melts?
Check yourself Which is a chemical change?
Check yourself Which state of matter has particles that move freely and fill the container?

Key terms

Physical change
A change in which no new substance is made.
Chemical change
A change in which one or more new substances are made.
Melting point
The temperature at which a solid becomes a liquid.
Boiling point
The temperature at which a liquid becomes a gas.
Sublimation
A solid changing straight to a gas.

Summary

  • Matter is made of moving particles; their arrangement decides the state.
  • Melting, freezing, boiling, condensing, sublimation and deposition are physical changes of state.
  • The temperature stays constant while a pure substance changes state.
  • A chemical change makes a new substance and is hard to reverse.
Changes of state