Last reviewed: 10 November 2026
Updated November 2026. These Grade 8 physical and chemical changes notes cover the kinetic theory of matter, heating curves, the effect of impurities on melting and boiling points, and how to tell physical changes from chemical changes. In a physical change no new substance forms and the change is usually easy to reverse, such as ice melting. In a chemical change a new substance forms and the change is usually hard to reverse, such as paper burning.
The questions are original practice for Kenyan junior school learners, not an official paper. For other Grade 8 science topics, see the Grade 8 Integrated Science revision guide and the Grade 8 notes hub.
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What the Grade 8 Curriculum Covers
Physical and chemical changes is a sub-strand under Mixtures, Elements and Compounds in the Kenya Institute of Curriculum Development’s Grade 8 Integrated Science design. It covers the kinetic theory of matter, heating curves, effects of impurities on boiling and melting points, physical and chemical changes (temporary and permanent), and their applications. The design is listed on the KICD Grade Eight curriculum designs page.
Kinetic Theory of Matter
All matter is made of tiny particles that are always moving. How they are arranged and how fast they move explains the three states:
| State | Arrangement | Movement | Properties |
|---|---|---|---|
| Solid | Closely packed in a regular pattern | Vibrate about fixed positions | Fixed shape and volume; not easily compressed |
| Liquid | Close together but irregular | Move past each other | Fixed volume; takes the container’s shape; flows |
| Gas | Far apart | Move fast in all directions | No fixed shape or volume; easily compressed |
Diffusion is evidence for moving particles: a drop of ink spreads through still water, and the smell of cooking spreads through a house.
The Heating Curve
If you heat ice steadily and record its temperature every minute, the graph has rising parts and flat parts:
- Ice warms up to 0 °C (temperature rises).
- Ice melts at 0 °C (flat: heat is used to separate particles, not to raise temperature).
- Water warms from 0 °C to 100 °C (rises).
- Water boils at 100 °C at sea level (flat again).
- Steam warms above 100 °C if heating continues in a closed system.
The flat parts are the melting point and boiling point. OpenStax Chemistry’s section on phase transitions explains why temperature stays constant during a change of state. Water boils below 100 °C at high altitude, such as in Nairobi, because air pressure is lower.
Effect of Impurities
- A pure substance melts and boils at fixed temperatures (sharp).
- Impurities lower the melting point and make melting happen over a range. Salt spread on icy roads in cold countries uses this effect.
- Impurities raise the boiling point. Salty water boils above 100 °C at sea level.
So measuring melting or boiling points is a simple test of purity.
Physical and Chemical Changes
The difference between physical and chemical properties and changes is set out in OpenStax Chemistry’s section on physical and chemical properties.
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance formed? | No | Yes |
| Reversible? | Usually easily (temporary) | Usually not easily (permanent) |
| Mass change? | No change in total mass | Total mass is conserved, though gases may escape in open containers |
| Energy | Small heat changes | Often large heat or light given out or taken in |
| Examples | Melting butter, dissolving sugar, boiling water, crushing chalk | Burning charcoal, rusting, cooking an egg, souring milk |
Class activity
Heat a small amount of candle wax in a test tube: it melts and solidifies again on cooling (physical). Burn a piece of paper in a metal dish under supervision: ash and smoke form that cannot become paper again (chemical).
Applications
Refrigerators and ice-cream vendors use freezing and melting; jua kali artisans melt metals to cast them; cooking, baking and fermentation are chemical changes; distillation of water uses boiling and condensation.
Original Practice Questions With Worked Answers
- Why is a gas easy to compress? Its particles are far apart with lots of space between them.
- Why is the heating curve flat while ice melts? Heat is used to break the forces holding particles in the solid, not to raise temperature.
- What happens to the boiling point of water when salt is added? It rises.
- Is dissolving sugar in tea a physical or chemical change? Physical; the sugar can be recovered by evaporating the water.
- Give two signs that a chemical change has happened. Any two of: new substance, gas given off, colour change, heat or light produced.
Cover the answers and attempt each question first. For timed practice, use the Grade 8 revision papers or the Grade 8 CBC Challenge. These notes are free; full revision access is available with the KSh 20 Day Pass or the KSh 500 Monthly Pass.
Frequently Asked Questions
What is the difference between a physical and a chemical change?
A physical change forms no new substance and is usually reversible. A chemical change forms a new substance and is usually not easily reversible.
What does the kinetic theory of matter say?
Matter is made of particles that are always moving; their arrangement and speed explain the properties of solids, liquids and gases.
How do impurities affect melting and boiling points?
They lower the melting point and raise the boiling point.
Why does water boil below 100 °C in Nairobi?
At higher altitude the air pressure is lower, so water boils at a lower temperature.