Learning Objectives
- Differentiate between physical and chemical changes
- Identify examples of each type of change
- Understand rusting and crystallisation
- Know how to prevent rusting
Key Concepts
Physical Changes
Physical change: A change in which no new substance is formed. The change is usually reversible.
Examples: Melting of ice, dissolving sugar in water, tearing paper, breaking glass, evaporation of water, folding paper.
Properties: Generally reversible, no new substance formed, no change in chemical composition.
Chemical Changes
Chemical change: A change in which one or more new substances are formed. Usually irreversible.
Examples: Burning of paper, rusting of iron, cooking food, curdling of milk, digestion of food, ripening of fruit.
Indicators of chemical change: Change in colour, evolution of gas, change in temperature, formation of precipitate, change in smell.
Rusting of Iron
Rusting: Iron reacts with oxygen and moisture to form iron oxide (rust): Iron + Oxygen + Water → Iron oxide (Rust)
Rust is reddish-brown and flaky. It weakens iron objects over time.
Prevention of rusting: Painting, oiling, greasing, galvanisation (coating with zinc), chrome plating, making alloys (stainless steel).
Crystallisation
Crystallisation: Process of forming crystals from a saturated solution on cooling. Example: Making crystals of copper sulphate from its solution. It is a physical change.
Galvanisation
Galvanisation: Process of depositing a layer of zinc on iron to prevent rusting. Zinc does not react with oxygen and moisture like iron does.
Summary
Physical changes are reversible and no new substance is formed. Chemical changes are usually irreversible and new substances are formed. Rusting is a chemical change that can be prevented by painting, oiling, or galvanisation. Crystallisation is a physical change used to obtain pure crystals.
Important Terms
- Physical change: Reversible change, no new substance
- Chemical change: Usually irreversible, new substance formed
- Rusting: Iron + Oxygen + Water → Iron oxide
- Galvanisation: Coating iron with zinc
- Crystallisation: Forming crystals from solution
Quick Revision
- Physical = Reversible, no new substance (melting, dissolving)
- Chemical = Irreversible, new substance (burning, rusting)
- Rusting needs both oxygen and moisture
- Prevent rusting: paint, oil, galvanise, alloy
- Signs of chemical change: colour, gas, temperature, precipitate, smell