Chapter 5: Changes Around Us: Physical and Chemical

A change is any difference we can notice in a material: its shape, size, colour, state, smell, temperature or the way it behaves. To study a change we ask two questions: what exactly changed? and is a new substance formed? If the material only looks different but is still the same substance, the change is a physical change. If a new substance with different properties is formed, the change is a chemical change. Careful observation, and sometimes a small test, is needed to decide which kind it is.

A physical change is a change in which no new substance is formed. Only the shape, size, state or appearance changes. Melting, freezing, boiling, evaporation, condensation, dissolving, cutting, bending, tearing and crushing are all physical changes. Many physical changes can be reversed by simple means: ice melts to water and water freezes back to ice, and sugar dissolved in water can be recovered by evaporating the water. Some physical changes cannot be undone easily, such as cutting a rope into small pieces, but the pieces are still rope.

A chemical change is a change in which one or more new substances are formed with properties different from the original materials. Chemical changes are usually permanent. Common signs are a change of colour, release of a gas (bubbles), a new smell, production of heat or light, a sound, or formation of a solid. Cooking food, burning fuel, rusting, rotting and the souring of milk are chemical changes. A harmless kitchen example is baking soda and vinegar: they fizz as a new gas is formed.

Rusting is the slow chemical change in which iron slowly reacts with oxygen and water (moisture) in the air to form a reddish-brown substance called rust. Rust is not iron; it is a new substance. It is flaky and weak, so it falls off and exposes fresh iron below, which then also rusts. Rusting is faster in damp places and near the sea, where salty air speeds it up. Rusting weakens iron objects such as railings, tools and bridges, and costs a great deal to repair. We can prevent it by keeping iron away from air and water: painting, oiling or greasing, coating with another metal such as zinc (galvanisation), or making stainless steel by mixing iron with other materials.

Combustion is the chemical change in which a substance reacts quickly with oxygen and gives out heat and usually light. The substance that burns is called a fuel: wood, coal, cooking gas, petrol and candle wax are examples. Burning needs three things at the same time: a fuel, oxygen (air) and enough heat to reach the ignition temperature. This is the fire triangle. Removing any one of the three puts out a fire. Burning a carbon fuel makes carbon dioxide and water vapour, and if the air supply is poor it also makes black soot and carbon monoxide, a poisonous gas. Fire is useful but dangerous, so we must use it carefully.

Many everyday processes contain both physical and chemical changes. To decide, we look at each step separately and ask whether a new substance is formed. In a burning candle, melting of wax is physical (the wax stays wax) but burning of the wax vapour is chemical (new gases and heat are produced). When sugar is heated, melting is physical and turning brown and smelling of caramel is chemical. In frying an egg, heating is the cause, and the white turning solid is a chemical change. Judging step by step avoids mistakes.

A change that can be undone to get back the original material is a reversible change. Examples are melting and freezing, dissolving and recovering, and bending a wire. A change that cannot be undone is a permanent (irreversible) change, such as baking a cake, burning paper or rotting fruit. Most physical changes are reversible and most chemical changes are permanent, but there are exceptions: crushing a chalk is physical but cannot be undone easily. Changes can also be desirable, which help us (cooking, ripening of fruit, making curd), or undesirable, which harm us (rusting, spoiling of food, forest fires). We try to slow undesirable changes by cooling, covering, painting and proper storage.

Weathering is the slow breaking down of rocks into smaller pieces by sun and cold, rain, wind, ice and plant roots. Day heat and night cold make rocks expand and contract until they crack. Water freezes in cracks, expands and splits the rock. Roots grow into cracks and push them wider. Slightly acidic rain can slowly dissolve rocks such as limestone and marble. Erosion is the carrying away of broken rock and soil by running water, wind or ice. Weathering and erosion slowly make soil, shape valleys, build river deltas and wear down mountains. They can also harm farmland and monuments. Planting trees and grass holds soil in place and reduces erosion.