Physical and Chemical Changes

The world around us is full of changes — ice melts, paper tears, food cooks, and iron rusts. To make sense of these, scientists divide changes into two broad kinds: physical changes and chemical changes. A physical change is a change in which no new substance is formed, and only the physical properties of a substance — such as its shape, size, or state — are altered. The substance remains chemically the same material before and after the change.

A key feature of most physical changes is that they are reversible: the original substance can usually be recovered by a simple physical method. When water freezes into ice and the ice later melts, it is still water throughout; only its state has changed. When salt dissolves in water, the salt is not destroyed — it can be recovered by evaporating the water. In each case, no new substance appears, and the change can be undone.

Common examples of physical changes include the melting of ice, boiling and freezing of water, dissolving of sugar or salt in water, cutting paper or cloth, stretching a rubber band, breaking a piece of chalk, making a paper boat, and the magnetisation of an iron bar. In all of these, the material stays the same kind of substance — paper stays paper, water stays water — even though its shape, size, or state has changed.

Physical changes are very common in everyday life and in nature. The water cycle, in which water evaporates, forms clouds, and falls again as rain, is a giant set of physical changes. Recognising a physical change is straightforward: ask whether a new substance has been formed. If the answer is no, and only the appearance or state has changed, it is a physical change.

A chemical change is a change in which one or more new substances are formed. During a chemical change, the original substances react and rearrange to produce entirely different materials with new properties. Because new substances are created, a chemical change is also called a chemical reaction. Unlike most physical changes, chemical changes are usually irreversible — the original substances cannot easily be got back by simple physical means.

A chemical change is often accompanied by an exchange of energy, usually as heat, light, or sound. Some chemical changes release energy (for example, burning, which gives out heat and light), while others absorb energy. The formation of new substances with different properties, often along with a change in energy, is the hallmark of a chemical change.

There are many familiar examples of chemical changes. The burning of paper, wood, or a candle produces ash, smoke, carbon dioxide, and water vapour — new substances quite different from the original fuel. The rusting of iron produces a new reddish-brown substance, rust. The cooking of food, the souring of milk into curd, the digestion of food in our body, the ripening of fruit, and photosynthesis in plants are all chemical changes. The burning of fuels in engines and the setting of cement are chemical changes too.

Chemical changes are central to life and to industry. The food we eat is digested by chemical changes; the energy our bodies use comes from chemical reactions; plants make their food by the chemical change of photosynthesis; and countless useful materials — from medicines to plastics to fertilisers — are manufactured through chemical changes. Recognising a chemical change is simple in principle: if a new substance with new properties has been formed, and the change usually cannot be reversed, it is a chemical change.

Since the key feature of a chemical change is the formation of a new substance, we need clues that help us recognise when this has happened. These clues are called the signs (or characteristics) of a chemical change. Observing one or more of these signs strongly suggests that a chemical reaction has taken place and a new substance has formed. There are several such signs to look for.

A common sign is a change in colour. When a shiny iron object rusts, it turns reddish-brown; when a slice of apple is left exposed, it turns brown. Another sign is the evolution (release) of a gas: when baking soda is added to vinegar or lemon juice, bubbles of gas fizz out. A third sign is the formation of a precipitate, which is an insoluble solid that appears when two solutions are mixed — for example, mixing certain salt solutions can produce a cloudy solid.

Two more important signs involve energy. A chemical change is often marked by a change in temperature: some reactions give out heat and feel warm (such as burning or neutralisation), while others absorb heat and feel cold. There may also be a change in smell (such as food going bad) or even the giving out of light or sound (as when fuels burn or crackers explode). The formation of a new substance with new properties ties all these signs together.

It is important to remember that no single sign is an absolute proof on its own — for instance, boiling water releases bubbles of vapour without any chemical change. Scientists look at the overall evidence: if a new substance with new properties has clearly formed, along with signs such as a colour change, gas evolution, a precipitate, a temperature change, or a change in smell, the change is chemical. These signs make it possible to identify chemical changes happening all around us.

One of the most important and costly chemical changes in everyday life is the rusting of iron. When iron or an iron object is left exposed to the surroundings for some time, a reddish-brown, flaky layer forms on its surface. This layer is called rust, and the process of its formation is called rusting. Rust is a new substance (chemically, hydrated iron oxide) that is quite different from the shiny iron beneath it — so rusting is a chemical change.

For rusting to occur, two conditions must both be present: water (moisture) and oxygen (air). Iron does not rust in dry air alone, nor in water that has no dissolved air. This can be shown by a simple experiment with three test tubes containing iron nails: one with air only (kept dry), one with boiled water (oxygen removed) sealed from air, and one with ordinary water exposed to air. Only the nail exposed to both water and air rusts. Rusting happens faster in humid (damp) conditions and near the sea, where salty, moist air speeds up the process.

Rusting is a serious problem because it weakens iron and steel, slowly eating away bridges, railings, machinery, vehicles, ships, and tools, causing huge losses every year. The good news is that rusting can be prevented by keeping away water, oxygen, or both from the iron surface. Several methods are used: painting the surface, applying grease or oil, coating with another metal that does not rust easily — such as a layer of zinc (a process called galvanisation) or chromium (chrome plating) — and mixing iron with other metals to make rust-resistant alloys such as stainless steel.

Understanding rusting and how to prevent it is very practical. Choosing the right protection method depends on the use of the object: painting is suitable for gates and railings, oiling for tools and machine parts, galvanising for pipes and sheets, and stainless steel for cutlery and surgical instruments. Preventing rust saves money, keeps structures safe, and makes iron objects last much longer.

Crystallisation is a special physical change used to obtain pure, large, well-shaped crystals of a substance from its solution. A crystal is a solid whose particles are arranged in a regular, repeating pattern, giving it a definite geometric shape and flat faces — for example, the cubic crystals of common salt or the colourless crystals of sugar. Crystallisation is one of the best methods for purifying solid substances, because the neat crystals that form leave impurities behind in the liquid.

The process works on a simple idea: a hot solution can dissolve more of a substance than a cold one. To crystallise a substance such as alum, copper sulphate, or sugar, we first prepare a hot saturated solution — a solution that has dissolved as much of the substance as it can at a high temperature. This hot solution is then allowed to cool slowly. As it cools, it can no longer hold so much dissolved substance, and the extra substance comes out of the solution in the form of crystals. Slow cooling is important, because it gives time for large, well-formed crystals to grow; fast cooling produces only tiny crystals.

Crystallisation is a physical change because the substance that crystallises is chemically the same before and after — only its physical form (from dissolved to solid crystal) has changed, and the substance could be redissolved. The crystals can be separated from the remaining liquid (the mother liquor) by filtering, and then dried. The result is a sample of the substance that is purer than the original, since most impurities stay dissolved in the liquid.

Crystallisation has many practical uses. It is used to purify substances in laboratories and industries, to obtain common salt from seawater by evaporation, to manufacture sugar crystals from sugarcane juice, and to grow the beautiful crystals of alum and copper sulphate seen in school experiments. Because it gives pure, attractive crystals, crystallisation is a favourite method wherever a clean, solid form of a substance is needed.