Chemical Changes and Reactions

In a physical change no new substance is made and the change can usually be reversed: ice melting to water, sugar dissolving, a wire being bent. The substance is the same before and after, only its state or shape differs. In a chemical change one or more new substances are formed, the change is usually hard to reverse, and it is often accompanied by a change in colour, the release of a gas, the formation of a precipitate, or an energy change. Iron rusting, paper burning and milk turning sour are chemical changes because the products are genuinely different substances from what we started with.

Substances do not always react just because they are placed together; a reaction usually needs the right condition. Bringing reactants into close contact helps (sodium reacts the instant it touches water). Heat starts many reactions, as when iron and sulphur are heated to form iron sulphide. Light drives others, such as hydrogen reacting explosively with chlorine in sunlight, or photosynthesis. Electricity decomposes water into hydrogen and oxygen (electrolysis). Pressure can trigger a reaction, as when a mixture explodes on being struck. A solution often lets ions meet and react, and a catalyst speeds a reaction up without being used up itself.

Most school reactions fall into four families. In a combination (synthesis) reaction two or more substances join to form a single product, as when hydrogen and oxygen combine to form water. In a decomposition reaction a single compound breaks into two or more simpler substances, as when heated calcium carbonate gives calcium oxide and carbon dioxide. In a displacement reaction a more reactive element pushes a less reactive one out of its compound, as when iron displaces copper from copper sulphate solution. In a double displacement reaction two compounds in solution swap partners, often forming a precipitate, as when silver nitrate and sodium chloride give silver chloride.

Every chemical reaction involves energy. In an exothermic reaction energy is released to the surroundings, so the surroundings get warmer: burning fuels, respiration, and adding water to quicklime all give out heat. In an endothermic reaction energy is absorbed from the surroundings, so they cool down: dissolving ammonium chloride in water makes the beaker feel cold, and photosynthesis stores absorbed light energy. Recognising whether heat is given out or taken in tells us about the energy stored in the bonds of the reactants compared with the products.