Chemical changes (or chemical reactions) are those that transform some chemical/s into new other chemical/s.
They normally occur when different substances, prone to react between them, are put into contact. And so, when iron (Fe) and oxygen (O2) are put together, they will combine and disappear to form a new chemical: rust (Fe2O3). But the wine and the nitrogen (kept in the upper part of the bottles of wine) will never react, allowing the wine to preserve its properties over time. Not all chemicals will react when put into contact.
The initial molecules that react in a chemical reaction are called reactants, and the final resulting molecules are called products. In the previous reaction, the oxygen and the iron are the reactants, whereas the rust is the only product.
Some chemical changes imply a loss of energy, i.e., they are exothermic, because they release energy (usually heat) to the environment. Some other chemical changes are endothermic, they absorb energy from the environment, and the products will have a greater amount of energy than the reactants.
Nevertheless, most chemical reactions, either exothermic or endothermic, require an initial contribution of external energy to take place. If you want a piece of paper to combine with the atmospheric oxygen and go into combustion, you need to heat up the paper (what you usually do with a flame). But during the reaction, the paper releases a lot of energy (heat, light and even sound), so the reaction is exothermic: the paper and the oxygen had more energy than the ashes, the smoke, the CO2 and the water vapour produced.
Quite often, chemical changes can be noticed by some conspicuous events, such as…
- A change in the temperature, as in any combustion;
- A change in the colour, as in the rusting of iron;
- The formation of bubbles, as when the calcite (CaCO3) reacts with hydrochloric acid (HCl);
- A change in the volume, as in baking bread;
Most chemical changes are irreversible: they can't be undone (e.g. a combustion), whereas most physical changes are reversible (e.g. the condensation of water).