• Look at two experiments that both show bubbles.
• In the first, dilute hydrochloric acid is poured onto a piece of marble (calcium carbonate).
• In the second, the same acid is poured onto a piece of zinc.
• Both show effervescence, yet they are not the same reaction.
• The gas from the marble is carbon dioxide; the gas from the zinc is hydrogen.
• In one case the acid reacts with a carbonate (a salt); in the other it reacts with a metal.
• The key question of this lesson is how can we sort the huge number of reactions into a few families so that we can predict their products?

• You already know the basic definition.
• A chemical reaction is a process in which the bonds in the molecules of the reactants are broken and new bonds form, giving products that are new substances with different properties.
• How do we know a reaction has happened? We look for one or more signs:

• a change in colour (for example blue copper sulphate solution fading);

a change in odour;

evolution of a gas (bubbles);

• formation of a precipitate (an insoluble solid that appears in a solution);
• release of heat and light (burning).

• Scientists classify reactions according to how the atoms or ions rearrange and re-bond.
• This classification is not just filing; it lets us predict the products of reactions we have never seen.
• The four main types in this lesson are:

1. Direct combination reactions
2. Decomposition reactions
3. Single replacement reactions
4. Double replacement reactions (which include neutralisation and precipitation)

• Later in the lesson we meet a different way of looking at reactions, oxidation–reduction (redox), which describes what happens to electrons.
• Combustion, studied in Lesson 2, is another important type.