A reaction may happen when two or more chemicals are mixed. Sometimes you have to add heat (energy) before the reaction will occur. Not all chemicals react together when they are mixed. We can tell a reaction has taken place if we see bubbles of gas, a change in colour, a change in temperature or a precipitate (a layer of solid in the bottom of the test tube that wasn’t there when you started). So vinegar (an acid) reacts with sodium bicarbonate – there are bubbles of gas and the test tube gets warm – but it doesn’t react with table salt (sodium chloride).