HOW TO IDENTIFY A CHEMICAL CHANGE
We can tell if a chemical reaction has taken place when one or more of the following things happen:
There has been a colour change in the reactants. When two reactants, usually liquids touch, the colour may change, meaning that a chemical reaction has occurred.
The reaction has produced a flame. Some times, the contact of two substances can cause a fire to appear. This also may be due to the air around the reactants.
A solid appears. We realise that when we can see a sludgy or cloudy layer of crystals forming, a solid substance has formed. Seeing some sort of solid that looks different than the reactants usually means a chemical reaction happened.
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Normally when we can see bubbles we know that gas has formed. When bubbles form, a temperature change could also be taking place. Temperature change and formation of bubbles often occur together, which is a sign of a chemical reaction.
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Each of the above signs is visible. Our other senses such as smell, touch and sound can also allow us to tell whether a chemical reaction occurred or not:
Sometimes chemical changes can be smelled, for example when a new material, that has a strong smell, is formed. Other chemical changes can be felt, like when the reaction creates heat. Some chemical changes can be heard, e.g. when an explosion takes place.