Five Things That Are Evidence of Chemical Reactions

Precipitate

• A precipitate is a solid which is formed when two liquids are combined and then react. It is a different substance to either of the reactants and does not dissolve in the reaction mixture. It may either sink to the bottom or remain suspended within the solution. The precipitate can generally be identified by looking at solubility rules: Certain groups of compounds are soluble in water, whereas others aren't.

Color

• Color changes are commonplace when substances are combined, but one of the identifying factors of a chemical reaction is an unexpected color change. For example, it would be expected that a blue and a yellow liquid would combine to produce a green liquid. However, if the same two liquids created a red product, that would be evidence of a chemical reaction. This can happen between two liquids and also when a solid dissolves into a liquid.

Gas

• A gas can be formed when two liquids are combined, or when a solid is added to a liquid. Sometimes this gas has an odor and other times it doesn't. It is generally visible as bubbles rising to the surface of the reaction mixture, but the gas can also be soluble. If this is the case it is generally very difficult to see its formation in the mixture.

Temperature

• It is very common for the substances involved in a chemical reaction to produce a change in temperature. This does not come from an external heat source, but instead is generated by an energy change within the reaction. The chemical change can cause either an increase or a decrease in the temperature of the reaction mixture.

Reversal

• With a chemical reaction, the outcome is very difficult or impossible to reverse. This is one of the easiest ways to tell the difference between a reaction which is purely physical and one in which chemical changes have taken place. For example, water can undergo several physical changes. It can be ice, water or steam and to move from one state to another it is frozen, melted or boiled. These are all easily reversible. In a chemical reaction there is a change to the chemical composition of the substances which is not so easy to reverse.