Indicators of a chemical reaction
A chemical reaction is when one or more substances change and produce one or more new chemical substances.
The substances that are involved in a chemical reaction and that are changed by it are called reactants.
The substances that are produced by a chemical reaction are called products.
Chemical reactions happen all around us (and inside us!) all the time. We might not always notice them but there are four indicators that show a chemical reaction has taken place:
- colour change
- effervescence
- precipitation
- temperature change
Colour change
A colour change might take place when two substances react. It can also happen when a compound is broken down by heating it. This is called thermal decomposition.
Watch this video to see how the colour of copper carbonate changes when it is heated.
Thermal decomposition of copper carbonate
Copper carbonate is green and copper oxide is black. You can see a colour change from green to black during the reaction. The carbon dioxide produced can be detected using limewater, which turns cloudy.
Effervescence
Some chemical reactions result in a product that is in a different state to the reactants.
An example of this is effervescence in which a reaction in a liquid produces bubbles of gas. Effervescence is an indicator of a chemical reaction taking place.
Watch this video to see how magnesium and dilute hydrochloric acid react to produce bubbles of hydrogen gas.
The reaction of magnesium and hydrochloric acid produces effervescence
Precipitation
Some liquids react together to produce an insoluble solid. This is called a precipitation reaction and the solid formed is called a precipitate.
Precipitation reaction of lead nitrate and potassium iodide
Temperature change
Different substances hold different amounts of energy.
Chemical reactions can take in energy or release energy, often in the form of heat. This causes a change in temperature.
Exothermic reactions
In an exothermic reaction the reactants hold more energy than the products, so exothermic reactions release energy in the form of heat.
Exothermic reactions can be identified by an increase in temperature.
Watch this video to see how an increase in temperature can be measured when magnesium reacts with hydrochloric acid.
Exothermic reaction between magnesium and hydrochloric acid
Endothermic reactions
In an endothermic reaction, the reactants have less energy than the products, so more energy is needed for the reaction to take place.
Endothermic actions take in heat. So endothermic reactions can be identified by a decrease in temperature.
Watch this video to see how a decrease in temperature can be measured when barium hydroxide and ammonium chloride react.
Endothermic reaction between barium hydroxide and ammonium chloride