A chemical reaction happens when one or more chemicals are changed into one or more other chemicals. Examples:
- iron and oxygen combining to make rust
- vinegar and baking soda combining to make sodium acetate, carbon dioxide and water
- things burning or exploding
- many reactions that happen inside living things
- electrochemical reactions when discharging or recharging batteries
Some reactions are fast, and others are slow. Some happen at different speeds, depending on temperature or other things. For example, wood does not react with air when it is cold, but if it is made hot enough, it will start to burn. Some reactions give out energy. These are exothermic reactions. In other reactions, energy is taken in. These are endothermic reactions.
Look at the images below. What’s happening in each type? For instance, in the first one, two separate chemicals join to make one chemical.
Four basic types
If you can’t tell someone what an endothermic and exothermic reaction is, watch this video.
In chemistry, concentration is how much of a substance is mixed with another substance.
These glasses containing red dye show changes in concentration. The solutions on the left are more diluted, and the solutions on the right are more concentrated.
The substance that is dissolved (shown as the red dye in the example) is known as the solute. The substance in which the solute is dissolved (shown as the water in the example) is known as the solvent.
To concentrate a solution, one must add more solute, or reduce the amount of solvent. To dilute a solution, one must add more solvent, or reduce the amount of solute.