Chemical reactions

 

What are chemical reactions?

A reaction is an event that occurs as a result of something else. In science, a reaction is a process where chemicals change into different chemicals. This happens when chemicals break their bonds and form new bonds. Chemical reactions are very common and essential for life. Some examples of chemical reactions you may know are fire burning wood, cake baking and glass making.

Chemical reactions are constantly happening in the core of the Earth, in the air and deep in the oceans. Chemical reactions also happen all the time in living things, which help them to survive. Cells change waste products into other chemicals and split molecules into other chemicals.

Examples

These are some simple examples of chemical reactions. In these reactions, the chemicals on the left side of the equation are the reactants and the chemicals on the right side of the equation are the products.

Carbon + oxygen ––> Carbon dioxide
Iron + oxygen ––> Iron oxide
Hydrogen + oxygen ––> Water

 

How can we tell it is a chemical reaction?

Some signs of a chemical reaction are:

 

• Gas bubbles – Many chemical reactions in the science lab produce a gas, so you can see bubbles forming.
• A different colour – If the new chemicals have a different colour than the original chemicals, there will be a change in colour.
• A lot of energy change – Many chemical reactions release a lot of energy, like burning, and some take in energy, so they feel cold.

 

There is a difference between physical processes, where chemicals change state, such as when water freezes to become ice. This is not a chemical reaction, but a physical change, because the substance changes its physical properties but not its chemical composition.

Example:

The bubbles in the water inside a kettle are not a sign of a chemical reaction, but a sign of boiling. Boiling water makes steam bubbles. These bubbles do not mean a new substance has been made because, when they cool down, the steam turns back into liquid water. Boiling is a physical change, not a chemical reaction.

 

🧪 Knowledge Check: Chemical Reactions

Use the information from the webpage to test your knowledge of chemical changes and equations.

1. Which of the following best defines a chemical reaction?

• A change that is very easy to reverse.
• A process where new substances are formed with different properties.
• A change in state, such as ice melting into water.

2. In a word equation, where are the 'reactants' always placed?

• On the right-hand side of the arrow.
• Underneath the arrow.
• On the left-hand side of the arrow.

3. Which of these is one of the five main signs that a chemical reaction has occurred?

• A change in colour.
• A substance dissolving in water.
• A substance changing shape.

4. If you see fizzing or bubbles during an experiment, which sign of a chemical reaction are you observing?

• Formation of a precipitate.
• Production of a gas.
• Production of light.

5. What does the arrow represent in a chemical equation?

• The direction of the reaction ("reacts to form").
• An equals sign showing the weight is the same.
• The speed at which the chemicals are mixed.

Click to Reveal Answers

1. A process where new substances are formed (Chemical reactions result in new products with different properties).
2. The left-hand side (Reactants are the starting materials that react together).
3. A change in colour (This is one of the five specific observations used to identify a chemical change).
4. Production of a gas (Fizzing indicates that a gas is being created as a new substance).
5. The direction of the reaction (It shows the transformation from reactants into products).

 

Summary:

• New chemicals are formed by chemical reactions.
• The number of atoms stays the same, but they are rearranged in different ways during a chemical reaction.
• A large change in temperature, the formation of bubbles, or a change in colour are signs of chemical reactions.
• Chemical reactions can be shown using equations.