My World QuestColor & Learn
Pioneers Science

Science · Chemistry basics

Chemical reactions

What happens when substances react.

Not started

What is a chemical reaction?

A chemical reaction happens when substances change into completely new substances. The starting materials are called reactants. The new substances made are called products. Chemical reactions are different from just mixing things together. In a mixture, you can still see the original substances and get them back easily. In a chemical reaction, the original substances are gone forever and you cannot easily reverse the change.

Signs a chemical reaction has happened

You can spot a chemical reaction by looking for clues. A colour change often means new substances have formed. Bubbles or fizzing show a gas is being produced. The mixture might get hotter or colder as energy is released or absorbed. A smell might appear or disappear. Sometimes a solid called a precipitate forms in a liquid. One or more of these signs usually means a chemical reaction has occurred.

Reactants become products

During a chemical reaction, atoms in the reactants rearrange to make products. The atoms themselves do not disappear or appear from nowhere. They just join together in different ways to make new substances with different properties. This is why the total mass stays the same before and after a reaction. We can show this with a word equation: reactants arrow products.

Everyday chemical reactions

Chemical reactions happen all around us every day. Burning wood or fuel is a reaction that releases heat and light. Cooking an egg changes its proteins permanently through chemical reactions. Rusting is a slow reaction between iron and oxygen that makes a new substance. Baking a cake involves reactions that produce carbon dioxide gas to make it rise. Your body uses chemical reactions to digest food and release energy.

Worked examples

  1. 1. Vinegar is added to baking soda in a beaker. Bubbles form and the mixture feels cold. Has a chemical reaction happened?

    1. Look for signs of a chemical reaction happening.
    2. Bubbles forming means a gas is being produced, which is a new substance.
    3. The temperature change shows energy is being absorbed during the reaction.
    4. Yes, a chemical reaction has happened because new substances formed.

  2. 2. Write a word equation for burning methane gas in air to produce carbon dioxide and water.

    1. Identify the reactants: methane and oxygen from the air.
    2. Identify the products: carbon dioxide and water.
    3. Write reactants on the left, products on the right, with an arrow between.
    4. Word equation: methane + oxygen → carbon dioxide + water

  3. 3. Sugar is dissolved in water. The sugar disappears but the water tastes sweet. Is this a chemical reaction?

    1. Check if new substances have formed with different properties.
    2. The sugar has not changed into a new substance, just broken into tiny pieces.
    3. You can get the sugar back by evaporating the water.
    4. No, this is not a chemical reaction, just a physical change called dissolving.

Try it yourself

A student heats a piece of magnesium metal with a Bunsen burner. It burns with a bright white flame and turns into a white powder. The powder has different properties from the original metal. Has a chemical reaction occurred? Explain your answer.

Ready to test yourself?

Take a short topic test — every question maps to a syllabus point so you’ll see exactly which parts you’ve mastered.