Chemical Reactions and Conservation of Mass

A chemical reaction is a process where substances change into new substances with different properties. Wood burning, iron rusting, and baking a cake all involve chemical reactions. A key rule governs all of them — the law of conservation of mass — and it shows up on the test again and again.

What Happens in a Reaction

In a chemical reaction, the starting substances are called reactants, and the new substances formed are called products. During the reaction, atoms are rearranged into new combinations, but no atoms are created or destroyed. Signs that a chemical reaction has occurred include a color change, a gas forming (bubbles), heat or light being released, or a solid forming from liquids. If you see a brand-new substance with new properties, a chemical change has happened.

The Law of Conservation of Mass

The law of conservation of mass says that matter is neither created nor destroyed in a chemical reaction. That means the total mass of the reactants equals the total mass of the products. If 10 grams of reactants go in, 10 grams of products come out — the atoms are just rearranged, not lost.

This explains a puzzle students often ask about. When wood burns and seems to lose mass, the “missing” mass did not vanish — it left as gases (like carbon dioxide and water vapor) that floated away. If you could capture those gases, the total mass would still be the same as before.

Using the Law on the Test

The most common test use is simple subtraction or addition. If a reaction starts with a known total mass of reactants and you know the mass of all but one product, you can find the missing mass, because the totals must match. Whenever a mass seems to disappear or appear, look for a gas entering or leaving the system — the mass is conserved once you account for everything.

Watch: A Short Video Lesson

FuseSchool walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:

A Routine for Reaction Questions

1. Reactants change into products; atoms are rearranged, not lost.
2. Look for signs of a reaction: color change, gas, heat/light, or a new solid.
3. Total mass of reactants = total mass of products.
4. If mass seems to change, account for a gas entering or escaping.

Practice

1. What are the starting substances in a reaction called?
2. What does the law of conservation of mass state?
3. If 20 g of reactants react completely, what is the mass of the products?
4. Name one sign that a chemical reaction has happened.
5. Why does burning wood seem to lose mass?
6. Are atoms created or destroyed in a reaction?

Answers

1. Reactants.
2. Matter is neither created nor destroyed; mass of reactants equals mass of products.
3. 20 g.
4. Any of: color change, gas/bubbles, heat or light, a new solid forming.
5. Some mass leaves as gases that float away.
6. Neither — they are only rearranged.

Where This Fits in Your Science Prep

Reactions build on atoms and lead into balanced equations and reaction energy, where conservation of mass is written as a balanced equation. See all topics on the Science Topics Hub.