Coaching Centre
Chemical equations and conservation of mass
Reactants and products, writing word equations, reading symbol equations, common types of reaction, and the law of conservation of mass in open and closed systems.
When a 100 kg log burns in a campfire, only a few kilograms of ash are left. Where did the rest go? Not a single atom was destroyed. Most of the wood's atoms floated away into the air as invisible gases.
Reactants and products
The substances you start with in a chemical reaction are the reactants. The new substances made are the products. A reaction is written with an arrow, which means "react to make":
reactants → products
The general form is A + B → C + D. The "+" on the left means "reacts with"; on the right it means "and".
Word equations
A word equation uses the names of the substances. When magnesium ribbon burns with a brilliant white flame, it combines with oxygen from the air to make a white powder, magnesium oxide:
magnesium + oxygen → magnesium oxide
A few tips for naming the products of simple reactions:
- A metal combining with oxygen makes a metal oxide: copper + oxygen → copper oxide.
- A metal combining with sulfur makes a sulfide; with chlorine, a chloride.
- Fuels made of carbon and hydrogen (like methane, petrol and candle wax) burn in plenty of oxygen to make carbon dioxide and water.
When hydrogen peroxide solution is poured onto a cut, it fizzes. It breaks down into water and oxygen gas. Write a word equation, and name the reactant(s) and product(s).
Only one substance starts the reaction: hydrogen peroxide. It goes on the left.
Two new substances form: water and oxygen. They go on the right, joined by "+".
hydrogen peroxide → water + oxygen
Symbol equations
A symbol equation uses chemical formulas instead of names. You don't need to write these yet, but you should be able to read them. The big numbers in front of formulas tell you how many molecules react:
2Mg + O2 → 2MgO
This says two magnesium atoms react with one oxygen molecule (two oxygen atoms) to make two "units" of magnesium oxide. Count the atoms: 2 Mg and 2 O on the left; 2 Mg and 2 O on the right. The atoms balance, as they must.
- Start with Hydrogen burning. With one of each molecule, which atom doesn't balance?
- Change the big numbers until every row has a tick. Why aren't you allowed to change H₂O into H₂O₂ instead?
- Try methane burning and photosynthesis. How many oxygen molecules does one glucose molecule need when it is made?
Some common types of reaction
| Type | What happens | Example |
|---|---|---|
| Combination (synthesis) | Two or more substances join to make one | iron + sulfur → iron sulfide |
| Decomposition | One substance breaks down into simpler ones | copper carbonate → copper oxide + carbon dioxide |
| Combustion (burning) | A substance reacts with oxygen, giving out heat and light | methane + oxygen → carbon dioxide + water |
| Corrosion | A metal slowly reacts with substances around it | iron + oxygen + water → rust |
| Displacement | A more reactive metal takes the place of a less reactive one | magnesium + copper sulfate → magnesium sulfate + copper |
| Neutralisation | An acid reacts with a base | hydrochloric acid + sodium hydroxide → sodium chloride + water |
Conservation of mass
Because atoms are only rearranged in a reaction, never created or destroyed, the total mass of the products always equals the total mass of the reactants. This is the law of conservation of mass. It was established in the 1770s and 1780s by the French chemist Antoine Lavoisier, who weighed reactants and products very carefully in sealed glass vessels. His wife, Marie-Anne Paulze Lavoisier, worked alongside him, recording results and drawing the apparatus.
So why does a burning log seem to lose mass, and rusting steel wool seem to gain it? Because gases are involved. In an open system, gases can escape into the air or be taken in from it. In a closed system, such as a sealed bottle, nothing can get in or out, and the mass stays exactly the same.
- Run vinegar + baking soda in an open flask, then in a sealed one. Explain the difference.
- Burn steel wool in the open. Why does the mass increase?
- Which reaction keeps the same mass even when open? Why?
Test yourself
In the reaction magnesium + oxygen → magnesium oxide, the product is
Products are on the right of the arrow.
10 g of calcium carbonate is heated in an open crucible and breaks down into calcium oxide and carbon dioxide. Afterwards the solid has a mass of 5.6 g. The best explanation is that
The mass is still there, just not on the balance.
In 2H2 + O2 → 2H2O, how many hydrogen atoms are on each side?
2 × H₂ = 4 H on the left; 2 × H₂O = 4 H on the right.
Copper carbonate → copper oxide + carbon dioxide is an example of
One substance breaks into two simpler ones.
Who showed that mass is conserved by weighing reactions in sealed vessels?
Lavoisier is often called the father of modern chemistry.
Which is the correct word equation for burning methane?
Fuels burn in oxygen to make CO₂ and water.
Sort the substances in: zinc + hydrochloric acid → zinc chloride + hydrogen, and wood + oxygen → carbon dioxide + water + ash.
Put the steps for writing a word equation in order.
The substances at the start of a reaction are the . In a reaction, atoms are , not destroyed. The total of the products equals that of the reactants. In a system, no gas can escape. Metals react with oxygen to form .
A student puts vinegar and baking soda in a flask on a balance. The mass starts at 152.4 g and drops to 150.2 g. (a) Write a word equation for the reaction. (b) Explain why the mass decreased. (c) Describe how the experiment could be changed to show that mass is conserved.
(a) acetic acid + sodium hydrogen carbonate → sodium acetate + water + carbon dioxide
(b) One of the products, carbon dioxide, is a gas. The flask was open, so the gas bubbled out into the room. The 2.2 g that "disappeared" is the mass of carbon dioxide that escaped. No atoms were destroyed; they just left the balance.
(c) Do the reaction in a closed system, such as a plastic bottle with the lid screwed on (tip the bottle to mix the baking soda in once it is sealed). The carbon dioxide can't escape, so the balance should still read 152.4 g at the end, showing that the mass of the products equals the mass of the reactants.