Conservation of Mass

Introduction

When wood burns, only a small pile of ash is left. When iron rusts, it seems to get heavier. Does matter disappear or appear from nowhere? More than two hundred years ago, careful weighing experiments showed that it does not. The law of conservation of mass is one of the cornerstones of chemistry, and the particle theory explains why it is true.

Core explanation

The law. In any physical change or chemical reaction, the total mass of the substances at the end is the same as the total mass at the start, provided nothing enters or leaves. Matter is neither created nor destroyed; it is only rearranged.

History. In the 1770s and 1780s, the French chemist Antoine Lavoisier carried out reactions in sealed glass vessels and weighed them carefully before and after. The total mass did not change, even when substances were heated and reacted. His work helped overturn older ideas and established chemistry as a quantitative science.

Physical changes. When ice melts, the water has the same mass as the ice. When salt dissolves, the solution's mass equals the mass of salt plus water. No particles are lost or gained.

Chemical reactions in a closed system. When solutions of silver nitrate and sodium chloride are mixed in a sealed flask, a white precipitate forms, yet the total mass on the balance stays the same. The atoms have rearranged into new substances, but every atom is still present.

Why mass seems to change in open systems. - Mass seems to decrease when a gas escapes. Burning wood forms carbon dioxide and water vapour that drift away, leaving light ash. If a marble chip reacts with acid in an open flask, carbon dioxide escapes and the reading on the balance falls. - Mass seems to increase when a gas from the air is taken in. When magnesium burns, it combines with oxygen from the air, so the magnesium oxide weighs more than the original magnesium. Rusting iron gains mass by combining with oxygen and water.

If all the substances, including gases, are collected and weighed, the total mass is always conserved.

The particle explanation. In a physical change, the same particles are rearranged. In a chemical reaction, atoms are separated and joined in new combinations, but no atom is created or destroyed. Since each atom keeps its mass, the total mass must stay the same.

Step-by-step reasoning

To explain why a balance reading falls when marble chips react with acid in an open flask:

1. Marble (calcium carbonate) reacts with acid to form carbon dioxide gas, among other products. 2. The flask is open, so the gas escapes into the air. 3. The escaping gas carries mass away from the flask. 4. The balance reading falls. 5. If a cotton-wool plug or a sealed balloon trapped the gas, the total mass would stay constant.

Visual explanation

Two balances side by side. On the left, an open flask with marble chips and acid: bubbles rise and escape, and the reading drops from 150.00 g to 148.80 g. On the right, the same reaction in a flask sealed with a balloon: the balloon inflates as it collects the gas, and the reading stays at 150.00 g. A particle diagram below shows the same atoms before and after, arranged differently.

Real-world analogy

Rebuilding a model from the same set of building bricks can produce a completely different shape — a house becomes a car — but the number of bricks, and so the total mass, stays the same. Chemical reactions rearrange atoms like bricks.

Real-world example

Chemical engineers use conservation of mass to design and monitor factories. By measuring the masses of raw materials going in and products, by-products and waste going out, they can detect leaks, calculate yields and check that pollutants are not escaping unrecorded. These "mass balance" calculations are required by environmental regulations in many countries.

Why?

Why is the law so important to chemistry? It means that chemical equations must be balanced — the same number of each type of atom on both sides — and that chemists can calculate exactly how much product a given mass of reactants can form. All quantitative chemistry, from medicine manufacture to fuel calculations, depends on it.

Common misconception

"When something burns, the matter is destroyed." Burning converts the fuel into gases such as carbon dioxide and water vapour, which spread into the air. The atoms still exist; they have just been rearranged and moved. Collecting all the products shows no loss of mass.

Worked example

Question: 2.4 g of magnesium burns completely in oxygen, forming 4.0 g of magnesium oxide. What mass of oxygen reacted?

Reasoning: By conservation of mass, mass of magnesium + mass of oxygen = mass of magnesium oxide. Oxygen = 4.0 − 2.4.

Answer: 1.6 g of oxygen.

Quick check

1. State the law of conservation of mass. Answer: In a chemical reaction or physical change, the total mass of the substances stays the same; matter is neither created nor destroyed.

Exam focus

Explain apparent mass changes in open systems by identifying the gas that escapes or is gained. Calculation questions often give three of four masses and ask for the missing one. Use the particle explanation: atoms are rearranged, not created or destroyed.

Advanced insight

Nuclear reactions are the exception at the level of mass: when nuclei split or fuse, a tiny fraction of mass is converted into a large amount of energy according to E = mc². In ordinary chemical reactions the corresponding mass change is far too small to measure, so chemists treat mass as conserved.

Summary

The law of conservation of mass states that total mass is unchanged in physical changes and chemical reactions. Mass only appears to change in open systems, when gases escape or are taken in from the air. Atoms are rearranged but not created or destroyed, which is why chemical equations must balance and why quantitative chemistry is possible.

Practice questions

1. Why does a burning candle appear to lose mass? Answer: The wax reacts to form carbon dioxide and water vapour, which escape into the air. 2. Why does iron wool gain mass when it rusts or burns? Answer: It combines with oxygen (and in rusting, water) from the air, adding their mass to the product. 3. 10.0 g of calcium carbonate decomposes to form 5.6 g of calcium oxide and carbon dioxide. What mass of carbon dioxide is produced? Answer: 10.0 − 5.6 = 4.4 g. 4. How could you show conservation of mass for a reaction that produces a gas? Answer: Carry it out in a sealed container (for example, a flask with a balloon over the top) on a balance; the total mass stays constant.