Conservation of Mass in Changes

Introduction

When a log burns, a heap of light ash is all that remains. It looks as though most of the wood has simply vanished. Yet careful experiments by chemists in the eighteenth century, especially Antoine Lavoisier, showed that matter is never lost. The gases produced carry the "missing" mass away. This idea — the conservation of mass — is one of the foundations of modern chemistry.

Core explanation

The law. The law of conservation of mass states that in any physical or chemical change, the total mass of the substances present at the end equals the total mass at the start. Mass is neither created nor destroyed.

Explaining it with atoms. In a physical change, the particles are unchanged and simply move, so the mass cannot alter. In a chemical change, bonds break and new bonds form, but every atom in the reactants ends up somewhere in the products. No atoms are made and none disappear. Because each atom has a fixed mass, the total mass stays the same:

total mass of reactants = total mass of products

Closed systems. Conservation is easiest to see in a closed system , where nothing can get in or out. For example, when solutions of lead nitrate and potassium iodide are mixed in a sealed flask, a bright yellow precipitate forms, but the balance reading does not change.

Open systems: apparent changes. In an open container, gases can escape or be taken in, and the balance reading may change:

- Mass seems to decrease when a gas is given off and escapes. When marble chips (calcium carbonate) react with dilute acid in an open flask, carbon dioxide bubbles out and the reading falls. - Mass seems to increase when a gas from the air joins a solid. When magnesium burns in air, it combines with oxygen, and the white magnesium oxide has more mass than the original metal.

In both cases, mass has not really been created or destroyed. It has simply moved between the container and the air. If all the gases were collected and weighed, the totals would match exactly.

Using the law. Conservation of mass lets chemists calculate an unknown mass. If 2.4 g of magnesium makes 4.0 g of magnesium oxide, then the mass of oxygen that combined must be 4.0 − 2.4 = 1.6 g.

Mass in dissolving and state changes. Dissolving 10 g of salt in 90 g of water gives 100 g of solution. Melting 50 g of ice gives 50 g of water. Physical changes also obey the law.

Step-by-step reasoning

To explain a change in balance reading:

1. Decide whether the container is open or closed. 2. Identify any gases taken in or given off. 3. If a gas escapes, the reading falls; if a gas from the air combines, the reading rises. 4. Conclude that total mass, including gases, is unchanged.

Visual explanation

Imagine a sealed plastic bottle containing vinegar with a small bag of baking soda hanging inside, sitting on a balance. Tip the bottle so the two mix: it fizzes and the bottle swells with carbon dioxide, but the reading stays the same. Remove the cap and the gas escapes: now the reading drops.

Real-world analogy

Mass in a reaction is like money in a bank that only moves between accounts. Transfers can make one account look smaller and another look bigger, but the bank's total stays the same. Gases escaping into the air are like money moving to an account you forgot to check.

Real-world example

A burning candle on a balance gets lighter because the wax reacts with oxygen to form carbon dioxide and water vapour, which escape into the room. In careful demonstrations where the gases are trapped by absorbent chemicals above the flame, the total mass actually increases, because oxygen from the air has been added.

Why?

Why must mass be conserved in a chemical reaction? Reactions only rearrange atoms; they do not change atoms into other atoms or make them vanish. Since each atom's mass stays the same, the total mass must stay the same too.

Common misconception

"When something burns, it loses mass because matter is destroyed." The products of burning are mainly gases that escape. Counting the mass of oxygen used and gases produced shows that no matter is lost at all.

Worked example

Question: 10.0 g of calcium carbonate is heated strongly in an open dish until it fully decomposes. 5.6 g of calcium oxide remains. What mass of carbon dioxide was given off?

Reasoning: calcium carbonate → calcium oxide + carbon dioxide. Mass of reactant = mass of products, so mass of carbon dioxide = 10.0 − 5.6.

Answer: 4.4 g of carbon dioxide.

Quick check

1. Why does the mass of an open flask decrease when marble reacts with acid? Answer: Carbon dioxide gas is produced and escapes into the air.

Exam focus

Be ready to state the law of conservation of mass, explain it using atoms, and explain apparent gains or losses in open containers using gases. Calculation questions often ask you to find one missing mass by subtraction.

Advanced insight

Einstein's equation E = mc² shows that energy changes are linked to tiny changes in mass. In chemical reactions these changes are far too small to measure, well under a millionth of a gram for typical laboratory amounts, so the law of conservation of mass holds for all practical purposes. Only in nuclear reactions do measurable amounts of mass convert into energy.

Summary

In every physical and chemical change, the total mass stays the same because atoms are neither created nor destroyed, only rearranged. In closed systems the balance reading does not change. In open systems, mass may appear to decrease when a gas escapes or increase when a gas from the air combines, but the total including gases is always conserved.

Practice questions

1. State the law of conservation of mass. Answer: In any change, the total mass of the substances at the end equals the total mass at the start. 2. Explain in terms of atoms why mass is conserved in a chemical reaction. Answer: Atoms are only rearranged into new substances; none are created or destroyed, and each atom keeps its mass. 3. Iron wool gains mass when it is heated in air. Explain why. Answer: The iron combines with oxygen from the air to form iron oxide, so the mass of the oxygen is added to the solid. 4. 3.2 g of sulfur reacts completely with 5.6 g of iron. What mass of iron sulfide forms? Answer: 3.2 + 5.6 = 8.8 g.