Dalton and the Conservation of Mass

Introduction

When wood burns, only a small pile of ash is left. When an iron nail rusts, it becomes heavier. It can look as if reactions destroy or create matter. In the late 1700s Antoine Lavoisier showed by careful weighing that they do neither, and a few years later Dalton explained why, using atoms. Understanding this connection is essential, because every chemical equation and every reacting-mass calculation depends on it.

Core explanation

The law. The law of conservation of mass states that in a chemical reaction the total mass of the products equals the total mass of the reactants. Mass is neither created nor destroyed.

Lavoisier's evidence. Antoine Lavoisier, working in France, carried out reactions in sealed glass vessels and weighed them before and after. He found no change in total mass, even when the substances inside changed dramatically. He published this conclusion in 1789. Weighing sealed containers was the key, because it stopped gases from escaping or entering unnoticed.

Dalton's explanation. Dalton's theory gave the reason. In a chemical reaction:

- atoms are not created — no new atoms appear from nowhere; - atoms are not destroyed — none disappear; - atoms do not change into atoms of other elements; - atoms are only rearranged into new combinations.

Since each atom has a fixed mass, and the number of each kind of atom stays the same, the total mass cannot change.

Balanced equations show this. Consider the reaction of hydrogen with oxygen to form water:

2H₂ + O₂ → 2H₂O

On the left there are 4 hydrogen atoms and 2 oxygen atoms. On the right there are also 4 hydrogen atoms and 2 oxygen atoms. The same atoms are present, grouped differently, so the mass is the same.

Apparent changes in mass. Sometimes a reaction seems to break the law:

- Mass seems to decrease when a gas product escapes. When wood burns, carbon dioxide and water vapour leave into the air, so the ash is lighter than the wood. - Mass seems to increase when a gas from the air joins the product. When iron rusts or magnesium burns, oxygen from the air combines with the metal, so the solid product is heavier than the metal.

In each case, if everything including the gases is accounted for — as in a closed system — the total mass is unchanged.

Step-by-step reasoning

To check whether a reaction obeys conservation of mass:

1. Add up the masses of all the reactants, including any gases. 2. Add up the masses of all the products, including any gases. 3. Compare the totals; they should be equal. 4. If they seem different, look for a gas that escaped or was taken in from the air.

Visual explanation

Picture a balance with a sealed flask on one side. Inside, coloured spheres representing atoms separate from their partners and regroup into new particles. The balance pointer stays perfectly still throughout, because the same atoms remain inside the flask.

Real-world analogy

Think of taking apart a LEGO car and building a LEGO house from exactly the same bricks. The model looks completely different, but if you weigh it, the house has the same mass as the car, because no bricks were added or lost — they were just rearranged.

Real-world example

Car engines burn petrol, and the fuel tank gets lighter as the journey goes on. The mass has not vanished: the carbon and hydrogen atoms in the fuel combine with oxygen atoms from the air to form carbon dioxide and water, which leave through the exhaust. The exhaust gases actually have a greater mass than the fuel burnt, because they include oxygen from the air.

Why?

Why does the total mass stay the same even though the substances change completely? Chemical reactions only break and make bonds between atoms. The atoms themselves, which carry almost all of the mass, pass unchanged from reactants to products, so the total mass is conserved.

Common misconception

"When something burns, matter is destroyed." Burning turns solids and liquids into gases that spread into the air. The atoms still exist in those gases. If they were collected, the total mass would equal the mass of fuel plus the oxygen that reacted.

Worked example

Question: 2.4 g of magnesium reacts completely with oxygen to form 4.0 g of magnesium oxide. What mass of oxygen reacted?

Reasoning: By conservation of mass, mass of reactants = mass of products. So mass of magnesium + mass of oxygen = mass of magnesium oxide. Mass of oxygen = 4.0 − 2.4 = 1.6 g.

Answer: 1.6 g of oxygen.

Quick check

1. State the law of conservation of mass. Answer: In a chemical reaction, the total mass of the products equals the total mass of the reactants.

Exam focus

Examiners often describe a reaction in an open container and ask why the mass changed. Always name the gas that escaped or was gained from the air, and state that in a closed system the total mass would stay the same because atoms are only rearranged.

Advanced insight

Einstein's equation E = mc² shows that energy released in a reaction has a tiny mass equivalent. In chemical reactions this change is far too small to measure, roughly a millionth of a millionth of a percent or less, so conservation of mass holds for all practical chemistry. In nuclear reactions the mass change is measurable.

Summary

The law of conservation of mass states that the total mass of products equals the total mass of reactants. Lavoisier showed it by weighing sealed vessels, and Dalton explained it: atoms are neither created nor destroyed in reactions, only rearranged. Apparent mass changes in open containers are caused by gases escaping or joining from the air.

Practice questions

1. Who first showed by careful weighing that mass is conserved in reactions? Answer: Antoine Lavoisier. 2. Use Dalton's ideas to explain why mass is conserved in a chemical reaction. Answer: Atoms are not created or destroyed; they are only rearranged into new substances, and each atom keeps its mass, so the total mass stays the same. 3. When a candle burns in an open room, its mass decreases. Does this break the law of conservation of mass? Explain. Answer: No. The wax reacts with oxygen to form carbon dioxide and water vapour, which escape into the air; including these gases, total mass is conserved. 4. 5.6 g of iron reacts completely with 3.2 g of sulfur. What mass of iron sulfide forms? Answer: 5.6 + 3.2 = 8.8 g. 5. Check that 2H₂ + O₂ → 2H₂O is balanced by counting atoms. Answer: Both sides have 4 hydrogen atoms and 2 oxygen atoms, so the equation is balanced and mass is conserved.