1.3 Law of conservation of mass

Modern chemistry emerged in the eighteenth century, when chemists began to use the balance systematically as a tool in research. Balances measure mass, which is the quantity of matter in a material. Matter is the general term for the material things around us; we can define it as whatever occupies space and can be perceived by our senses.

Antoine Lavoisier (1743 – 1794), a French chemist, insisted on the use of the balance in chemical research. His experiments demonstrated the law of conservation of mass, a law that states that the total mass remains constant during a chemical change (chemical reaction). A magnesium flash bulb gives us a convenient illustration of this law. The flash from the bulb accompanies a chemical reaction triggered by the heat of an electric current. A flash bulb that weighs 11.2 grams before it is flashed still weighs 11.2 grams afterward; the mass (11.2 grams) remains constant.

In a series of experiments, Lavoisier showed that when a metal or any other substance burns, something in the air combines chemically with it. He called this component of air oxygen. For example, Lavoisier found that the liquid metal mercury was transformed in air into a red-orange substance. The substance had greater mass than the original mercury. This was due, he said, to the chemical combination of mercury with oxygen. Furthermore, he could heat the new substance, mercury (II) oxide, to recover the original mass of mercury. Lavoisier's explanation of the burning or combustion, of mercury can be written

Mercury + oxygen — mercury (II) oxide

The following example illustrates how you can use the law of conservation of mass to investigate chemical changes.

Example 1.1

Using the Law of Conservation of Mass

You heat 2.53 grams of metallic mercury in air to give 2.73 grams of a red-orange residue. Assume that the chemical change is the reaction of the metal with oxygen in air. What is the mass of the oxygen that reacts? Suppose you strongly heat the red-orange residue, and it decomposes to give metallic mercury again. What is the mass if the oxygen lost during this heating?

Solution

According to the law of conservation of mass, the mass remains constant during a chemical reaction. Therefore, the mass of the mercury metal plus the mass of the oxygen that reacts with it must equal the mass of the resulting red-orange residue.

2.53 grams + mass of oxygen = 2.73 grams or

Mass of oxygen = (2.73 - 2.53) grams = 0.20 grams

The mass of the oxygen lost when the red-orange residue decomposes equals the mass of the oxygen that originally reacted (0.20 grams)

Exercise 1.1

You place 1.85 grams of wood in a vessel with 9.45 grams of air and seal it. Then you heat the vessel strongly so that the wood burns. After the experiment, you weigh the ash that remains after the wood burns and find that its mass is 0.28 grams. What is the mass of the gases in the vessel at the end of the experiment?

Measurements of mass before and after the combustion of various substances were necessary to convince chemists that Lavoisier's views were correct. Thereafter, measurements of mass became indispensable to chemical research.

Before leaving this section, you should note the distinction between the terms mass and weight in precise usage. The weight of an object is the force of gravity exerted on it. It is proportional to the mass of the object divided by the square the distance between the center of mass of the object and that of the earth. Because the earth is slightly flattened at the poles, an object weighs more at the North Pole, where it is closer to the center of the earth, than it does at the equator. The mass of an object is the same wherever it is measured.