Law of Conservation of Mass

When salt disappears into water, it can look as though the salt has ceased to exist. When vinegar reacts with baking soda, bubbles rush out and the mass shown on a balance can even appear to fall. These observations raise a basic question: during a change, is matter actually lost, or has it simply moved or changed form? Careful weighing gives a reliable way to answer that question.

Investigating a Physical Change

Consider a clean beaker containing water and a measured amount of common salt. Before the salt dissolves, the balance supports the beaker, the water and the salt. After swirling, the visible grains disappear because the salt becomes distributed through the water, but the same matter is still present in the beaker. The mass of the solution is therefore practically equal to the combined masses of water and salt taken initially.

The same reasoning applies if a piece of paper is weighed, torn into many pieces and weighed again without losing any pieces. The shape and size of the pieces have changed, but the amount of matter has not. A physical change may alter appearance, state or distribution without changing the total mass of the material being measured.

Investigating a Chemical Change

A chemical change is more challenging because new substances can form. When baking soda reacts with vinegar or lemon juice, brisk effervescence occurs because carbon dioxide gas is produced. If the reaction takes place in an open flask, some of this gas leaves the flask and spreads into the surrounding air. A balance supporting only the flask and its remaining contents will then show a smaller final reading.

This smaller reading does not mean matter has been destroyed. It means part of the product has left the part of the system that is being weighed. To test conservation fairly, every reactant and every product must remain inside the measured system.

Open and Closed Systems

In the closed setup, vinegar is placed in the conical flask and baking soda is held inside a balloon attached securely to the mouth of the flask. The complete apparatus is weighed before mixing. The balloon is then lifted so that the baking soda falls into the vinegar. Carbon dioxide forms and inflates the balloon, but it does not escape. When the entire apparatus is weighed again, the final reading matches the initial reading within normal measurement uncertainty.

Law of Conservation of Mass

The law does not say that every individual substance keeps the same mass. Reactants are converted into products, so the amount of one substance may decrease while another appears. What remains constant is the total mass of all matter participating in the reaction.

Verifying the Law With a Precipitation Reaction

When sodium sulfate solution and barium chloride solution are mixed, a white precipitate of barium sulfate forms along with sodium chloride. No gas is produced in this reaction, so the two flasks can be kept on the balance before and after mixing and the total mass can be compared directly. The observed constancy of mass gives another verification of the law.

Designing a Fair Test

Suppose zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas. Because a gas is produced, an open beaker would not be a reliable setup for comparing mass. A fair design must trap the hydrogen while still allowing the reactants to mix. A stoppered flask connected to a gas-tight balloon is one possible approach. The complete apparatus must be weighed before and after the reaction.