Same mass before and after
The Law of Conservation of Mass.
A reaction in a closed box
In a group activity, students mix calcium carbonate and hydrochloric acid in a closed container. They weigh everything before and after the reaction.
How will the total mass after the reaction compare with the total mass before?
What this lesson covers
The idea
The total mass of the reactants equals the total mass of the products: matter can neither be created nor destroyed in a chemical reaction.
A reaction in a closed box
In a group activity, students mix calcium carbonate and hydrochloric acid in a closed container. They weigh everything before and after the reaction.
How will the total mass after the reaction compare with the total mass before?
- The same
- Smaller, because a gas is made
- Bigger, because new substances are made
Weigh it before and after
Press Next step to follow Example 9.1.
Matter is not made or lost
In a closed container the total mass stayed the same. Antoine Lavoisier proposed this in 1789. In an open container the mass seems to be lost because the gas escapes.
The total mass of the reactants equals the total mass of the products. Matter can neither be created nor destroyed in a chemical reaction. This is the Law of Conservation of Mass.
Notes
In a chemical reaction the total mass of reactants = total mass of products. Matter is neither created nor destroyed. This is the Law of Conservation of Mass.
Check yourself
Who proposed the Law of Conservation of Mass in 1789?
In Example 9.1 the reactants had a total mass of 6.92 g. The products were 1.76 g of carbon dioxide, 0.72 g of water and some calcium chloride. What mass of calcium chloride, in g, was formed?
Answer: 4.44 g
Products must also total 6.92 g. Calcium chloride = 6.92 − 1.76 − 0.72 = 4.44 g.
Why does the reading drop when baking soda reacts with vinegar in an open container?
12 g of carbon reacts with 32 g of oxygen. By the Law of Conservation of Mass, how many g of carbon dioxide are formed?
Answer: 44 g
Mass of product = 12 g + 32 g = 44 g.
- Antoine Lavoisier — correct. Yes!
- Joseph Proust. Proust proposed the Law of Constant Proportions, about the fixed ratio of elements in a compound.
- John Dalton. Dalton proposed the atomic theory, in 1808.
- Matter is destroyed in the reaction. Matter can neither be created nor destroyed in a chemical reaction. The mass left the container as a gas.
- The carbon dioxide gas made in the reaction escapes — correct. Yes!
- The vinegar turns into nothing. No substance turns into nothing. The gas that escapes carries away its mass.