When you add calcium carbonate to hydrochloric acid, there is a chemical reaction.

Mariam places a flask of hydrochloric acid on a top pan balance and carefully adds calcium carbonate. She measures the mass of the flask and the contents at the beginning of the reaction and after ten minutes. These are her results:

Not what you expect?

The law of conservation of mass tells you that there must be the same mass at the end of a reaction as at the start. But in Mariam's experiment the mass appears to decrease. Why is this?
The word equation for this reaction is:
calcium carbonate + hydrochloric acid => calcium chloride + water + carbon dioxide
The carbon dioxide gas escapes into the air because the flask is open. You then cannot measure its mass. So it appears as if the mass decreases as the reaction continues.
The elements present in the reactants are all present in the products. Remember that water is made from hydrogen and oxygen atoms.

Another surprising result

Here is another reaction that produces a result which is a surprise to some people.
Some magnesium is placed in a crucible, and the mass of the crucible with the magnesium is recorded. The crucible is then heated very carefully. The lid of the crucible is lifted from time to time during the heating to allow the air in. After heating, the mass of the crucible with contents is found again.

While heating the magnesium, lift the lid to allow the oxygen in.
A reaction takes place when the magnesium is heated. After the reaction, the crucible contains white ashes. The mass of the ashes is more than the mass of the magnesium at the start of the reaction.
Some people think that the ashes will be lighter because the ashes look smaller than the magnesium. They see flames escaping from the crucible, so they think something has been lost.
However, the word equation explains the situation.
magnesium + oxygen => magnesium oxide
There is an increase in mass because oxygen from the air has combined with the magnesium.
A French scientist called Antoine Lavoisier carried out this experiment in 1772. He repeated it many times and found he had an increase in mass every time. He could not explain why this happened. Finally he came up with the idea that when something burns it combines with a gas from the air. He also found that the gas from the air that is involved in burning is involved in respiration as well. He named this gas oxygen.