Mass is conserved because atoms are only rearranged

1. During a reaction, bonds between atoms break and new bonds form, so the atoms rearrange into new substances.
2. The same atoms are present at the end as at the start, so the total mass cannot change.
   1. The total mass of the products therefore equals the total mass of the reactants.
3. A balanced equation shows this, because it has the same number of each type of atom on both sides.
   1. In $2Mg+O_2\to 2MgO$, there are two magnesium atoms and two oxygen atoms on each side.
4. The law applies to every reaction, including reactions that give off a gas or form a precipitate.

Conservation of mass lets you calculate a missing mass

1. Add the masses of all the reactants to find the total mass of the products.
2. If one product mass is unknown, subtract the known masses from the total.
   1. Heating $10.0g$ of calcium carbonate gives $5.6g$ of calcium oxide, so the mass of carbon dioxide made is $10.0-5.6=4.4g$.
3. The masses in a reaction are always in the same proportion, so using twice the mass of reactant gives twice the mass of each product.
   1. Heating $20.0g$ of calcium carbonate therefore gives $11.2g$ of calcium oxide and $8.8g$ of carbon dioxide.
4. Check that the product masses add up to the reactant total, since any difference points to an arithmetic slip or a substance that was left out.

Mass falls in an open container as gas escapes

1. Marble chips react with hydrochloric acid in an open flask to make calcium chloride, water and carbon dioxide gas.
2. The balance reading falls as the reaction goes on.
3. Gas particles are far apart and move quickly in random directions, so they spread out of the open flask into the room.
   1. The balance no longer weighs these escaped particles, so the measured mass drops by the mass of carbon dioxide lost.
4. Heating copper carbonate in an open tube also loses mass, because carbon dioxide leaves: $${CuCO}_3\to CuO+{CO}_2$$
5. If the escaped gas were collected and weighed, the total mass of all the products would equal the mass of the reactants.

Mass rises when a product gains gas from air

1. When magnesium ribbon is heated in a crucible, it reacts with oxygen from the air to form magnesium oxide.
2. The magnesium oxide has a greater mass than the magnesium that was heated.
   1. Oxygen particles from the air bond to the magnesium atoms and stay in the solid product, so the balance now weighs them.
3. The mass gained equals the mass of oxygen that reacted.
4. Iron wool heated in air also gains mass, because the iron combines with oxygen to form iron oxide.
5. Before the reaction, the oxygen was part of the air and was not on the balance, so no mass was created.

In an enclosed system, mass stays the same

1. In a sealed container, no particles can leave or enter.
2. Marble chips and acid in a flask closed with a bung give the same balance reading before and after the reaction.
   1. The carbon dioxide particles stay inside the flask, so they are still weighed.
3. A reaction with no gas involved keeps the same mass, even in an open beaker.
   1. Mixing silver nitrate and sodium chloride solutions forms a precipitate, but the total mass of the beaker and its contents does not change.
4. An observed change in mass therefore always means that a substance has crossed the boundary of the container.