Law of conservation of mass
The law of conservation of mass states that no The smallest part of an element that can exist. are lost or made in a chemical reaction. Instead, the atoms join together in different ways to form A substance formed in a chemical reaction.. This is why, in a balanced symbol equation, the number of atoms of each element is the same on both sides of the equation.
Since atoms are not lost or made in a chemical reaction, the total The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of the products is equal to the total mass of the A substance that reacts together with another substance to form products during a chemical reaction.. The sum of the The sum of the relative atomic masses of the atoms in a chemical formula. of the reactants is equal to the sum of the relative formula masses of the products.
Calculations using the law
The mass of one substance in a reaction can be calculated if the masses of the other substances are known. For example:
Image caption, 1. Calcium carbonate is made up of 28 grams of calcium oxide and 22 grams of carbon dioxide
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Reactions in closed systems
No substances can enter or leave a In chemistry, a system in which no substances can enter or leave during a reaction., such as a stoppered flask. Sometimes, reactions that happen in open beakers are closed systems, for example acid-alkali The reaction between an acid and a base to form a salt plus water. reactions. Since all the reactants and products stay in the beaker, the total mass of the beaker and the substances in it stay the same during the reaction.
Reactions in non-enclosed systems
Substances can enter or leave a In chemistry, a system in which gases or other substances can enter or leave during a reaction.. These systems include open flasks, boiling tubes or crucibles that let gases enter or leave. For example:
- copper carbonate decomposes on heating to make solid copper oxide, which stays in the boiling tube, and carbon dioxide gas, which escapes
- magnesium reacts with oxygen, gained from the air, to produce magnesium oxide
If a gas escapes, the total mass will look as if it has decreased. If a gas is gained, the total mass will look as if it has increased. However, the total mass stays the same if the mass of the gas is included.
Worked example
Question
10.0 g of calcium carbonate, CaCO3, was heated in a Type of reaction in which a compound breaks down to form two or more substances when it is heated. reaction. 5.60 g of solid remained after heating. The equation below represents the reaction:
CaCO3(s) → CaO(s) + CO2(g)
Explain the change in mass.
The mass appears to have decreased because one of the products, carbon dioxide, escapes to the air. The total mass of the solid and gas products will be 10.0 g.