A change in which two or more substances join together to form one new substance is called combination. In combination, substances bond in fixed ratios, so the law of conservation of mass and the law of definite proportions hold.
Basic Knowledge
Combination is a chemical change in which several substances become one substance (the opposite of decomposition). Representative examples:
・Combination of iron and sulfur: Fe + S → FeS (iron sulfide)
・Combination of copper and oxygen: 2Cu + O2 → 2CuO (copper oxide)
・Combination of hydrogen and oxygen: 2H2 + O2 → 2H2O (water)
Law of conservation of mass (Antoine Lavoisier, 1774): The total mass of substances does not change before and after a chemical change. This is because the types and numbers of atoms do not change before and after the reaction.
Law of definite proportions (Joseph Proust): The elements in the same compound are always bonded in a fixed mass ratio. For example, copper and oxygen always combine in a mass ratio of 4:1 to form copper oxide CuO. Magnesium and oxygen combine in a ratio of 3:2 to form magnesium oxide MgO.
Combination（a change in which two or more substances join to form one new substance）
Law of conservation of mass（the total mass of substances does not change before and after a chemical change）
Law of definite proportions（the mass ratio of elements forming the same compound is always constant）
Compound（a pure substance formed by chemical bonding between two or more elements）
Iron sulfide（FeS. A substance formed by the combination of iron and sulfur, with properties completely different from those of iron）
Copper oxide（CuO. Formed when copper and oxygen combine in a mass ratio of 4:1）
Further Study
Comparing a mixture of iron and sulfur with iron sulfide after heating is a standard experiment.
・Mixture: attracted to a magnet (iron is present), and only a small amount of hydrogen sulfide (rotten-egg odor) is produced when hydrochloric acid is added
・Iron sulfide: not attracted to a magnet (it is a different substance), and a large amount of hydrogen sulfide is produced when hydrochloric acid is added
This shows that combination is completely different from mixing.
In the oxidation of copper, completely oxidizing 0.8 g of copper produces 1.0 g of copper oxide. The combined oxygen has a mass of 0.2 g, so copper:oxygen = 0.8:0.2 = 4:1. This ratio is always constant (the law of definite proportions). Even if excess copper is present, the reaction stops when the oxygen is used up.
- Combination = two or more types → one type of substance (the opposite of decomposition)
- Law of conservation of mass = total mass does not change before and after a reaction
- Law of definite proportions = the mass ratio in a compound is always constant
- Copper:oxygen = 4:1 produces copper oxide CuO
- Mg:oxygen = 3:2 produces magnesium oxide MgO
- Iron＋sulfur→iron sulfide (not attracted to a magnet)
- Mixtures and compounds are completely different
Points to Note
① Combination is a chemical change (a new substance is produced), while mixing is a physical change (the substances do not change). ② The law of conservation of mass must be confirmed in a closed container (in an open system, gas may enter or leave, causing the apparent mass to change). ③ Under the law of definite proportions, even when one substance is in excess, the mass ratio that combines does not change. The excess does not react. ④ Remember the values copper:oxygen = 4:1 and Mg:oxygen = 3:2.
Practice
- When 3.2 g of copper is completely oxidized, find the mass of copper oxide produced. (Copper:oxygen = 4:1)
- Explain the difference between combination and mixing.
- Why is a closed container used in an experiment confirming the law of conservation of mass?