Everything Science Grade 10

CHAPTER 13. PHYSICAL AND CHEMICAL CHANGE 13.3

conserved. For each reaction give the total molecular mass of the reactants and

the products.

1. Hydrogen gas combines with nitrogen gas to form ammonia.

2. Hydrogen peroxide decomposes (breaks down) to form hydrogen and oxy-

gen.

3. Calcium and oxygen gas react to form calcium oxide.

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(1.) 0058 (2.) 0059 (3.) 005a

Law of constant
composition

ESADW

In any given chemical compound, the elements always combine in the same proportion

with each other. This is thelaw of constant composition.

Thelaw of constant compositionsays that, in any particular chemical compound, all sam-

ples of that compound will be made up of the same elements in the same proportion or

ratio. For example, any water molecule is always made up of two hydrogen atoms and one

oxygen atom in a2 : 1ratio. If we look at the relative masses of oxygen and hydrogen in

Chemistry: Chemical change 225