Law of Constant Composition Revision Notes for Grade 10 NSC Matric Physical Sciences

Law of Constant Composition

What is the law of constant composition?

The law of constant composition is a fundamental principle in chemistry that tells us something very important about chemical compounds. This law states that in any given chemical compound, the elements always combine in the same proportion with each other, no matter how much of the compound you have or where it comes from.

Put simply, if you have a chemical compound like water ( $H_2O$ ), every single water molecule will always contain exactly the same ratio of hydrogen to oxygen atoms. This ratio never changes.

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Think of it like a recipe - just as a cake recipe always requires the same proportions of ingredients to make the same cake, chemical compounds always require the same proportions of elements to form that specific compound.

Key definition

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Law of constant composition: In any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio.

This means that whether you have a drop of water or a whole swimming pool full of water, the proportion of hydrogen to oxygen will always be exactly the same.

Understanding through examples

Water molecules ( $H_2O$ )

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Worked Example: Water Composition

Let's analyze water as our main example:

Step 1: Identify the atomic composition

Every water molecule contains 2 hydrogen atoms and 1 oxygen atom
The atom ratio is always H : O = 2 : 1

Step 2: Calculate the mass composition

When we look at mass, oxygen is much heavier than hydrogen
In a water molecule, 94% of the mass comes from oxygen and 6% comes from hydrogen

Step 3: Apply the law

This mass proportion stays the same for any amount of water, anywhere in the world

Hydrogen peroxide ( $H_2O_2$ )

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Worked Example: Different Compound, Same Elements

Hydrogen and oxygen can also combine in different proportions to form a different compound:

In hydrogen peroxide:

The ratio is H : O = 1 : 1 (equal numbers of each atom)
The mass ratio of hydrogen to oxygen is 1 : 16
This ratio will be the same for any sample of hydrogen peroxide, anywhere

Key insight: This shows us that the same elements can form different compounds, but each specific compound always has its own fixed ratio.

Practical investigation

Scientists can test this law through experiments. Here's how it works:

Aim

To investigate the ratios in which compounds combine.

Method

The investigation uses three different reactions:

Silver nitrate + sodium chloride: Mix different amounts (5 ml, 10 ml, 15 ml) of silver nitrate with the same amount of sodium chloride
Lead nitrate + sodium iodide: Mix different amounts of lead nitrate with sodium iodide
Sodium hydroxide + iron chloride: Mix different amounts of these reactants

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Key Finding from the Investigation

No matter how much of each reactant you add, the same products form with the same compositions. However, if reactants aren't added in the correct ratios, some unreacted material will be left over.

This proves that compounds always form in fixed proportions - you can't force them to combine in different ratios.

Volume relationships in gases

The law of constant composition also applies to gases, and there's a special relationship called Gay-Lussac's Law.

Gay-Lussac's Law

In chemical reactions between gases, the volumes of gases involved are in ratios of small whole numbers (when measured at the same temperature and pressure).

Examples

Water formation: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$

2 volumes of hydrogen react with 1 volume of oxygen to produce 2 volumes of water

Ammonia formation: $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$

1 volume of nitrogen reacts with 3 volumes of hydrogen to produce 2 volumes of ammonia

These volume ratios are always the same, just like the mass ratios we discussed earlier.

Why this law matters

Understanding the law of constant composition helps us:

Predict how much of each element we need to make a compound
Calculate how much product we'll get from known amounts of reactants
Identify unknown compounds by analysing their composition
Understand that chemical formulas represent exact, fixed ratios

Common exam tips

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Essential Exam Tips

Remember that this law applies to compounds, not mixtures
The ratios are fixed for each specific compound, but different compounds of the same elements can have different ratios
Mass ratios and atom ratios are related but different - always check which one the question is asking for
In calculations, use the molar masses of elements to convert between atom ratios and mass ratios

Remember!

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Key Points to Remember:

Every chemical compound has a fixed composition - the elements always combine in the same proportion
Water is always $H_2O$ - 2 hydrogen atoms to 1 oxygen atom, giving a mass ratio where oxygen makes up 94% and hydrogen 6%
Different compounds of the same elements exist - like $H_2O$ (water) and $H_2O_2$ (hydrogen peroxide), each with their own fixed ratios
Volume ratios in gas reactions follow the same principle - gases combine in simple whole number ratios
This law is fundamental for chemical calculations - it allows us to predict exactly how much of each reactant we need and how much product we'll get

Law of Constant Composition (Grade 10 NSC Matric Physical Sciences): Revision Notes