Conservation of Mass

Section 1 of 7The law

Atoms are not created or destroyed

• Atoms are not created or destroyed In a chemical reaction the atoms present in the reactants are rearranged into new substances in the products.
• Because the same atoms are present before and after, the total mass of reactants equals the total mass of products.

Balanced equations

• Balanced equations A balanced symbol equation has the same number of atoms of each element on both sides, which reflects this law.

Section 2 of 7Balancing an equation

1

Write the correct formula for every reactant and product. Never change a formula to balance an equation.

2

Count the atoms of each element on both sides.

3

Place numbers in front of formulae (coefficients) to balance one element at a time. Leave oxygen and hydrogen until last if they appear in several substances.

4

Recount all atoms to confirm the equation balances.

5

Add state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.

Section 3 of 7Example: magnesium and oxygen

Stage	Equation	Comment
Unbalanced	Mg + O2 → MgO	2 O atoms on left, 1 on right
Balance O	Mg + O2 → 2MgO	Now 2 Mg needed on the left
Balanced	2Mg + O2 → 2MgO	2 Mg and 2 O on each side
With states	2Mg(s) + O2(g) → 2MgO(s)	Complete answer

Section 4 of 7Why mass seems to change

Open vs closed systems

Mass appears to increase

• Mass appears to increase A metal burning in air gains oxygen from the air, so the solid product weighs more than the metal.

Mass appears to decrease

• Mass appears to decrease A reaction that releases a gas, such as carbonate with acid in an open flask, loses mass because the gas escapes.

Closed system

• Closed system In a sealed container, nothing can escape or enter, and the total mass stays the same.

Explanation

• Explanation The missing or extra mass is that of a gas that was not weighed.
  • Include it and the masses balance.

Section 5 of 7Using the law in calculations

1Known masses

• Known masses Write down the masses of all substances you know.

2Add reactants

• Add reactants Total mass of reactants = total mass of products.

3Subtract

• Subtract Find the unknown by subtracting the known products from the total of the reactants.

4Example

• Example If 10 g of calcium carbonate gives 5.6 g of calcium oxide, then 4.4 g of carbon dioxide must have been formed.

Section 6 of 7Key terms

Term	Meaning
Law of conservation of mass	In a chemical reaction no atoms are made or lost, so the mass of the products equals the mass of the reactants.
Reactant	A substance that is used up in a reaction.
Product	A substance that is made in a reaction.
Coefficient	The big number in front of a formula showing how many of that substance are involved.
State symbol	A letter in brackets showing whether a substance is solid, liquid, gas or aqueous.

Section 7 of 7Exam technique

1

Changing a subscript, such as writing MgO₂, to balance an equation. Only numbers in front of formulae may change.

2

Stating that mass is lost in a reaction because it 'disappears'. It is carried away as a gas.

3

Forgetting that a gas from the air, such as oxygen, adds mass to a solid product.

4

Leaving out state symbols when the question asks for a full equation.