How are state symbols used?

State symbols in brackets after each formula show the physical state. For example HCl(aq) + NaOH(aq) - NaCl(aq) + H2O(l). State symbols are shown at the temperature of the reaction.

Conservation of Mass Notes

Oxford AQA IGCSE Chemistry: Revision notes

Key facts

Reactions can be shown as word equations or balanced symbol equations.
State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
No atoms are lost or made in a reaction, only rearranged.
Total mass of reactants = total mass of products.
Apparent mass changes in open containers are due to a gas entering or leaving.

Showing reactions

Use a word equation to name substances and a balanced symbol equation to show the atoms.

Both show reactants on the left and products on the right, with an arrow between.

Reactants
Products

2Mg + O₂ → 2MgO: a balanced equation has the same number of each atom on both sides.

\ce{2Mg + O2 -> 2MgO}

	Word equation	Balanced symbol equation
Example	magnesium + oxygen → magnesium oxide	$\ce{2Mg + O2 -> 2MgO}$
Shows	Names the substances	Shows the exact atoms

Word equation

Example:: magnesium + oxygen → magnesium oxide
Shows:: Names the substances

Balanced symbol equation

Example:: $\ce{2Mg + O2 -> 2MgO}$
Shows:: Shows the exact atoms

Which is a balanced symbol equation?

State symbols

State symbols in brackets after each formula show the physical state.

For example $\ce{HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)}$ . State symbols are shown at the temperature of the reaction.

(s)

(l)

(g)

(aq)

State symbols: solid (s), liquid (l), gas (g) and dissolved in water (aq).

\ce{Mg(s)}

	(s)	(l)	(g)	(aq)
Meaning	Solid	Liquid	Gas	Dissolved in water
Example	$\ce{Mg(s)}$	$\ce{H2O(l)}$	$\ce{CO2(g)}$	$\ce{NaCl(aq)}$

(s)

Meaning:: Solid
Example:: $\ce{Mg(s)}$

(l)

Meaning:: Liquid
Example:: $\ce{H2O(l)}$

(g)

Meaning:: Gas
Example:: $\ce{CO2(g)}$

(aq)

Meaning:: Dissolved in water
Example:: $\ce{NaCl(aq)}$

What does (aq) mean in $\ce{NaCl(aq)}$ ?

The law

Atoms are rearranged in a reaction, so the total mass of products equals the total mass of reactants.

No atoms are lost or made. A balanced equation shows equal numbers of each atom on both sides, and this principle underlies all reacting-mass calculations.

Magnesium (24 g) burns with oxygen (16 g) to give magnesium oxide: 24 + 16 = 40 g.

Mass of reactants= mass of products

Worked example

3 g of magnesium burns completely in 2 g of oxygen. What mass of magnesium oxide forms?

1
Mass is conserved, so product mass = reactant mass.
2
Add the reactant masses: 3 g + 2 g.

A student heats a metal carbonate in an open crucible and the mass goes down. Why?

Apparent mass changes

A gas entering or leaving an open container changes the mass you measure.

Mass increases when magnesium burns because it gains oxygen from the air. Mass decreases when a carbonate is heated because carbon dioxide escapes. Overall mass is still conserved.

Open container: Mg gains mass from O₂; CaCO₃ (100 g) loses CO₂ (44 g), leaving 56 g of CaO.

Magnesium burns in air Carbonate heated in open crucible Measured mass Mass goes up Mass goes down Reason Oxygen gas is gained Carbon dioxide escapes

	Magnesium burns in air	Carbonate heated in open crucible
Measured mass	Mass goes up	Mass goes down
Reason	Oxygen gas is gained	Carbon dioxide escapes

Magnesium burns in air

Measured mass:: Mass goes up
Reason:: Oxygen gas is gained

Carbonate heated in open crucible

Measured mass:: Mass goes down
Reason:: Carbon dioxide escapes

Magnesium is burned in air and the mass of the solid increases. Why?

Try an exam question

5.0 g of calcium carbonate is heated strongly in an open crucible. The mass of the solid left is less than 5.0 g. Explain why.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.