Conservation of Mass (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
Written by: Alexandra Brennan
Reviewed by: Stewart Hird
Chemical Equations
Word Equations
Word equations show the reactants and products of a chemical reaction using their full chemical names
reactants → products
The reactants are those substances on the left-hand side of the arrow
They can be thought of as the chemical ingredients of the reaction
They react with each other to form new substances, which are the products
The products are on the right-hand side of the arrow
The arrow (which is spoken as “to form” or “produces”) implies the conversion of reactants into products
Reaction conditions or the name of a catalyst (a substance added to make a reaction go faster) can be written above the arrow
An example is the reaction of sodium hydroxide (a base) and hydrochloric acid to produce sodium chloride (common table salt) and water:
sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water
Symbol Equations
A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction
When writing symbol equations, you should:
Ensure reactants are on the left of the equation and products are on the right
Write the following non-metals as molecules: H2, N2, O2, F2, Cl2, Br2 and I2
Include state symbols
Solid = (s)
Liquid = (l)
Gas = (g)
Aqueous = (aq)
A symbol equation must be balanced to give the correct ratio of reactants and products:
For example, the combustion of sulfur:
S (s) + O2 (g)→ SO2 (g)
This equation shows that one atom of sulfur, S, reacts with one molecule of oxygen, O2, to make one molecule of sulfur dioxide, SO2
When balancing equations, there must be the same number of atoms of each element on either side of the equation following the law of conservation of mass (see below)
To balance an equation you work across the equation from left to right, checking one element after another
If there is a group of atoms such as a nitrate group (NO3–) that has not changed from one side to the other, then count the whole group as one entity rather than counting the individual atoms
Examples of balanced symbol / chemical equations include:
Acid-base neutralisation reaction:
NaOH (aq) + HCl (aq) ⟶ NaCl (aq) + H2O (l)
Redox reaction:
2Fe2O3 (aq) + 3C (s) ⟶ 4Fe (s) + 3CO2 (g)
In each equation, there are equal numbers of each atom on either side of the reaction arrow so the equations are balanced
The best approach is to practice lot of examples of balancing equations
This can be by trial and error - changing the coefficients (numbers) in front of the formulae one by one and checking the result on the other side
Balance elements that appear on their own, last in the process
Worked Example
Ammonia reacts with nitric acid to form the fertiliser ammonium nitrate. Write a word equation for the reaction taking place.
Answer:
Ammonia + nitric acid → ammonium nitrate
This question has all the information in the correct order
Ammonia reacts with nitric acid
This becomes ammonia + nitric acid
to form
This is the arrow in the equation
to form the fertiliser ammonium nitrate
This tells you that the product is ammonium nitrate
Worked Example
When magnesium oxide, MgO, reacts with nitric acid, HNO3, it forms magnesium nitrate, Mg(NO3)2, and water.
Write a symbol equation for this reaction.
Answer:
The balanced symbol equation is:
MgO (s) + 2HNO3 (aq) ⟶ Mg(NO3)2 (aq) + H2O (l)
Step 1 - writing the unbalanced equation
Magnesium oxide, MgO, reacts with nitric acid, HNO3, it forms magnesium nitrate, Mg(NO3)2, and water
MgO + HNO3 ⟶ Mg(NO3)2 + H2O
The Mg and O atoms (not including the O in the NO3 group appear to be balanced), so we should focus on the H atoms and NO3 groups
Step 2 - balancing hydrogen atoms
There are 2 hydrogen atoms on the product side, so 2 hydrogen atoms are needed on the reactant side
This means that 2HNO3 will be needed as we cannot change the chemical formula
MgO + 2HNO3 ⟶ Mg(NO3)2 + H2O
This also balances the nitrate, NO3, groups
Step 3 - checking the equation
The equation appears balanced so we need to check that it is:
Reactant side:
Mg atom
1 O atom - not including those in the NO3 group
2 H atoms
2 NO3 groups - remember to keep groups as a single entity if they are unchanged on both sides of the equation
Product side:
1 Mg atom
2 NO3 groups - remember to keep groups as a single entity if they are unchanged on both sides of the equation
2 H atoms
1 O atom - not including those in the NO3 group
The equation is now balanced
Examiner Tips and Tricks
You need to be confident using the state symbols (s), (l), (g) and (aq)
You will not need to include them in all equations unless you are specifically asked to
However, it is good practice to include state symbols in your equations so that you don't miss any marks
Conservation of Mass
The conservation of mass states:
No atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants
For example, the reaction between sodium and chlorine:
The reaction between sodium and chlorine
In the balanced chemical equation, there are:
2 atoms of sodium and 2 atoms of chlorine on the reactants side
2 atoms of sodium and 2 atoms of chlorine on the products side
This shows that there are the same number of each atom throughout the chemical reaction
The only thing that has changed is how the atoms are arranged
Demonstrating conservation of mass
A precipitation reaction is one where two solutions react to form an insoluble solid called a precipitate
If the reaction flask is closed and no other substance can enter or leave the system, then the total mass of the reaction flask will remain constant
For example, the reaction between calcium chloride and sodium sulfate produces a precipitate of calcium sulfate
The balanced chemical equation is:
CaCl2 (aq) + Na2SO4 (aq) ⟶ CaSO4 (s) + 2NaCl (aq)
If the reaction is carried out in a closed system, then the mass before and after the reaction will be the same
Diagram showing the conservation of mass
If the reaction flask is open and a gaseous product is allowed to escape, then the total mass of the reaction flask will decrease
This is because the gas is lost from the system to the surroundings
For example, the reaction between hydrochloric acid and calcium carbonate produces carbon dioxide gas:
2HCl (aq) + CaCO3 (s) ⟶ CaCl2 (aq) + H2O (l) + CO2 (g)
Mass will be lost from the reaction flask unless it is closed
Examiner Tips and Tricks
The mass of the reactants is always equal to the mass of the product however mass can appear to have been lost or gained during a reaction. If a gas is given off during the reaction, mass appears to have been lost. If a substance reacts with a gas, mass appears to have been gained.
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