IGCSEChemistryOxford AQARevision NotesQuantitative ChemistryConservation of Mass Including the Quantitative Interpretation of Chemical EquationsConservation of Mass

Conservation of Mass (Oxford AQA IGCSE Chemistry)

Revision Note

Alexandra Brennan

Expertise

Chemistry

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

Exam Tip

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

The arrangement of atoms in the reaction between sodium and chlorine
There are the same number of sodium and chlorine atoms on both sides of the chemical equation

  • 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

law-of-conservation-of-mass-
The mass at the start and end of the reaction is the same

  • 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

Exam Tip

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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    Alexandra Brennan

    Author: Alexandra Brennan

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.