Atoms are the building blocks that can be combined in different ways to form different substances.
It is impossible for any atom to be changed into a different type of atom by a chemical reaction.
All of the atoms in the reactants of a chemical reaction are reorganised to form all of the products of the reaction.
It is impossible to make a particular product without all the atoms that it needs.
The law of conservation of mass can be applied to a closed system, or a non-enclosed system that takes in/gives out gas.

Keywords

Conservation of mass - Conservation of mass means that the combined mass of the starting reactants equals the combined mass of the products formed.
Closed system - A closed system is one in which matter cannot enter or leave the observed environment, allowing only energy transfer between the system and its surrounding environment.
Reactant - Reactants are the elements or compounds before a reaction takes place.
Product - Products are the elements or compounds that are formed after a reaction has taken place.
Atom - Atoms are the building blocks of all matter.

Common misconception

Pupils can struggle to link chemical formulae to what is happening on the atomic level and to what they observe during a chemical reaction.

Reinforce via demonstrations, modelling and diagrams via Johnstone's triangle. One simple demo (e.g. magnesium + oxygen) = one model, one diagram and one chemical reaction equation. Use progressively more complicated demos and build up from there.

Challenge pupils to use moly mods (or Lego or plasticine balls + toothpicks) to model the rearrangement of atoms in specific reactions, e.g. oxidation, decomposition or precipitation. Using Lego easily extends the activity to conservation of mass whereby each dot = 1 unit of mass per 'atom'.

Teacher tip

Content guidance

Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

What are the building blocks of all matter?

molecules

compounds

Correct answer: atoms

Q2.

Atoms cannot be created or ...

Correct Answer: destroyed

Q3.

Match the formulae of the substances to the name of the substance. Use your periodic table to help.

Correct Answer:CO₂,carbon dioxide

carbon dioxide

Correct Answer:MgO,magnesium oxide

magnesium oxide

Correct Answer:CaCO₃,calcium carbonate

calcium carbonate

Correct Answer:NaCl,sodium chloride

sodium chloride

Correct Answer:AgBr,silver bromide

silver bromide

Correct Answer:NH₃,ammonia

ammonia

Q4.

Name the elements and number of atoms of each type in (NH₄)₂SO₄

nitrogen (1) hydrogen (4) sodium (4)

nitrogen hydride (2) sulfur oxide (4)

Correct answer: nitrogen (2), hydrogen (8), sulfur (1) and oxygen (4)

nitrogen (2), hydrogen (8) and oxygen (4)

ammonium (2), sulfur (1) and oxygen (4)

Q5.

Hydrogen reacts with oxygen to form water as steam. Match up the following statements to make complete sentences about this reaction.

Correct Answer:All the reactants and products are ...,gases shown by the state symbol (g).

gases shown by the state symbol (g).

Correct Answer:Water is ... ,a gas as it is steam.

a gas as it is steam.

Correct Answer:The symbol equation for the reaction is:,2H₂ (g) + O₂(g) → 2H₂O(g)

2H₂ (g) + O₂(g) → 2H₂O(g)

Correct Answer:There are the same number of atoms ...,of oxygen (2) and hydrogen (4) in the reactants and products.

of oxygen (2) and hydrogen (4) in the reactants and products.

Correct Answer:The word equation for the reaction is:,hydrogen + oxygen → water (dihydrogenoxide)

hydrogen + oxygen → water (dihydrogenoxide)

Q6.

Methane (CH₄) burns to produce carbon dioxide (CO₂) and water (H₂O). Count how many atoms of each element are on the left and right hand side of the equation. Are they equal? CH₄ + 2O₂ → CO₂ + 2H₂O

carbon (1), hydrogen (4) oxygen (4)-> carbon (1), hydrogen (4) oxygen (4) No

carbon (1), hydrogen (4) oxygen (2)-> carbon (1), hydrogen (4) oxygen (3) Yes

carbon (4), hydrogen (4) oxygen (2)-> carbon (2), hydrogen (4) oxygen (3) Yes

Correct answer: carbon (1), hydrogen (4) oxygen (4)-> carbon (1), hydrogen (4) oxygen (4) Yes

Exit quiz

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6 Questions

Q1.

What is the name of the starting chemicals in a reaction?

ingredients

products

Correct answer: reactants

Q2.

Order the steps in a chemical reaction.

1 - Presence of reactants

2 - Rearrangement of atoms

3 - Formation of products

Q3.

Which of the following statements explains why mass is conserved in a chemical reaction?

atoms are destroyed

atoms change into different elements

Correct answer: atoms are not lost or gained, but rearranged

Q4.

10 g of magnesium is heated in a crucible and 12g of magnesium oxide is obtained at the end of the reaction. Match the questions to the answers below about this reaction.

Correct Answer:What other reactant was present?,oxygen

oxygen

Correct Answer:How much of this reactant reacted?,12 g - 10 g = 2 g

12 g - 10 g = 2 g

Correct Answer:What type of system was this?,open

open

Correct Answer:What could have happened to the product?,Some could have escaped into the atmosphere.

Some could have escaped into the atmosphere.

Q5.

Some open system reactions where a gas is used as a reactant or formed as a product, may appear as if there has been a gain or loss in mass. Choose the reactions below where this could be the case.

Correct answer: Na(s) + Cl₂(g) → NaCl(s)

Fe(s) + S(s) → FeS(s)

Correct answer: 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)

2Al(s) + 3Br₂(l) → 2AlBr₃(s)

Correct answer: Ca(s) + 2HCl(aq) → CaCl₂(aq) + H₂(g)

Q6.

Balance the following equation showing the combustion of a hydrocarbon by replacing X, Y and Z and add state symbols to show the expected state of the reactants and products. C₃H₈ + XO₂ → YCO₂ + ZH₂O

X = 5 (g), Y = 3 (g) and Z = 4 (l)

X = 4 (g), Y = 2 (g) and Z = 2 (g)

X = 3 (g), Y = 3 (g) and Z = 3 (g)

Correct answer: X = 5 (g), Y = 3 (g) and Z = 4 (g)

X = 2 (g), Y = 2 (g) and Z = 4 (g)