S3 Science — Chemical Reactions and Equations: Balancing, Conservation of Mass and Energy Change (Lesson Prep Guide)

2026-11-15

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Students can write that magnesium plus oxygen gives magnesium oxide, then start changing subscripts the moment symbols appear, turning water into hydrogen peroxide just to match the atom count. This lesson blocks that with one rule: change coefficients only, never subscripts.

Objectives — Students can: (1) turn a word description of a reaction into a symbol equation with reactants and products in the right places; (2) balance an equation by counting atoms element by element and explain why only coefficients may change; (3) use conservation of mass to calculate a missing mass and decide from a temperature change whether a reaction is exothermic or endothermic.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Symbols and subscripts	How many hydrogen atoms are in H2O?	Two
Conservation of mass	Does the total number of atoms change during a reaction?	No
Reaction types	Give one example each of combustion, neutralisation and decomposition	Burning magnesium, acid plus alkali, heating calcium carbonate

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	One wrong line	Write H2 + O2 gives H2O2 and ask whether it is acceptable	No, the product is a different substance
5-14 min	Words to symbols	Turn methane plus oxygen gives carbon dioxide and water into CH4 + O2 gives CO2 + H2O	Write magnesium plus hydrochloric acid themselves
14-28 min	Balancing	Count each element, carbon and hydrogen first, oxygen last: CH4 + 2O2 gives CO2 + 2H2O	Balance H2 + O2 themselves
28-38 min	Conservation of mass	100 g of calcium carbonate decomposes into 56 g of calcium oxide and the rest carbon dioxide	Work out the mass from magnesium and acid
38-42 min	Independent practice	Three questions on balancing, mass and energy; tour asking whether both sides match	Recount atoms after every answer
42-45 min	Exit check	Collect answers and ask whether subscripts may be changed	No, only coefficients may change

03 Teaching script (read it out)

Scene 1: Words to symbols: position decides everything

Reactants go left of the arrow and products go right, which is the step everybody rushes. Methane plus oxygen gives carbon dioxide and water becomes CH4 and O2 on the left, CO2 and H2O on the right. Have students read the sentence aloud, saying plus and gives, and draw the arrow at the word gives, so the position falls out naturally. Get this wrong and no amount of careful balancing will save the answer.

Students say which side holds reactants and which products

Scene 2: Change coefficients, never subscripts

Balancing means making the count of each element equal on both sides by changing the numbers written in front, the coefficients. For H2 + O2 gives H2O, write 2 in front of H2O to get 2H2O, which needs 2H2 on the left for four hydrogens, and the oxygen count is then 2 on each side: 2H2 + O2 gives 2H2O. A student who writes H2O2 has not balanced anything, they have invented hydrogen peroxide. Say that sentence out loud and nobody does it twice.

Students explain why subscripts cannot be changed

Scene 3: Mass is conserved and energy leaves a trace

Atoms are neither created nor destroyed, so mass must balance: 100 g of calcium carbonate giving 56 g of calcium oxide leaves 44 g of carbon dioxide. For energy, trust the thermometer only: surroundings get warmer means exothermic, such as combustion, neutralisation and magnesium with acid; surroundings get colder means endothermic, such as heating calcium carbonate or dissolving ammonium nitrate. Insist on two separate steps, the mass equation first, then the energy judgement.

Students give one exothermic and one endothermic example

04 Question types and worked steps

Type	Example	Steps	Answer
Words to symbols	Write magnesium plus hydrochloric acid giving magnesium chloride and hydrogen as an equation.	1) Mg + HCl on the left. 2) MgCl2 + H2 on the right. 3) Do not balance yet, just fix positions and formulae	Mg + HCl gives MgCl2 + H2
Balancing	Balance H2 + O2 giving H2O.	1) Write 2 in front of H2O. 2) Four hydrogens on the right need 2H2. 3) Oxygen is 2 on each side	2H2 + O2 gives 2H2O
Conservation of mass	100 g of calcium carbonate decomposes into 56 g of calcium oxide. What mass of carbon dioxide forms?	1) Write CaCO3 gives CaO + CO2. 2) Subtract the known product mass. 3) 100 minus 56	44 g
Exothermic or endothermic	Magnesium and acid take the temperature from 20 to 45 degrees Celsius. Which is it?	1) Compare the start and end temperatures. 2) A rise means heat is released. 3) The rise is 45 minus 20	Exothermic, a rise of 25 degrees

Sanity check (teach this): Three checks: first, count every element on both sides after balancing, carbon, hydrogen and oxygen alike; second, confirm no subscript was altered, because a different subscript means a different substance; third, check the mass, since reactants must equal products and a negative answer means the sides were swapped. Numeric anchors: 16 plus 64 equals 80, and 44 plus 36 also equals 80 for methane combustion; 100 equals 56 plus 44 for calcium carbonate; 24 plus 73 equals 95 plus 2 for magnesium and acid.

05 Independent practice (3 questions)

Question	Answer	What to watch
Balance H2 + O2 giving H2O.	2H2 + O2 gives 2H2O	Check H2O was not changed to H2O2
24 g of magnesium reacts fully with acid. What mass of magnesium chloride forms?	95 g	Check the 2 g of hydrogen was not forgotten
Dissolving ammonium nitrate lowers the water temperature. Exothermic or endothermic?	Endothermic	Check a fall in temperature was not called exothermic

06 Common mistakes and what to say

Mistake	Student writes	Say this
Changing subscripts to balance	H2 + O2 gives H2O2	Change coefficients only: 2H2 + O2 gives 2H2O
Reactants and products swapped	Writing CO2 + H2O gives CH4 + O2	Follow the word gives and point the arrow from reactants to products
Dropping a product in mass work	24 g of magnesium giving 95 g with the hydrogen ignored	Every product counts: 24 plus 73 equals 95 plus 2
Judging energy by feel	Touching the tube and guessing	Read the thermometer: a rise is exothermic, a fall is endothermic

07 Homework (paper, section and question numbers)

Combustion and oxidation, question 2 — Same atom-counting explanation of a mass change
Acids, alkalis and neutralisation, question 3 — Neutralisation is the commonest exothermic example
Energy forms and conversion, question 4 — Chemical energy converts into heat and light

08 Exit check (last 5 minutes)

Question	Answer	Decision
Balance CH4 + O2 giving CO2 + H2O.	CH4 + 2O2 gives CO2 + 2H2O	Answering CH4 + O2 gives CO2 + H2O means hydrogen and oxygen were never counted
100 g of calcium carbonate gives 56 g of calcium oxide. Mass of carbon dioxide?	44 g	Answering 156 g means the masses were added instead of subtracted
Photosynthesis absorbs light energy. Exothermic or endothermic?	Endothermic	Answering exothermic means absorbing energy was read as releasing it

09 Tutor FAQ

Must coefficients be the smallest whole numbers?

Yes. 4H2 + 2O2 giving 4H2O is mathematically fine but not accepted, because the smallest whole-number ratio is required.

Are conservation of mass and energy related?

Both are conservation laws, but at junior level keep them separate: mass is explained with atom counts, energy with temperature changes.

Should equations be memorised?

Only the common ones: combustion, neutralisation and the decomposition of calcium carbonate. Everything else follows from words to symbols, then balancing.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.