Edexcel GCSE Chemistry Equations You Must Know
Edexcel GCSE Chemistry Equations You Must Know
Equations are the language of chemistry. In the Edexcel GCSE Chemistry (1CH0) exam, you are expected to recall, write and balance a significant number of word equations and symbol equations across both papers. There is no equation sheet provided, so you need to know them from memory.
This guide lists every key equation organised by reaction type, with the general word equation first, then specific balanced symbol equations. Each section includes a note on where the equation appears in the specification and common mistakes to avoid. If you learn these thoroughly, you will be well prepared for any equation-based question the exam throws at you.
1. Acid Reactions
Acid reactions form the backbone of Topic 3 (Chemical Changes). Examiners frequently test whether you can predict products and write balanced equations for acids reacting with metals, metal oxides, metal hydroxides and metal carbonates.
Acid + Metal
Word equation: acid + metal -> salt + hydrogen
Symbol equations:
- Mg + 2HCl -> MgCl2 + H2
- Zn + H2SO4 -> ZnSO4 + H2
- Fe + 2HCl -> FeCl2 + H2
Exam note: You may be asked to predict the salt produced. Remember: hydrochloric acid produces chlorides, sulfuric acid produces sulfates, and nitric acid produces nitrates. A common error is forgetting to balance the HCl -- magnesium needs two moles of HCl because it forms a 2+ ion.
Acid + Metal Oxide
Word equation: acid + metal oxide -> salt + water
Symbol equations:
- CuO + H2SO4 -> CuSO4 + H2O
- MgO + 2HCl -> MgCl2 + H2O
- ZnO + H2SO4 -> ZnSO4 + H2O
Exam note: This reaction is a neutralisation. The metal oxide acts as a base. Questions often ask you to describe how to make a pure, dry sample of a copper sulfate crystal -- this reaction is the starting point for that required practical.
Acid + Metal Hydroxide
Word equation: acid + metal hydroxide -> salt + water
Symbol equations:
- NaOH + HCl -> NaCl + H2O
- 2NaOH + H2SO4 -> Na2SO4 + 2H2O
- Ca(OH)2 + 2HCl -> CaCl2 + 2H2O
Exam note: This is another neutralisation reaction. Sodium hydroxide and hydrochloric acid is a classic titration pair. Watch the balancing when sulfuric acid reacts with NaOH -- you need two moles of NaOH for every mole of H2SO4.
Acid + Metal Carbonate
Word equation: acid + metal carbonate -> salt + water + carbon dioxide
Symbol equations:
- CaCO3 + 2HCl -> CaCl2 + H2O + CO2
- Na2CO3 + H2SO4 -> Na2SO4 + H2O + CO2
- MgCO3 + 2HCl -> MgCl2 + H2O + CO2
Exam note: These reactions produce three products. Students often forget the carbon dioxide or fail to balance the HCl. The fizzing observed when acid is added to a carbonate is a standard observation question.
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2. Combustion
Combustion equations appear in Topic 8 (Fuels and Earth Science). You need to know both complete and incomplete combustion.
Complete Combustion
Word equation: hydrocarbon + oxygen -> carbon dioxide + water
Symbol equations:
- CH4 + 2O2 -> CO2 + 2H2O
- C3H8 + 5O2 -> 3CO2 + 4H2O
- C2H6 + 3.5O2 -> 2CO2 + 3H2O (or multiply through: 2C2H6 + 7O2 -> 4CO2 + 6H2O)
Exam note: Balance carbon first, then hydrogen, then oxygen last. If you end up with a fractional coefficient for O2, multiply the entire equation by 2 to get whole numbers. Examiners accept either form at GCSE, but whole numbers are safer.
Incomplete Combustion
Word equation: hydrocarbon + limited oxygen -> carbon monoxide (+ carbon) + water
Symbol equations:
- 2CH4 + 3O2 -> 2CO + 4H2O
- CH4 + O2 -> C + 2H2O (severe oxygen shortage)
Exam note: Incomplete combustion produces carbon monoxide (toxic) and possibly carbon (soot). Questions often ask you to explain why incomplete combustion is dangerous or why it wastes fuel. You are expected to know that CO is a poisonous gas because it binds to haemoglobin.
3. Thermal Decomposition
Thermal decomposition appears in Topics 1 and 4. You must know that many metal carbonates decompose on heating, and this is a standard core practical context.
Word equation: metal carbonate -> metal oxide + carbon dioxide
Symbol equations:
- CaCO3 -> CaO + CO2
- ZnCO3 -> ZnO + CO2
- CuCO3 -> CuO + CO2
- MgCO3 -> MgO + CO2
Exam note: These equations are straightforward to balance because they are already balanced as written. The key exam skill is linking the observable change (colour change for copper carbonate from green to black, limewater turning milky from the CO2) to the equation. Higher-tier questions may ask you to compare the thermal stability of different metal carbonates.
4. Displacement Reactions
Displacement reactions feature in Topic 4 (reactivity series) and Topic 3 (Group 7 halogens). A more reactive element displaces a less reactive one from a compound.
Metal Displacement
Word equation: more reactive metal + salt solution of less reactive metal -> new salt + less reactive metal
Symbol equations:
- Mg + CuSO4 -> MgSO4 + Cu
- Fe + CuSO4 -> FeSO4 + Cu
- Zn + CuSO4 -> ZnSO4 + Cu
Exam note: You should be able to use the reactivity series to predict whether a displacement reaction will occur. If the metal is higher in the reactivity series than the metal in the compound, displacement happens. A common error is writing the wrong formula for the new salt -- make sure the charges balance.
Halogen Displacement
Word equation: more reactive halogen + halide salt -> new halide salt + less reactive halogen
Symbol equations:
- Cl2 + 2KBr -> 2KCl + Br2
- Cl2 + 2KI -> 2KCl + I2
- Br2 + 2KI -> 2KBr + I2
Exam note: Reactivity decreases as you go down Group 7: chlorine displaces bromine and iodine, bromine displaces iodine, but iodine cannot displace either. Observation questions will ask about colour changes in solution.
5. Group 1 Reactions
Group 1 alkali metals reacting with water is a core part of Topic 6 (Groups in the Periodic Table). These equations follow an identical pattern.
Word equation: alkali metal + water -> metal hydroxide + hydrogen
Symbol equations:
- 2Li + 2H2O -> 2LiOH + H2
- 2Na + 2H2O -> 2NaOH + H2
- 2K + 2H2O -> 2KOH + H2
Exam note: The pattern is always the same: 2M + 2H2O -> 2MOH + H2. Examiners test whether you can describe the observations (fizzing, metal moves on the surface, dissolves, potassium may ignite with a lilac flame) and link them to the equation. Reactivity increases down Group 1.
6. Electrolysis Half-Equations
Half-equations appear in Topic 3 (Chemical Changes) and are considered one of the more challenging areas of the specification. You need to write separate equations for what happens at each electrode.
At the Cathode (Reduction -- Gain of Electrons)
- Cu2+ + 2e- -> Cu (electrolysis of copper sulfate solution)
- 2H+ + 2e- -> H2 (electrolysis of acidified water or dilute acid)
- Al3+ + 3e- -> Al (extraction of aluminium from aluminium oxide)
At the Anode (Oxidation -- Loss of Electrons)
- 2Cl- -> Cl2 + 2e- (electrolysis of brine)
- 4OH- -> 2H2O + O2 + 4e- (electrolysis of aqueous solutions when halide ions are absent)
- 2O2- -> O2 + 4e- (extraction of aluminium)
Exam note: Remember the mnemonic OILRIG: Oxidation Is Loss, Reduction Is Gain (of electrons). At the cathode, positive ions gain electrons (reduction). At the anode, negative ions lose electrons (oxidation). The most common mistake is getting the electron direction wrong or forgetting to balance the charges. Check that the total charge on each side of the equation is equal.
7. Test Reactions
The specification requires you to know chemical tests for specific gases and substances. These appear across both papers and are commonly tested in the context of core practicals.
Test for Hydrogen
Equation: 2H2 + O2 -> 2H2O
Method: Hold a burning splint at the mouth of a test tube. A squeaky pop confirms hydrogen.
Test for Carbon Dioxide
Equation: CO2 + Ca(OH)2 -> CaCO3 + H2O
Method: Bubble the gas through limewater. If it turns milky (cloudy), carbon dioxide is present. The cloudiness is caused by the formation of insoluble calcium carbonate.
Test for Chlorine
Method: Chlorine bleaches damp litmus paper, turning it white. No specific equation is required, but you should know that chlorine is a toxic, yellow-green gas.
Test for Oxygen
Method: Hold a glowing splint in the gas. It relights if oxygen is present.
Exam note: These tests are short, but the marks add up. Make sure you state the correct test method and the expected observation. Saying "the splint pops" without specifying "burning splint" and "squeaky pop" can cost a mark.
8. Key Formulae for Calculations
Topic 1 and Topic 9 (Separate Chemistry 2) include quantitative chemistry. You will not be given these formulae, so learn them.
- Moles = mass / Mr
- Concentration (mol/dm3) = moles / volume (dm3)
- Concentration (g/dm3) = mass / volume (dm3)
- Atom economy = (Mr of desired product / sum of Mr of all products) x 100
- Percentage yield = (actual yield / theoretical yield) x 100
- Rf value = distance moved by substance / distance moved by solvent
Exam note: Calculation questions typically carry 3 to 4 marks. Show every step of your working clearly. If you make an arithmetic error but your method is correct, you can still pick up method marks. Always include units in your final answer. A common mistake with concentration is confusing mol/dm3 with g/dm3 -- read the question carefully to see which unit is required.
9. Cracking
Cracking is part of Topic 8 (Fuels and Earth Science). It is the process of breaking long-chain hydrocarbons into shorter, more useful molecules.
Word equation: long-chain alkane -> shorter alkane + alkene
Symbol equations:
- C10H22 -> C8H18 + C2H4
- C10H22 -> C7H16 + C3H6
- C6H14 -> C4H10 + C2H4
Exam note: The key rule is conservation of atoms -- the total number of carbon and hydrogen atoms on each side must be equal. Cracking always produces at least one alkene (which contains a C=C double bond). Questions may ask you to identify the products or explain why cracking is economically important (short-chain hydrocarbons are more useful as fuels, and alkenes are used to make polymers).
10. Oxidation and Reduction (Ionic Equations)
Redox concepts run through multiple topics but are tested most directly in Topic 4 (electrolysis and extraction) and Topic 3 (displacement reactions).
Oxidation (loss of electrons):
- M -> M(n+) + ne-
- Example: Zn -> Zn2+ + 2e-
- Example: Fe -> Fe2+ + 2e-
Reduction (gain of electrons):
- M(n+) + ne- -> M
- Example: Cu2+ + 2e- -> Cu
- Example: Ag+ + e- -> Ag
Exam note: You may be asked to write ionic equations for displacement reactions. For example, when zinc displaces copper from copper sulfate solution, the ionic equation is: Zn + Cu2+ -> Zn2+ + Cu. The sulfate ion is a spectator ion and does not appear. Higher-tier questions often ask you to identify what has been oxidised and what has been reduced, and to justify your answer in terms of electron transfer.
How to Balance Equations: A Step-by-Step Method
Balancing equations is a skill that comes up repeatedly across the entire specification. Here is a reliable method.
Step 1: Write the unbalanced equation. Start with the correct formulae for all reactants and products. Never change a formula to balance an equation -- only change the numbers (coefficients) in front of formulae.
Step 2: Count atoms on each side. Make a list of every element and count how many atoms of each appear on the left and right.
Step 3: Balance one element at a time. Start with the element that appears in the fewest formulae. Adjust coefficients to make the atom count equal on both sides.
Step 4: Leave hydrogen and oxygen until last. These often appear in multiple compounds, so they are easier to balance once everything else is in place.
Step 5: Do a final check. Count every atom on both sides one more time. Make sure all coefficients are whole numbers and are in the simplest ratio.
Common mistakes to avoid:
- Changing the small (subscript) numbers in a formula. You must never do this -- H2O is water, H2O2 is hydrogen peroxide. Changing the subscript changes the substance.
- Forgetting to multiply through brackets. In Ca(OH)2, there are 2 oxygen atoms and 2 hydrogen atoms, not 1 of each.
- Leaving coefficients as fractions. While technically correct, GCSE mark schemes expect whole-number coefficients.
Worked example:
Balance: Fe + O2 -> Fe2O3
- Count: Left has 1 Fe and 2 O. Right has 2 Fe and 3 O.
- Balance Fe: put 2 in front of Fe. Now left has 2 Fe and 2 O. Right has 2 Fe and 3 O.
- Balance O: you need 3 O on the left, but O2 comes in pairs. The lowest common multiple of 2 and 3 is 6. Put 3 in front of O2 (giving 6 O) and 2 in front of Fe2O3 (giving 6 O and 4 Fe).
- Re-balance Fe: put 4 in front of Fe.
- Final equation: 4Fe + 3O2 -> 2Fe2O3
- Check: 4 Fe on each side, 6 O on each side. Balanced.
Pulling It All Together
The equations in this guide cover the core reactions you need for both Paper 1 and Paper 2 of Edexcel GCSE Chemistry. Do not try to memorise them all in one sitting. Instead, work through them topic by topic as you revise each area of the specification. Write them out by hand, test yourself using flashcards, and practise applying them in exam-style questions.
When you sit down with a past paper, you will find that the same types of equations appear again and again. Once you recognise the patterns -- acid plus carbonate always gives three products, Group 1 metals always follow the 2M + 2H2O pattern, combustion always starts with balancing carbon -- the equations become second nature.
For a broader overview of the specification, see our Edexcel GCSE Chemistry Revision Guide. If you want to understand what examiners are looking for in your written answers, read Edexcel GCSE Exam Command Words Explained and How Edexcel Mark Schemes Work.
Explore all Edexcel GCSE Chemistry courses on LearningBro and start practising with exam-style questions today.