IGCSE Chemistry 0620 · Night-before sheet · ~13 min read
Chemistry Facts, Equations & Tests
Every test and colour, the mole calculations, key equations, industrial processes and the reactivity series on one sheet. Learn the tests word for word, because examiners mark the exact observation.
TestsColoursMolesEquationsElectrolysisIndustryOrganic
RSby Riyaz Sir
mentorriyaz.in
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againHANDY worth memorisingTest yourself at the end
In 30 seconds
- Tests are worth easy marks if you write the reagent, the observation and the colour change. "Limewater turns milky" scores. "It goes cloudy" often does not.
- Mole calculations all start with n = m ÷ Mr. Turn everything into moles first, use the equation ratio, then turn the answer back into grams, dm³ or concentration.
- Know the conditions for the Haber and Contact processes, the extraction of iron and aluminium, and what is made at each electrode.
1Tests to learn word for word
Tests for gases
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen, H₂ | lighted splint | squeaky pop |
| Oxygen, O₂ | glowing splint | relights |
| Carbon dioxide, CO₂ | bubble through limewater | turns milky |
| Ammonia, NH₃ | damp red litmus paper | turns blue |
| Chlorine, Cl₂ | damp litmus paper | bleached white |
| Sulfur dioxide, SO₂ | acidified aqueous potassium manganate(VII) | purple to colourless |
- Water: anhydrous copper(II) sulfate turns white to blue. Anhydrous cobalt(II) chloride turns blue to pink.
- Unsaturated compound (alkene): bromine water turns from orange to colourless.
Tests for cations and flame colours
- Ammonium, NH₄⁺: add NaOH(aq) and warm gently. Ammonia gas is given off and turns damp red litmus blue.
- Zinc or aluminium? Both give a white precipitate that dissolves in excess NaOH. Use NH₃(aq): the zinc precipitate dissolves in excess, but the aluminium one does not.
- Copper(II) with excess ammonia: the light blue precipitate dissolves to give a deep blue solution.
- Flame tests: use a clean nichrome wire or splint. Say the colour of the flame, not just "it burns".
Tests for anions
| Ion | Test | Positive result |
|---|---|---|
| Carbonate, CO₃²⁻ | add dilute acid | fizzing, gas turns limewater milky |
| Chloride, Cl⁻ | dilute nitric acid, then silver nitrate | white precipitate |
| Bromide, Br⁻ | dilute nitric acid, then silver nitrate | cream precipitate |
| Iodide, I⁻ | dilute nitric acid, then silver nitrate | yellow precipitate |
| Sulfate, SO₄²⁻ | dilute nitric acid, then barium nitrate | white precipitate |
| Nitrate, NO₃⁻ | NaOH(aq) and aluminium foil, warm | ammonia gas (turns damp red litmus blue) |
- Why acidify first? Dilute nitric acid removes carbonate ions, which would give a false white precipitate with silver or barium ions.
Indicators and pH
| pH | Universal indicator colour | Type |
|---|---|---|
| 0 to 2 | red | strong acid |
| 3 to 6 | orange to yellow | weak acid |
| 7 | green | neutral |
| 8 to 11 | blue | weak alkali |
| 12 to 14 | purple | strong alkali |
- Litmus: red in acid, blue in alkali. Methyl orange: red in acid, yellow in alkali. Phenolphthalein: colourless in acid, pink in alkali.
Exam tipWrite the reagent AND the observation AND the colour change. "Limewater turns milky" scores, but "it goes cloudy" often does not.
2Moles and calculations
Mole map
n = mMr
m in grams, Mr = relative formula massn = V24
V in dm³ for a gas at room temperature and pressuren = c × V
c in mol/dm³ and V in dm³. Divide cm³ by 1000percentage yield = actual yieldtheoretical yield × 100
always less than 100%percentage purity = mass of pure substancetotal mass of sample × 100
- Empirical formula EXT: divide each mass (or percentage) by its Ar, divide by the smallest answer, then make whole numbers.
- Limiting reactant: the substance that runs out first decides how much product forms.
Worked example
Calculate the mass of 0.25 mol of calcium carbonate, CaCO₃ (Ar: Ca 40, C 12, O 16).
- Mr = 40 + 12 + (3 × 16) = 100
- mass = n × Mr = 0.25 × 100 = 25 g
Worked example
What volume of carbon dioxide at room temperature and pressure forms when 5.0 g of calcium carbonate reacts with excess acid? CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
- n(CaCO₃) = 5.0 ÷ 100 = 0.05 mol
- ratio 1 : 1 so n(CO₂) = 0.05 mol
- V = 0.05 × 24 = 1.2 dm³
Worked example
4.0 g of sodium hydroxide is dissolved to make 500 cm³ of solution. Find the concentration in mol/dm³.
- Mr(NaOH) = 23 + 16 + 1 = 40
- n = 4.0 ÷ 40 = 0.10 mol
- V = 500 ÷ 1000 = 0.50 dm³
- c = 0.10 ÷ 0.50 = 0.20 mol/dm³
Worked example
25.0 cm³ of 0.100 mol/dm³ NaOH is neutralised by 20.0 cm³ of hydrochloric acid, HCl. Find the concentration of the acid.
- n(NaOH) = 0.100 × 0.0250 = 0.00250 mol
- NaOH + HCl → NaCl + H₂O, ratio 1 : 1, so n(HCl) = 0.00250 mol
- c(HCl) = 0.00250 ÷ 0.0200 = 0.125 mol/dm³
Worked example
Find the mass of magnesium needed to make 10 g of magnesium oxide. 2Mg + O₂ → 2MgO (Ar: Mg 24, O 16)
- Mr(MgO) = 24 + 16 = 40
- n(MgO) = 10 ÷ 40 = 0.25 mol
- ratio 2 : 2 so n(Mg) = 0.25 mol
- mass = 0.25 × 24 = 6.0 g
Worked example
The theoretical yield of a product is 10.0 g but only 8.0 g is collected. Find the percentage yield.
- percentage yield = 8.0 ÷ 10.0 × 100 = 80%
Common mistakeUsing cm³ instead of dm³ in n = c × V. Divide the volume in cm³ by 1000 first.
Common mistakeUsing the mass ratio from the equation. The numbers in an equation are mole ratios.
3Equations to know
Reactions and half-equations
- Acid + metal → salt + hydrogen. Acid + base → salt + water. Acid + carbonate → salt + water + carbon dioxide.
- Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Thermal decomposition of limestone: CaCO₃ → CaO + CO₂. Displacement: Mg + CuSO₄ → MgSO₄ + Cu.
- Iron extraction: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
- Haber process EXT: N₂ + 3H₂ ⇌ 2NH₃. Contact process EXT: 2SO₂ + O₂ ⇌ 2SO₃.
- At the cathode: Cu²⁺ + 2e⁻ → Cu, 2H⁺ + 2e⁻ → H₂ and Al³⁺ + 3e⁻ → Al. At the anode: 2Cl⁻ → Cl₂ + 2e⁻ and 4OH⁻ → 2H₂O + O₂ + 4e⁻. EXT
- Balance by changing the big numbers in front of formulas only, never the small numbers inside a formula.
4Industry and the reactivity series
Reactivity and extraction
- A more reactive metal displaces a less reactive one from its compound. Magnesium displaces copper from copper(II) sulfate, but copper cannot displace magnesium.
- Rusting needs both water and oxygen. Prevent it with paint, oil, plastic coating, galvanising (zinc) or sacrificial protection with a more reactive metal.
- Aluminium is protected by a thin layer of aluminium oxide, so it seems unreactive.
Industrial processes
| Process | Raw materials | Conditions | Product |
|---|---|---|---|
| Haber | nitrogen from air and hydrogen from natural gas | iron catalyst, 450 °C, 200 atm | ammonia |
| Contact | sulfur (burned to SO₂) and air | vanadium(V) oxide catalyst, 450 °C, 2 atm | sulfuric acid |
| Blast furnace | haematite, coke, limestone and hot air | carbon monoxide reduces iron(III) oxide | molten iron and slag |
| Aluminium | aluminium oxide dissolved in molten cryolite | electrolysis with carbon electrodes | aluminium at the cathode |
| Brine electrolysis | concentrated sodium chloride solution | electrolysis | hydrogen, chlorine and sodium hydroxide |
| Cracking | long-chain alkanes | high temperature and a catalyst | shorter alkanes and alkenes |
| Fermentation | glucose and yeast | about 30 °C, no oxygen | ethanol and carbon dioxide |
| Hydration of ethene | ethene and steam | phosphoric acid catalyst, 300 °C, 60 atm | ethanol |
- Slag in the blast furnace is calcium silicate. Limestone decomposes to CaO, which reacts with silicon dioxide impurities.
- The carbon anodes burn away in aluminium extraction, because oxygen reacts with them to make carbon dioxide, so they must be replaced.
Electrolysis rules
- PANIC: Positive Anode, Negative Is Cathode. OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
- Cathode (negative): a metal, or hydrogen if the metal is more reactive than hydrogen. Anode (positive): a halogen if a halide ion is present, otherwise oxygen.
- Molten ionic compound: the metal forms at the cathode and the non-metal forms at the anode.
- Copper electrodes in copper(II) sulfate solution: the anode dissolves and copper is deposited on the cathode. This purifies copper.
5Structure, bonding and organic facts
Structure and properties
| Structure | Examples | Melting point | Conducts electricity |
|---|---|---|---|
| Ionic lattice | sodium chloride | high | only when molten or dissolved |
| Simple molecular | water, carbon dioxide | low | never |
| Giant covalent | diamond, graphite, silicon dioxide | very high | graphite only, because of free electrons |
| Metallic | copper, iron | high | yes, in solid and liquid state |
- Group I metals become more reactive going down. Group VII elements become less reactive going down. Noble gases are unreactive because their outer shells are full.
- Air is about 78% nitrogen, 21% oxygen, 1% argon and 0.04% carbon dioxide.
Organic chemistry
| Family | General formula | Key feature |
|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | single bonds only (saturated) |
| Alkenes | CₙH₂ₙ | a C=C double bond (unsaturated) |
| Alcohols | contain the OH group | made by fermentation or hydration of ethene |
| Carboxylic acids | contain the COOH group | weak acids, react with carbonates |
- First four alkanes: methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. Alkenes: ethene C₂H₄ and propene C₃H₆.
- Complete combustion gives carbon dioxide and water. Incomplete combustion gives carbon monoxide (toxic) or carbon.
- Addition polymerisation: many alkene molecules join to form one long polymer. Poly(ethene) is made from ethene.
6Test yourself
Test yourself — cover the answers first
Quick check
- What is the test for hydrogen gas?
- What is the test for carbon dioxide?
- What colour is the precipitate of copper(II) ions with sodium hydroxide?
- A white precipitate forms with silver nitrate and dilute nitric acid. Which ion is present?
- What colour flame does a sodium compound give?
- Which gas is released at the anode in concentrated sodium chloride solution?
- Work out the number of moles in 4.4 g of CO₂ (Mr = 44).
- What volume does 0.5 mol of any gas occupy at room temperature and pressure?
- EXT What are the conditions of the Haber process?
- EXT Which catalyst is used in the Contact process?
- Why is aluminium extracted using electrolysis and not carbon?
- Which general formula do alkenes have?
Exam-style
- Calculate the volume of hydrogen at room temperature and pressure made when 2.4 g of magnesium reacts with excess dilute hydrochloric acid. Mg + 2HCl → MgCl₂ + H₂ (Ar: Mg 24) [3]
- 25.0 cm³ of 0.200 mol/dm³ sodium hydroxide is neutralised by 20.0 cm³ of sulfuric acid. 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O. Find the concentration of the acid. [3]
- Heating 12.5 g of zinc carbonate, ZnCO₃, gives zinc oxide, ZnO. The yield is 6.48 g. Find the percentage yield. (Ar: Zn 65, C 12, O 16) [4]
- A solution gives a white precipitate with NaOH that dissolves in excess. With NH₃(aq) the white precipitate also dissolves in excess. Name the metal ion and explain. [2]
Show answers
- lighted splint gives a squeaky pop
- it turns limewater milky
- light blue
- chloride, Cl⁻
- yellow
- chlorine
- 0.1 mol
- 12 dm³
- iron catalyst, 450 °C, 200 atm
- vanadium(V) oxide
- aluminium is more reactive than carbon
- CₙH₂ₙ
- n(Mg) = 2.4 ÷ 24 = 0.10 mol
ratio 1 : 1 so n(H₂) = 0.10 mol
V = 0.10 × 24 = 2.4 dm³ - n(NaOH) = 0.200 × 0.0250 = 0.00500 mol
ratio 2 : 1 so n(H₂SO₄) = 0.00250 mol
c = 0.00250 ÷ 0.0200 = 0.125 mol/dm³ - Mr(ZnCO₃) = 125 and Mr(ZnO) = 81
n(ZnCO₃) = 12.5 ÷ 125 = 0.100 mol
theoretical mass of ZnO = 0.100 × 81 = 8.10 g
percentage yield = 6.48 ÷ 8.10 × 100 = 80.0% - the ion is zinc, Zn²⁺
aluminium, Al³⁺ also dissolves in excess NaOH but its precipitate does not dissolve in excess ammonia
Stuck on a question?
Share with a friendAsk Riyaz SirSend it to me on WhatsApp. I read every message.
Want regular help? Send this to your parent →