Acids, Bases and Salts: IGCSE Chemistry 0620

Acids, bases and salts appears on every 0620 paper: definitions and pH on the multiple choice, salt preparation and neutralisation equations on Papers 3 and 4, and titration technique on Paper 6. Examiner reports repeat the same complaint year after year: students know the chemistry but describe salt preparation steps out of order, or write “add acid” without saying which acid or how they know the reaction is finished. The marks here are method marks, and method marks need precise wording.

Acids, bases and indicators

A Core definition you must state exactly: acids produce hydrogen ions, H+, in aqueous solution; alkalis produce hydroxide ions, OH−. Bases are oxides or hydroxides of metals that neutralise acids; alkalis are the soluble bases.

The characteristic reactions of acids earn equation marks across the whole syllabus:

• acid + metal → salt + hydrogen
• acid + base → salt + water
• acid + carbonate → salt + water + carbon dioxide

Indicators are tested as straight recall. Litmus turns red in acid and blue in alkali. Thymolphthalein is colourless in acid and blue in alkali. Methyl orange is red in acid and yellow in alkali. Write both colours: “methyl orange changes colour” scores nothing.

At Supplement level (S), acids are defined as proton donors and bases as proton acceptors. In the reaction between HCl and water, HCl donates a proton to H2O. Supplement students must also separate strong acids (fully dissociated in solution, like hydrochloric, sulfuric and nitric acid) from weak acids (partially dissociated, like ethanoic acid). A weak acid of the same concentration has a higher pH and reacts more slowly. Link this to rate of reaction and you have a classic Paper 4 question.

The pH scale

pH runs from below 0 to 14. Below 7 is acidic, 7 is neutral, above 7 is alkaline. Universal indicator gives an approximate pH from its colour: red around pH 1–2, orange and yellow through the weakly acidic range, green at 7, blue around 9–11, violet at 13–14. The key exam link: the lower the pH, the higher the concentration of H+ ions (S). Questions also ask how farmers control soil acidity: calcium oxide or calcium hydroxide (lime) neutralises acidic soil.

Oxides

Classify oxides by their behaviour with acids and alkalis. Metal oxides are basic: they neutralise acids. Non-metal oxides such as CO2 and SO2 are acidic: they react with alkalis. At Supplement level (S), amphoteric oxides react with both: aluminium oxide and zinc oxide are the two named examples. A one-mark definition question on “amphoteric” appears regularly on Paper 4, and the answer must mention reacting with both acids and bases.

Preparation of salts

This is the heart of the topic and the most common 4–6 mark question. The Core method makes a soluble salt from an acid and an excess of an insoluble base, metal or carbonate:

1. Warm the dilute acid, then add the solid in excess.
2. The excess shows all the acid has reacted (unreacted solid remains visible).
3. Filter to remove the excess solid.
4. Heat the filtrate to evaporate water until crystals start to form (the point of crystallisation), then leave to cool and crystallise.
5. Dry the crystals between filter paper.

Each numbered step is typically one mark. “Boil until dry” loses the crystallisation mark. Hydrated salts like CuSO4·5H2O lose their water of crystallisation if overheated.

At Supplement level (S), you choose the method from solubility. All nitrates are soluble; all sodium, potassium and ammonium salts are soluble; chlorides are soluble except silver and lead; sulfates are soluble except barium, calcium and lead; carbonates and hydroxides are insoluble except sodium, potassium and ammonium. Insoluble salts are made by precipitation: mix two soluble solutions, filter, wash the residue with distilled water, dry. Soluble salts of sodium, potassium and ammonium are made by titration, because there is no insoluble solid to add in excess. The definitions of hydrated, anhydrous and water of crystallisation are also Supplement.

Titration and solubility

The titration method is examined on Paper 6 as much as on the theory papers. Use a pipette to measure a fixed volume of alkali into a conical flask, add a few drops of indicator, then add acid from a burette until the indicator just changes colour. Read the burette at eye level, repeat until you have concordant titres within 0.1 cm³, and use the mean of concordant results. The calculation that follows (moles, ratio, concentration) is pure stoichiometry, and it is where Extended candidates separate themselves.

Salt identification questions overlap heavily with qualitative analysis ion and gas tests, so revise the two together: prepare the salt, then prove which salt you made.

Worked exam question

Question (Paper 4 style, 5 marks). Describe how you would prepare a pure, dry sample of copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid.

Model answer. Warm the dilute sulfuric acid. Add copper(II) oxide a little at a time until it is in excess and black solid remains. Filter the mixture to remove the unreacted copper(II) oxide. Heat the filtrate until crystals begin to form on a glass rod, then leave the solution to cool so blue crystals form. Dry the crystals between sheets of filter paper.

Mark-by-mark breakdown.

• M1: add copper(II) oxide to the (warm) acid until in excess.
• M2: reason or evidence for excess: ensures all the acid is used up / solid remains.
• M3: filter to remove the excess copper(II) oxide.
• M4: evaporate to the point of crystallisation, then cool to crystallise (not “evaporate to dryness”).
• M5: dry the crystals with filter paper (or in a warm oven at low temperature).

Five marks, five sequenced actions. Examiners report that the most commonly lost marks are M2 (no reason for the excess) and M4 (boiling to dryness).

The mistakes that cost marks

1. Evaporating to dryness. Hydrated copper(II) sulfate decomposes to the white anhydrous powder. The mark scheme wants “heat to the point of crystallisation, then cool”.
2. Confusing strong with concentrated. Strong = fully dissociated (S); concentrated = lots of acid per dm³. Using one word for the other loses the mark outright.
3. Giving one indicator colour. “Litmus turns blue” only scores if you state the conditions: blue in alkali. State the colour in acid and in alkali.
4. Wrong method for the salt. Trying to make barium sulfate (insoluble) by the excess-base method, or sodium chloride by adding excess solid. Check solubility first, then choose: precipitation, excess insoluble base, or titration.
5. Titration calculations without the mole ratio. Students compute moles of acid then copy the number across. If the equation says 2NaOH : 1H2SO4, the ratio mark is a real mark. Show it.

How to phrase it for full marks

Student wording	Mark-scheme wording
”Add the base until it stops reacting"	"Add the base in excess so that all the acid is used up"
"Boil off the water"	"Heat until crystals start to form, then leave to cool and crystallise"
"The acid is strong because there’s a lot of it"	"The acid is strong because it is fully dissociated into ions in solution"
"pH shows how acidic it is"	"The lower the pH, the higher the concentration of H+ ions"
"Filter it"	"Filter to remove the excess (named) solid”

The pattern: name the substance, state the purpose of each step, and use the syllabus word (excess, dissociated, crystallisation, filtrate, residue).

The Malaysia note

In Malaysian international schools, this topic usually lands in Year 10, well before students have rehearsed full practical write-ups, so by the May/June exam in Year 11, the salt-prep method has gone fuzzy. Students who also sat SPM-track science sometimes carry over kertas exam habits of one-word answers; 0620 mark schemes want full method sentences. We rebuild this topic with past-paper drills until the five-step method is automatic, and you can see how that works in a free trial lesson before committing to anything.

Every sub-topic in Acids, Bases and Salts

• Acids, Bases and Indicators
• The pH Scale
• Oxides
• Preparation of Salts
• Titration and Solubility