IGCSE Chemistry 0620 · Topic 7: Acids, Bases and Salts
Acids, Bases and Salts
Acids and bases neutralise each other to form salts. Which method you use to prepare a given salt depends entirely on one property: is it soluble in water or not?
1. Key Reactions of Acids
acid + metal → salt + hydrogen
acid + base (oxide/hydroxide) → salt + water
acid + carbonate → salt + water + carbon dioxide
2. pH and Strength
The pH scale runs from 0 (strongly acidic) to 14 (strongly alkaline), with 7 neutral. A strong acid ionises completely in water; a weak acid only partially ionises, so a weak acid of the same concentration has a higher pH than a strong acid.
3. Preparing Salts
Salt type
Method
Soluble salt
React acid with excess insoluble base/carbonate/metal, filter off the excess, then evaporate and crystallise the solution
Insoluble salt
Precipitation — mix two soluble solutions so the desired salt forms as a solid, then filter, wash, and dry
Solubility rules to remember
All Group I and ammonium compounds are soluble. All nitrates are soluble. Most chlorides are soluble, except silver chloride and lead(II) chloride. Most sulfates are soluble, except barium sulfate and lead(II) sulfate. Most carbonates are insoluble, except Group I carbonates and ammonium carbonate.
Worked example — finding the formula of a hydrated salt
2.50 g of hydrated copper(II) sulfate, CuSO₄·xH₂O, is heated until all the water of crystallisation is driven off, leaving 1.60 g of anhydrous CuSO₄. Find x. (Ar: Cu=64, S=32, O=16, H=1; Mr CuSO₄=160, Mr H₂O=18)
Mass of water lost = 2.50−1.60 = 0.90 g. Moles of water = 0.90/18 = 0.050 mol.
Moles of CuSO₄ = 1.60/160 = 0.010 mol.
Ratio of water : CuSO₄ = 0.050 : 0.010 = 5 : 1 → x = 5 (CuSO₄·5H₂O)
Common mistakes to watch for
Trying to make an insoluble salt using the “excess solid + filter” method — that only works for soluble salts.
Assuming a weak acid is automatically “safe” — weak acids can still be corrosive, they just don’t ionise fully.
Forgetting the acid+carbonate reaction always produces a gas (CO₂) as well as a salt and water.
Key Vocabulary Recap
Neutralisation — reaction of an acid with a base to form a salt and water
Water of crystallisation — water chemically bonded within a crystal’s structure
Syllabus Reference
This note covers IGCSE Chemistry 0620 Topic 7, Acids, Bases and Salts: properties of acids and bases, pH, neutralisation, strong and weak acids, oxides, salt preparation, solubility rules, and hydrated/anhydrous substances.
Practice questions
IGCSE Chemistry 0620 · Topic 7: Acids, Bases and Salts — Practice Question Bank (no answers)
Acids, Bases and Salts — Practice
Covers: acid reactions, pH, salt preparation, and hydrated salts. 10 questions.
Q1 Core
Write the general word equation for the reaction between an acid and a metal.
Q2 Core
Write the general word equation for the reaction between an acid and a carbonate.
Q3 Core
State the pH range that is acidic, the pH range that is alkaline, and the pH value that is neutral.
Q4 Extended
Explain the difference between a strong acid and a weak acid.
Q5 Core
Describe the method used to prepare a pure, dry sample of a soluble salt from an insoluble base and an acid.
Q6 Core
Describe the method used to prepare an insoluble salt.
Q7 Core
State whether lead(II) chloride and sodium chloride are soluble or insoluble in water.
Q8 Extended
2.50 g of hydrated copper(II) sulfate, CuSO₄·xH₂O, is heated until only 1.60 g of anhydrous CuSO₄ remains. Find x. (Mr CuSO₄=160, Mr H₂O=18)
Q9 Core
Classify each oxide as acidic or basic: (a) sodium oxide, (b) sulfur dioxide.
Q10 Core
Explain what is meant by “water of crystallisation.”
✓A strong acid ionises (dissociates) completely in water, releasing the maximum possible number of H⁺ ions. A weak acid only partially ionises, so a solution of a weak acid contains mostly un-ionised acid molecules and has a higher pH than a strong acid of the same concentration.
Q5
Method for preparing a soluble salt.
✓Add excess insoluble base to the acid and warm/stir until no more reacts (excess base remains undissolved). Filter to remove the excess unreacted base. Gently heat the filtrate to evaporate some water and form a saturated solution, then leave it to cool and crystallise. Filter off the crystals and dry them.
Q6
Method for preparing an insoluble salt.
✓Mix two suitable soluble solutions (one providing each ion of the desired salt) so that the insoluble salt precipitates out as a solid. Filter the mixture to collect the precipitate, then wash it with distilled water and dry it.
Q7
Solubility of PbCl₂ and NaCl.
✓Lead(II) chloride is insoluble. Sodium chloride is soluble (all Group I compounds are soluble).
Q8
Find x in CuSO₄·xH₂O.
1
Mass of water lost = 2.50−1.60 = 0.90g. Moles of water
0.90÷18=0.050 mol
2
Moles of CuSO₄
1.60÷160=0.010 mol
✓Ratio = 0.050:0.010 = 5:1, so x = 5
Q9
Classify Na₂O and SO₂.
✓(a) Sodium oxide — basic (a metal oxide). (b) Sulfur dioxide — acidic (a non-metal oxide).
Q10
What is water of crystallisation?
✓Water of crystallisation is water chemically combined in fixed proportions within the crystal structure of certain salts (e.g. CuSO₄·5H₂O). Heating drives this water off, leaving the anhydrous salt.
