Properties of acids and bases

This page covers one Form 4 content standard: the properties of acids and bases. It is the foundation of the whole Acids, Bases and Salts chapter, because every later idea, the pH scale, neutralisation, titration and the preparation of salts, depends on understanding what actually makes a substance behave as an acid or an alkali. The examiner tests both the definitions and the typical reactions, so both are set out carefully below.

What makes a substance an acid or a base

In SPM Chemistry (code 4541) the working definitions are based on the ions a substance produces in water.

• An acid is a chemical compound that ionises in water to produce hydrogen ions, H⁺. It is the hydrogen ion (in water, the hydroxonium ion H₃O⁺) that is responsible for all acidic properties.
• A base is a substance that reacts with an acid to form a salt and water. Common bases are metal oxides and metal hydroxides.
• An alkali is a base that dissolves in water to produce hydroxide ions, OH⁻. Every alkali is a base, but only the soluble bases are alkalis.

The key word in each definition is ion. An acid without water cannot ionise, so it produces no H⁺ and shows no acidic behaviour.

The essential role of water

This is the single most examined idea in the standard. A substance shows acidic or alkaline properties only when it is dissolved in water, because only then can it ionise to release free H⁺ or OH⁻ ions.

• Dry hydrogen chloride gas, or hydrogen chloride dissolved in a non-aqueous solvent such as dry methylbenzene, does not ionise. It has no free H⁺ ions, so it does not turn blue litmus red and does not react with a metal.
• The same hydrogen chloride dissolved in water ionises completely: HCl → H⁺ + Cl⁻. Now the free H⁺ ions give all the usual acidic properties.

So water is not just a background liquid, it is the medium that allows ionisation to happen. Remember this for the classic “compare the two solvents” question.

The typical reactions of acids

An acid in water reacts in three characteristic ways, and you should know a word equation and a balanced chemical equation for each:

1. Acid + reactive metal → salt + hydrogen gas. For example, Zn + H₂SO₄ → ZnSO₄ + H₂. The gas gives a “pop” with a lighted splint.
2. Acid + base (metal oxide or hydroxide) → salt + water. For example, 2HCl + CuO → CuCl₂ + H₂O. This is neutralisation.
3. Acid + metal carbonate → salt + water + carbon dioxide. For example, 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. The gas turns limewater milky.

An alkali reacts with acids (neutralisation) and with ammonium salts to release ammonia gas, and it turns red litmus blue. These reactions are the practical evidence for the definitions above.

Basicity and the strength idea

Two further ideas often appear. The basicity of an acid is the number of ionisable hydrogen atoms per molecule, HCl is monobasic (1), H₂SO₄ is dibasic (2), H₃PO₄ is tribasic (3). Separately, a strong acid ionises completely in water while a weak acid ionises only partially; the same distinction applies to strong and weak alkalis. Strength is about the degree of ionisation, not about concentration.

Worked example

Question. Hydrogen chloride is dissolved separately in (a) dry methylbenzene and (b) water. Blue litmus paper is dipped into each. State and explain the observation in each case. Then write the balanced equation for the reaction of the aqueous solution with magnesium, and name the gas produced.

Step 1, Solution in dry methylbenzene. Methylbenzene is a non-aqueous solvent, so the hydrogen chloride does not ionise and produces no H⁺ ions. The blue litmus stays blue, the solution is not acidic.

Step 2, Solution in water. In water the hydrogen chloride ionises: HCl → H⁺ + Cl⁻. Free H⁺ ions are present, so the blue litmus turns red, the solution is acidic.

Step 3, Reaction with magnesium. An acid reacts with a reactive metal to give a salt and hydrogen: Mg + 2HCl → MgCl₂ + H₂.

Answer. In methylbenzene the litmus stays blue (no ionisation, no H⁺); in water it turns red (H⁺ produced). With magnesium the balanced equation is Mg + 2HCl → MgCl₂ + H₂, and the gas is hydrogen, which gives a squeaky “pop” with a lighted splint.

Practice question

Question. Excess calcium carbonate is added to dilute hydrochloric acid. (a) Write the balanced equation. (b) Name the gas released and the test for it. (c) Calculate the volume of gas produced at room conditions when 0.10 mol of the acid reacts fully. [Molar volume of gas at room conditions = 24 dm³ mol⁻¹]

Answer. (a) 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. (b) The gas is carbon dioxide; it turns limewater milky. (c) From the equation, 2 mol HCl gives 1 mol CO₂, so 0.10 mol HCl gives 0.050 mol CO₂. Volume = mol × molar volume = 0.050 × 24 dm³ mol⁻¹ = 1.2 dm³ at room conditions.

Exam tip

When a question mentions two different solvents, the mark is almost always about the role of water: say clearly that the substance ionises in water to produce H⁺ (or OH⁻) ions, and does not ionise in the non-aqueous solvent, so it shows acidic (or alkaline) properties only in the aqueous case. Never write that “the acid is stronger in water”, that confuses strength with ionisation. And keep the three definitions distinct: acid produces H⁺; a base reacts with an acid; an alkali is a soluble base producing OH⁻.

Reading the standard for the exam

Almost every question on this standard reduces to two skills. The first is choosing the correct ion: acidic properties come from H⁺, alkaline properties from OH⁻. If you can name the ion responsible, most explanation marks fall into place. The second is matching a reaction to its observation: hydrogen “pops”, carbon dioxide turns limewater milky, and ammonia (from an alkali plus an ammonium salt) turns damp red litmus blue and has a pungent smell. Learn these gas tests as fixed pairs, because Paper 2 loves to combine a property with the practical evidence for it.

You should also be ready to classify substances. Given a list, hydrochloric acid, ethanoic acid, sodium hydroxide, ammonia solution, copper(II) oxide, you should be able to say which are acids, which are alkalis, which is an insoluble base, and which contain the same responsible ion. This classification is quick marks once the definitions are secure, and it connects directly to the pH scale in the next standard, where the concentration of H⁺ or OH⁻ is turned into a number.

Where this fits

This is content standard 6.1, the opening of the Acids, Bases and Salts chapter. It underpins the pH scale, neutralisation and salt preparation, so it is worth securing first. Consolidate it with the chapter revision notes and practise the reaction equations in the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure every property answer names the ion responsible and pairs each reaction with its gas test, which is where students most often drop easy marks in this standard.

Quick recap

• Acid: ionises in water to produce H⁺; base: reacts with an acid; alkali: soluble base producing OH⁻.
• Water is essential, no water, no ionisation, no acidic or alkaline behaviour.
• Acid reactions: with reactive metals (→ H₂), with bases (→ salt + water), with carbonates (→ salt + water + CO₂).
• Basicity = number of ionisable H atoms; strength = degree of ionisation, not concentration.