Acids, Bases and Salts — Part 1: Ions, indicators and kinds of salts: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Sour, bitter and in between

Part 1 — what makes a solution acidic or alkaline, how we measure it, and the many kinds of salt formed when acids react.

In this lesson you will

Define acids and alkalis in terms of the ions they give in water
Write ionisation and dissociation equations
Use indicators and the pH scale to classify solutions
Classify salts as normal, acid, basic or complex

Table of five everyday substances and whether each is an acid, an alkali or a salt
Substance	Nature
Hydrochloric acid in the stomach	Strong acid
Vinegar (ethanoic acid)	Weak acid
Lime water (calcium hydroxide)	Weak alkali
Caustic soda (sodium hydroxide)	Strong alkali
Common salt (sodium chloride)	Neutral salt

Concept

Acids: hydronium ions in water

An acid is a compound which, when dissolved in water, gives hydronium ions (H₃O⁺) as the only positive ions. The acid molecule ionises: it gives H⁺, which joins a water molecule. Monobasic acids give one H⁺ per molecule (HCl, HNO₃); dibasic acids give two (H₂SO₄, in two steps). Strong acids (HCl, HNO₃, H₂SO₄) ionise almost completely; weak acids (ethanoic acid, carbonic acid) only partly.

\ce{HCl + H2O -> H3O+ + Cl-}

\ce{HNO3 + H2O -> H3O+ + NO3-}

\ce{H2SO4 + H2O -> H3O+ + HSO4-}

\ce{HSO4- + H2O -> H3O+ + SO4^2-}

Table of three mineral acids with their basicity and the ions formed in water
Acid	Basicity	Ions formed in water
Hydrochloric acid, HCl	Monobasic	H₃O⁺, Cl⁻
Nitric acid, HNO₃	Monobasic	H₃O⁺, NO₃⁻
Sulphuric acid, H₂SO₄	Dibasic	H₃O⁺, HSO₄⁻, SO₄²⁻

Concept

Bases and alkalis: hydroxide ions

A base is a compound that reacts with an acid to give a salt and water only (metal oxides and hydroxides: CuO, MgO, NaOH). An alkali is a base that is soluble in water and gives hydroxide ions (OH⁻) as the only negative ions. Alkalis dissociate (their ions already exist in the solid). All alkalis are bases, but not all bases are alkalis.

\ce{NaOH -> Na+ + OH-}

\ce{KOH -> K+ + OH-}

\ce{Ca(OH)2 -> Ca^2+ + 2OH-}

Table of five bases showing which are soluble alkalis
Base	Soluble in water?	Alkali?
Sodium hydroxide	Yes	Yes (strong)
Potassium hydroxide	Yes	Yes (strong)
Ammonium hydroxide	Yes	Yes (weak)
Copper(II) oxide	No	No
Iron(III) hydroxide	No	No

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