Acids, bases and salts: Class 10 Chemistry

Key points

Acids and bases were first described by their properties. Acids taste sour, turn blue litmus red and react with metals to give hydrogen. Bases taste bitter, feel soapy and turn red litmus blue. A base that dissolves in water is called an alkali, for example NaOH and KOH.

Three main concepts define acids and bases:

• Arrhenius concept: an acid gives H⁺ ions in water, and a base gives OH⁻ ions in water. HCl → H⁺ + Cl⁻ and NaOH → Na⁺ + OH⁻.
• Bronsted-Lowry concept: an acid is a proton (H⁺) donor and a base is a proton acceptor. This works in solvents other than water too, and it explains why NH₃ is a base.
• Lewis concept: an acid is an electron pair acceptor and a base is an electron pair donor. BF₃ is a Lewis acid, and NH₃ is a Lewis base because it has a lone pair.

In the Bronsted-Lowry idea, acids and bases come in conjugate pairs. When an acid loses a proton, what remains is its conjugate base. When a base gains a proton, it forms its conjugate acid. In HCl + H₂O → H₃O⁺ + Cl⁻, HCl is the acid and Cl⁻ is its conjugate base. H₂O is the base and H₃O⁺ is its conjugate acid. A strong acid has a weak conjugate base.

Substances such as water can act as both acid and base depending on the partner. They are called amphoteric (or amphiprotic) substances. Water gives H₃O⁺ with HCl and gives OH⁻ with NH₃.

Common properties of acids and bases:

• Acid + metal → salt + hydrogen, for example Zn + 2HCl → ZnCl₂ + H₂.
• Acid + carbonate → salt + water + carbon dioxide.
• Acid + base → salt + water (neutralization).
• Aqueous solutions of both conduct electricity because they contain ions.
• Bases react with ammonium salts on heating to release ammonia.

Strength depends on the degree of ionization, not on concentration. A strong acid such as HCl, H₂SO₄ or HNO₃ ionizes almost completely in water. A weak acid such as CH₃COOH or H₂CO₃ ionizes only slightly. In the same way NaOH and KOH are strong bases and NH₃ is a weak base. Concentration tells how much solute is present. Strength tells how fully it ionizes.

Water ionizes slightly: H₂O ⇌ H⁺ + OH⁻. At 25 °C the ionic product of water is Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ mol² dm⁻⁶. In pure water [H⁺] = [OH⁻] = 1 × 10⁻⁷ mol dm⁻³. An acid raises [H⁺], and a base raises [OH⁻].

The pH scale gives [H⁺] as a simple number: pH = −log[H⁺]. Similarly pOH = −log[OH⁻], and pH + pOH = 14 at 25 °C. A pH below 7 is acidic, 7 is neutral and above 7 is basic. One unit change in pH means ten times change in [H⁺].

Indicators change colour with pH. Litmus is red in acid and blue in base. Methyl orange is red in acid and yellow in base. Phenolphthalein is colourless in acid and pink in base. Universal indicator shows a range of colours and gives an approximate pH. A pH meter gives an accurate reading.

Neutralization is the reaction of an acid with a base to give salt and water, for example HCl + NaOH → NaCl + H₂O. In ionic form it is H⁺ + OH⁻ → H₂O. In titration, the volume of acid and base that just react is found using an indicator, and the unknown concentration is then calculated.

A buffer solution resists a change in pH when small amounts of acid or base are added. An acidic buffer is a weak acid with its salt, for example CH₃COOH with CH₃COONa. A basic buffer is a weak base with its salt, for example NH₄OH with NH₄Cl. Blood is buffered at about pH 7.4 by the carbonic acid and bicarbonate system.

Types of salts:

• Normal salt: all H⁺ replaced, for example NaCl and Na₂SO₄.
• Acid salt: some replaceable H⁺ remains, for example NaHCO₃ and NaHSO₄.
• Basic salt: contains OH⁻ as well, for example Mg(OH)Cl.
• Double salt: two salts crystallize together, for example potash alum.
• Complex salt: contains a complex ion, for example K₄[Fe(CN)₆].

Hydrolysis is the reaction of a salt with water. A salt of a strong acid and strong base (NaCl) gives a neutral solution. A salt of a strong base and weak acid (CH₃COONa) gives a basic solution. A salt of a weak base and strong acid (NH₄Cl) gives an acidic solution.

Uses: sulphuric acid is used in batteries and fertilizers, nitric acid in explosives, and hydrochloric acid in cleaning metals. Sodium hydroxide is used in soap making, and magnesium hydroxide (milk of magnesia) relieves acidity. Sodium bicarbonate is used in baking and as an antacid.

Study tip: Practise the three theories with one example each and be ready to write conjugate pairs. For numericals, write [H⁺] in powers of 10 first, then apply the log. Always write units for concentration and volume, and give the balanced equation before a titration calculation.

Definitions and flashcards

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Short questions with model answers

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Common mistakes

• Saying a strong acid is the same as a concentrated acid. Strength is about ionization and concentration is about amount.
• Forgetting that each unit change of pH is a tenfold change in [H⁺]. A solution of pH 3 is 100 times more acidic than pH 5.
• Writing pH = log[H⁺] without the negative sign.
• Saying a salt solution is always neutral. Salts of weak acids or weak bases undergo hydrolysis.
• Using pH + pOH = 14 at temperatures other than 25 °C without noting the condition.

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