The Characteristic Properties of Acids and Bases — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

Section 1 of 12Properties of acids

• Physical
  • acids have pH below 7, taste sour when edible and are corrosive.
• Neutralise
  • they react with bases to give a salt and water.
  • Ions
    • in water they form hydrogen ions, H⁺, which make the solution acidic.
  • Example
    • HCl(aq) → H⁺(aq) + Cl^-(aq).

Section 2 of 12Reactions of acids

Reactants	Products	Notes
Acid + metal	Salt + hydrogen	Only metals above hydrogen in the reactivity series react with dilute acids
Acid + base (metal oxide or hydroxide)	Salt + water	A neutralisation reaction
Acid + metal carbonate	Salt + carbon dioxide + water	Effervescence is seen

• Naming salts
  • the salt name comes from the metal and the anion of the acid.
• Hydrochloric acid
  • chlorides.
  • Sulfuric acid
    • sulfates.
  • Nitric acid
    • nitrates.

Section 3 of 12Indicators

• Two-colour indicators
  • show different colours in acid and alkali.
• Litmus
  • a natural indicator from lichens, red in acid and blue in alkali.
  • Synthetic
    • methyl orange and thymolphthalein are organic compounds sensitive to acidity.
• Titrations
  • synthetic indicators give a sharp colour change at the end point.
• Litmus unsuitable
  • it changes slowly and goes through purple in neutral solutions.
  • Litmus paper
    • useful in red and blue paper form for solutions and gases.

Section 4 of 12Properties of bases and alkalis

• Bases
  • have pH above 7 and are usually metal oxides or hydroxides that neutralise acids to give a salt and water.
• Alkali
  • a base that dissolves in water.
  • Colours in alkali
    • red litmus turns blue, methyl orange turns yellow and thymolphthalein turns blue.
  • Ions
    • alkalis form hydroxide ions, OH^-, in water.
  • Example
    • NaOH(s) → Na⁺(aq) + OH^-(aq).

Section 5 of 12Alkalis and ammonium salts

• Reaction
  • warming an ammonium salt with an alkali releases ammonia, a salt and water.
• Example
  • NH₄Cl + NaOH → NaCl + H₂O + NH₃.
  • Why it works
    • ammonia is volatile and is displaced by the stronger alkali.
• Test for ammonium ions
  • add alkali, warm gently and hold damp red litmus paper at the mouth.
• Positive result
  • the paper turns blue because ammonia is present.

Section 6 of 12Neutralisation and the ions involved

• Ionic equation
  • H⁺ + OH^- → H₂O.
• Result
  • a neutral solution because water has pH 7.
  • Not all acid reactions
    • metal plus acid forms a salt but no water, so it is not neutralisation.

Section 7 of 12The pH scale

• Range
  • pH 1 to 14, although very strong acids can be below 1.
• Acid
  • below 7, and lower pH means more acidic.
  • Neutral
    • 7, such as water.
  • Alkaline
    • above 7, and higher pH means more alkaline.
• Link to ions
  • pH measures the concentration of H⁺ ions, with an inverse relationship.
• Logarithmic
  • each pH unit is a factor of 10 in concentration.
  • Example
    • pH 3 has ten times the H⁺ of pH 4, and pH 2 has 100 times that of pH 4.
• Universal indicator
  • a mixture of indicators whose colour is matched to a chart to find pH.

Section 8 of 12Proton transfer

• Acids
  • proton donors, because they ionise to give H⁺.
• Bases
  • proton acceptors.
• Water as a base
  • in water an acid donates a proton and water accepts it.

Section 9 of 12Strong and weak acids

Feature	Strong acid	Weak acid
Ionisation	Completely ionises in water	Partially ionises
H+ in solution	High	Low
pH of solution	Very low	Closer to 7 but below 7
Examples	HCl, H2SO4	Ethanoic acid CH3COOH, propanoic acid

• Weak acid equilibrium
  • CH₃CH₂COOH ⇌ H⁺ + CH₃CH₂COO^- with the equilibrium far to the left.
• Concentration versus strength
  • concentrated means many acid molecules per dm³, not necessarily strong.
  • Example
    • dilute HCl is more acidic than concentrated ethanoic acid.

Section 10 of 12Worked examples

1. Write the ionic equation for neutralisation and explain why the product is neutral. [2]2 marks

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2. How does a solution of pH 2 compare with one of pH 4 in H⁺ concentration? [1]1 mark

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3. Describe a test for ammonium ions. [3]3 marks

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4. Explain the difference between a weak acid and a dilute acid. [2]2 marks

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Section 11 of 12Key terms

Term	Meaning
Acid	A substance that forms H+ ions in water; a proton donor.
Base	A substance that neutralises acids to give a salt and water; a proton acceptor.
Alkali	A base that dissolves in water, forming OH- ions.
pH	A scale showing how acidic or alkaline a solution is.
Strong acid	An acid that ionises completely in water.
Weak acid	An acid that ionises only partially in water.
Universal indicator	A mixture of indicators that shows pH by colour.

Section 12 of 12Exam technique

1. Saying all acid reactions are neutralisations — metal plus acid gives no water.
2. Confusing strong with concentrated — strength is about ionisation.
3. Thinking pH units are linear — each is ten times.
4. Forgetting that ammonia turns damp red litmus blue.