The characteristic properties of acids and bases

1. Overview

Acids and bases are chemical opposites that play a fundamental role in chemical reactions. This topic covers how these substances behave in aqueous solutions, their characteristic reactions with other compounds, and how we measure their strength using indicators and the pH scale.

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Key Definitions

• Acid: A substance that releases hydrogen ions ($H^+$) in aqueous solution. (Extended: A proton donor).
• Base: A metal oxide or hydroxide that neutralises an acid to form a salt and water. (Extended: A proton acceptor).
• Alkali: A soluble base that releases hydroxide ions ($OH^-$) in aqueous solution.
• Indicator: A chemical that changes colour depending on whether it is in an acidic or alkaline solution.
• Salt: A compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion.
• Neutralisation: A reaction between an acid and a base/alkali to produce a salt and water.

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Core Content

Properties of Acids

Acids have a sour taste, are corrosive, and have a pH of less than 7.

Reactions of Acids:

1. With Metals: Acid + Metal → Salt + Hydrogen $(g)$
   • Word: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
   • Symbol: $Mg (s) + 2HCl (aq) → MgCl_2 (aq) + H_2 (g)$
2. With Bases (Neutralisation): Acid + Base → Salt + Water $(l)$
   • Word: Copper(II) oxide + Sulfuric acid → Copper(II) sulfate + Water
   • Symbol: $CuO (s) + H_2SO_4 (aq) → CuSO_4 (aq) + H_2O (l)$
3. With Carbonates: Acid + Carbonate → Salt + Water $(l)$ + Carbon dioxide $(g)$
   • Word: Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide
   • Symbol: $CaCO_3 (s) + 2HCl (aq) → CaCl_2 (aq) + H_2O (l) + CO_2 (g)$

Properties of Bases and Alkalis

Bases are typically metal oxides (e.g., $CuO$) or metal hydroxides (e.g., $NaOH$). Alkalis are the subset of bases that dissolve in water.

Reactions of Bases:

1. With Acids: (See Neutralisation above).
2. With Ammonium Salts: Alkali + Ammonium Salt → Salt + Water $(l)$ + Ammonia $(g)$
   • Word: Sodium hydroxide + Ammonium chloride → Sodium chloride + Water + Ammonia
   • Symbol: $NaOH (aq) + NH_4Cl (s) → NaCl (aq) + H_2O (l) + NH_3 (g)$

Indicators and pH

• Acids in aqueous solution contain hydrogen ions: $H^+ (aq)$.
• Alkalis in aqueous solution contain hydroxide ions: $OH^- (aq)$.

Colour Changes Table:

Indicator	Colour in Acid	Colour in Neutral	Colour in Alkali
Litmus	Red	Purple	Blue
Thymolphthalein	Colourless	Colourless	Blue
Methyl Orange	Red	Orange	Yellow

Universal Indicator and pH Scale: The pH scale measures the concentration of $H^+$ ions.

• pH 0-2: Strongly acidic (Red)
• pH 3-6: Weakly acidic (Orange/Yellow)
• pH 7: Neutral (Green)
• pH 8-11: Weakly alkaline (Blue)
• pH 12-14: Strongly alkaline (Violet/Purple)

The Neutralisation Reaction

The essential ionic reaction between any acid and alkali to produce water is: $$H^+ (aq) + OH^- (aq) → H_2O (l)$$

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Extended Content (Extended Only)

Proton Transfer

• Acids are defined as proton donors. They give away an $H^+$ ion (which is just a single proton).
• Bases are defined as proton acceptors. They receive the $H^+$ ion.

The Ionic Equation for Neutralisation

When any acid reacts with any alkali, the reaction that actually happens at the particle level is always the same: $$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$ Everything else (the sodium, the chloride, the sulfate) are just spectator ions sitting in solution doing nothing. This single ionic equation describes every acid-alkali neutralisation, regardless of which specific acid and alkali you use.

Amphoteric Oxides

Most metal oxides are basic (they react with acids). But a few special oxides can react with both acids and alkalis — these are called amphoteric. The two you need to know are:

• Aluminium oxide ($Al_2O_3$) — reacts with HCl to form AlCl₃, but also reacts with NaOH to form sodium aluminate
• Zinc oxide ($ZnO$) — reacts with H₂SO₄ to form ZnSO₄, but also reacts with NaOH to form sodium zincate

Think of amphoteric as “two-faced” — it behaves as a base when it meets an acid, and as an acid when it meets a base.

Strong vs. Weak Acids

Strength is determined by the degree of dissociation (splitting into ions) in water.

1. Strong Acids: Completely dissociate in aqueous solution.

   • Example: Hydrochloric acid ($HCl$)
   • Equation: $HCl (aq) → H^+ (aq) + Cl^- (aq)$
   • Note the single arrow (→) indicating the reaction goes to completion.

2. Weak Acids: Partially dissociate in aqueous solution.

   • Example: Ethanoic acid ($CH_3COOH$)
   • Equation: $CH_3COOH (aq) ⇌ H^+ (aq) + CH_3COO^- (aq)$
   • Note the reversible arrow (⇌) indicating that only some molecules split into ions.

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Key Equations

Reaction Type	General Equation
Acid + Metal	$Acid + Metal → Salt + H_2 (g)$
Acid + Base	$Acid + Base → Salt + H_2O (l)$
Acid + Carbonate	$Acid + Carbonate → Salt + H_2O (l) + CO_2 (g)$
Alkali + Ammonium Salt	$Alkali + NH_4^+ Salt → Salt + H_2O (l) + NH_3 (g)$
Neutralisation (Ionic)	$H^+ (aq) + OH^- (aq) → H_2O (l)$

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Common Mistakes to Avoid

• ❌ Wrong: Writing $H$ for hydrogen gas.
• ✓ Right: Always write hydrogen gas as a diatomic molecule: $H_2 (g)$.
• ❌ Wrong: Confusing "Strong/Weak" with "Concentrated/Dilute".
• ✓ Right: Strong/Weak refers to dissociation (how many ions are made). Concentrated/Dilute refers to the amount of acid dissolved in a volume of water.
• ❌ Wrong: Forgetting state symbols in equations.
• ✓ Right: Pure water is always $(l)$, precipitated salts or carbonates are often $(s)$, and acids/alkalis are always $(aq)$.

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Exam Tips

• Command Word - "State": If a question asks you to "state the colour of methyl orange in acid," a single word ("Red") is sufficient.
• Command Word - "Name": If asked to name a gas, do not write the formula. Write "carbon dioxide," not $CO_2$.
• Gas Identification: Remember that the reaction of a base with an ammonium salt produces ammonia gas ($NH_3$). This is often used in exams to identify an unknown substance. Ammonia turns damp red litmus paper blue.
• Contexts: You may be asked to suggest how to treat acidic soil. Since bases neutralise acids, adding a base like Calcium oxide ($CaO$, lime) or Calcium hydroxide ($Ca(OH)_2$, slaked lime) is the standard real-world application.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define the term acid in terms of proton transfer. [1]

(b) State what is meant by a strong acid. [1]

(c) Describe how you could use universal indicator paper to distinguish between 0.1 mol/dm³ solutions of each acid. [3]

Worked Solution:

(a)

1. Acids are proton donors. $\boxed{\text{Acids are proton donors.}}$ By definition.

How to earn full marks:

• State that acids are proton donors.

(b)

1. A strong acid is one that completely dissociates in aqueous solution. $\boxed{\text{A strong acid completely dissociates in aqueous solution.}}$ Definition of a strong acid.

How to earn full marks:

• State that a strong acid completely dissociates in aqueous solution.

(c)

1. Dip universal indicator paper into each solution. This sets up the comparison.

2. Compare the colours produced with a pH chart. This links colour to pH value.

3. The hydrochloric acid will show a lower pH (more acidic) than the ethanoic acid. This identifies the difference between the two.

How to earn full marks:

• Dip universal indicator paper into each solution.
• Compare the colours produced with a pH chart.
• The hydrochloric acid will show a lower pH than the ethanoic acid.

Common Pitfall: Remember that universal indicator paper gives you a range of colours that correspond to different pH values. A strong acid will have a lower pH (more red/orange) than a weak acid of the same concentration (more yellow/green).

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) State two observations you would expect to see when a dilute acid reacts with a metal. [2]

(b) Aqueous solutions of alkalis contain a specific ion. State the formula of this ion. [1]

(c) Complete the following equation for the reaction between an acid and an alkali: $H^+ (aq) + $ ______ $ (aq) \rightarrow $ ______ $ (l)$ [3]

Worked Solution:

(a)

1. Effervescence (bubbles) occurs. $\boxed{\text{Effervescence/Bubbles}}$ Gas production is a key observation.

2. The metal dissolves/disappears. $\boxed{\text{Metal dissolves/disappears}}$ The metal is reacting away.

How to earn full marks:

• State effervescence or bubbles.
• State that the metal dissolves or disappears.

(b)

1. Hydroxide ions are present in aqueous solutions of alkalis. $\boxed{OH^-}$ Correct formula.

How to earn full marks:

• State the correct formula of the hydroxide ion.

(c)

1. Identify the missing reactant. $OH^-$

2. Identify the missing product. $H_2O$

3. Complete the equation. $H^+ (aq) + OH^- (aq) \rightarrow H_2O (l)$ $\boxed{H^+ (aq) + OH^- (aq) \rightarrow H_2O (l)}$ Correctly balanced and with state symbols.

How to earn full marks:

• Correctly identify and write the formula of the hydroxide ion, including the charge.
• Correctly identify and write the formula of water.
• Correctly complete the equation with the correct state symbols.

Common Pitfall: Make sure you include the charge when writing the formula for the hydroxide ion ($OH^-$). Also, remember the state symbols; they are important for showing the physical state of each substance in the reaction.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

Ammonia ($NH_3$) is a base. It reacts with acids to form ammonium salts.

(a) Define the term base in terms of proton transfer. [1]

(b) Describe what you would observe when ammonia gas is bubbled through hydrochloric acid. [2]

(c) Write the balanced chemical equation for the reaction between ammonia and hydrochloric acid. Include state symbols. [2]

(d) An experiment is carried out to determine the concentration of a hydrochloric acid solution. 25.0 cm³ of 0.100 mol/dm³ ammonia solution is neutralised by 20.0 cm³ of the hydrochloric acid. Calculate the concentration of the hydrochloric acid in mol/dm³. [3]

Worked Solution:

(a)

1. Bases are proton acceptors. $\boxed{\text{Bases are proton acceptors.}}$ By definition.

How to earn full marks:

• State that bases are proton acceptors.

(b)

1. White smoke is formed. $\boxed{\text{White smoke/fumes}}$ Ammonium chloride is a white solid.

2. The gas bubbles disappear. $\boxed{\text{Bubbles disappear.}}$ The ammonia is reacting.

How to earn full marks:

• State that white smoke/fumes are formed.
• State that the gas bubbles disappear.

(c)

1. Identify the reactants and products. $NH_3 (g) + HCl (aq) \rightarrow NH_4Cl (s)$

2. Write the balanced equation with state symbols. $\boxed{NH_3 (g) + HCl (aq) \rightarrow NH_4Cl (s)}$ Correct balancing and state symbols.

How to earn full marks:

• Correctly write the chemical formula for each reactant and product.
• Correctly balance the equation.
• Correctly write the state symbols.

(d)

1. Calculate moles of ammonia. $moles\ of\ NH_3 = \frac{25.0}{1000} \times 0.100 = 0.0025\ mol$ $\boxed{0.0025\ mol}$ Use volume in dm³ and concentration to find moles.

2. Moles of HCl = Moles of NH3 (1:1 stoichiometry). $moles\ of\ HCl = 0.0025\ mol$ Recognise the 1:1 mole ratio.

3. Calculate concentration of HCl. $concentration\ of\ HCl = \frac{0.0025}{\frac{20.0}{1000}} = 0.125\ mol/dm^3$ $\boxed{0.125\ mol/dm^3}$ Use moles and volume in dm³ to find concentration.

How to earn full marks:

• Correctly calculate the number of moles of ammonia.
• State that the number of moles of HCl is the same as the number of moles of ammonia.
• Correctly calculate the concentration of the HCl solution with correct units.

Common Pitfall: Remember to convert volumes from cm³ to dm³ by dividing by 1000 before using them in calculations. Also, pay attention to the stoichiometry of the reaction to ensure you're using the correct mole ratios.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A student investigates the reaction between dilute hydrochloric acid and zinc carbonate ($ZnCO_3$). The equation for the reaction is: $ZnCO_3(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2O(l) + CO_2(g)$

The student adds 5.00 g of zinc carbonate to 50.0 cm³ of 2.00 mol/dm³ hydrochloric acid. The carbon dioxide gas produced is collected in a gas syringe.

(a) State three observations the student would make during the reaction. [3]

(b) Calculate the number of moles of hydrochloric acid used in the experiment. [2]

(c) Calculate the number of moles of zinc carbonate used in the experiment ($M_r$ of $ZnCO_3 = 125.4$). [2]

(d) Determine which reactant, hydrochloric acid or zinc carbonate, is the limiting reactant. Explain your answer. [2]

Worked Solution:

(a)

1. Effervescence/Bubbles. $\boxed{\text{Effervescence/Bubbles}}$ Carbon dioxide gas is produced.

2. Zinc carbonate disappears. $\boxed{\text{Zinc carbonate disappears.}}$ The solid is reacting.

3. The solution becomes clearer. $\boxed{\text{Solution becomes clearer.}}$ The solid is being converted to aqueous zinc chloride.

How to earn full marks:

• State effervescence or bubbles.
• State that the zinc carbonate disappears.
• State that the solution becomes clearer.

(b)

1. Calculate the moles of HCl. $moles\ of\ HCl = \frac{50.0}{1000} \times 2.00 = 0.100\ mol$ $\boxed{0.100\ mol}$ Use volume in dm³ and concentration.

How to earn full marks:

• Correctly calculate the number of moles of HCl.

(c)

1. Calculate the moles of zinc carbonate. $moles\ of\ ZnCO_3 = \frac{5.00}{125.4} = 0.0399\ mol$ $\boxed{0.0399\ mol}$ Use mass and $M_r$.

How to earn full marks:

• Correctly calculate the number of moles of zinc carbonate.

(d)

1. Determine the mole ratio required for complete reaction using the balanced equation. From the equation, 1 mole of $ZnCO_3$ reacts with 2 moles of $HCl$. Therefore, 0.0399 moles of $ZnCO_3$ requires $2 \times 0.0399 = 0.0798$ moles of $HCl$ for complete reaction.

2. Compare the required moles of HCl to the available moles of HCl. We have 0.100 moles of $HCl$ which is more than the 0.0798 moles required. Therefore, zinc carbonate is the limiting reactant. $\boxed{\text{Zinc carbonate is the limiting reactant.}}$ Since the amount of $HCl$ is in excess, $ZnCO_3$ will limit the reaction.

How to earn full marks:

• Calculate the number of moles of HCl required to react completely with the zinc carbonate.
• Compare the required moles of HCl with the actual moles of HCl available.
• State that zinc carbonate is the limiting reactant and correctly justify your answer using the mole comparison.

Common Pitfall: When determining the limiting reactant, it's crucial to consider the stoichiometry of the balanced equation. Don't just compare the number of moles directly; you need to account for the mole ratios in the reaction.