The characteristic properties of acids and bases
Characteristic Properties of Acids (LO1)
Acids are defined by how they react with specific substances. You must describe the observations for each reaction type.
| Reactant | Observation / Product |
|---|---|
| Metals (e.g., Mg, Zn) | Effervescence (bubbles/fizzing) due to the production of hydrogen gas (H_2). The metal dissolves. |
| Bases (e.g., NaOH, CuO) | Neutralisation. Usually no visible change if both are clear solutions, but heat is evolved (exothermic). Salt and water are formed. |
| Carbonates (e.g., CaCO_3, Na_2CO_3) | Vigorous effervescence (bubbles/fizzing) due to the production of carbon dioxide gas (CO_2). The solid dissolves. |
Building on LO1, these reactions confirm the acidic nature. Note that 'effervescence' is the key keyword for gas production in exams.
Indicator Colours for Acids (LO2)
Acids change the colour of specific indicators. You must memorise these colours.
| Indicator | Colour in Acid |
|---|---|
| Litmus | Red |
| Thymolphthalein | Colourless |
| Methyl Orange | Red |
A common error is confusing the colours. Remember: Acids turn Methyl Orange Red, not yellow.
Bases and Alkalis (LO3, LO4)
Bases are oxides or hydroxides of metals. Alkalis are bases that dissolve in water to form alkaline solutions.
Characteristic properties of bases/alkalis:
| Reactant | Observation / Product |
|---|---|
| Acids | Neutralisation. Salt and water are formed. Heat is evolved. |
| Ammonium Salts (e.g., NH_4Cl) | Ammonia gas (NH_3) is produced when warmed. This gas has a pungent, choking smell and turns damp red litmus blue. |
Note: Not all bases are alkalis (e.g., Copper(II) oxide is a base but insoluble). Only soluble bases are alkalis.
Indicator Colours for Alkalis (LO5)
Alkalis change the colour of indicators in the opposite way to acids.
| Indicator | Colour in Alkali |
|---|---|
| Litmus | Blue |
| Thymolphthalein | Blue |
| Methyl Orange | Yellow |
Thymolphthalein is particularly useful for distinguishing alkalis from neutral solutions (colourless).
Ions in Solution (LO6)
The ions responsible for the properties are:
| Solution Type | Ion Present |
|---|---|
| Aqueous Acid | H^+(aq) ions (hydrogen ions) |
| Aqueous Alkali | OH^-(aq) ions (hydroxide ions) |
Note: In advanced chemistry, H^+(aq) is often written as H_3O^+(aq) (hydronium), but for this syllabus, H^+ is the required notation.
Universal Indicator and pH (LO7)
Universal Indicator (UI) paper is used to compare hydrogen ion concentration ([H^+]), neutrality, relative acidity, and relative alkalinity.
How to compare:
| Observation | Meaning |
|---|---|
| Red / Orange / Yellow | Acidic. Red indicates high [H^+] (low pH). Yellow indicates lower [H^+] (higher pH, but still acidic). |
| Green | Neutral (pH = 7). No excess H^+ or OH^-. |
| Blue / Purple | Alkaline. Blue indicates moderate [OH^-]. Purple indicates high [OH^-] (low [H^+], high pH). |
To quantify, match the colour to a pH chart. A lower pH number means higher acidity (higher [H^+]). A higher pH number means higher alkalinity (lower [H^+]).
Neutralisation Reaction (LO8)
Neutralisation is the reaction between an acid and an alkali.
| Word Equation | **Acid + Alkali → Salt + Water** |
|---|---|
| Ionic Equation | H^+(aq) + OH^-(aq) \rightarrow H_2O(l) |
The H^+ ions from the acid react with the OH^- ions from the alkali to form water. The salt remains in solution.
Acid: A proton donor.
Base: A proton acceptor.
This definition (Brønsted-Lowry) explains why ammonia (NH_3) is a base: it accepts a proton to become NH_4^+.
Strong Acid: An acid that is completely dissociated in aqueous solution. It breaks apart 100% into ions.
Weak Acid: An acid that is partially dissociated in aqueous solution. Only a small percentage of molecules break apart; an equilibrium exists.
Note: 'Strong' refers to the degree of dissociation, NOT concentration. A dilute strong acid has fewer H^+ ions than a concentrated weak acid.
Hydrochloric Acid (HCl) is a strong acid.
| Equation Type | Symbol Equation |
|---|---|
| Representation | HCl(aq) \rightarrow H^+(aq) + Cl^-(aq) |
Note the single arrow (\rightarrow) indicating complete dissociation.
Ethanoic Acid (CH_3COOH) is a weak acid.
| Equation Type | Symbol Equation |
|---|---|
| Representation | CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq) |
Note the equilibrium arrow (\rightleftharpoons) indicating partial dissociation.
Calculating pH of Strong vs Weak Acids
Scenario: Compare 0.1 mol/dm³ Hydrochloric Acid (Strong) and 0.1 mol/dm³ Ethanoic Acid (Weak).
Analysis:
| Acid Type | $[H^+]$ Concentration |
|---|---|
| Strong (HCl) | 0.1 mol/dm³ (Complete dissociation) |
| Weak (CH_3COOH) | < 0.1 mol/dm³ (Partial dissociation) |
Result: The strong acid has a higher [H^+], so it has a lower pH than the weak acid of the same concentration.
⚠︎ Confusing Strength and Concentration
Error: Assuming a 'weak' acid always has a higher pH than a 'strong' acid.
Correction: Strength is about dissociation; concentration is about amount per volume. A concentrated weak acid can have a lower pH (more acidic) than a dilute strong acid. Always check the concentration values.
⚠︎ Indicator Colours
Error: Saying Methyl Orange turns 'orange' in acid.
Correction: Methyl Orange is red in acid and yellow in alkali. The orange colour is only seen at the transition point (end-point) during titration, not in a stable acidic solution.
Describing Neutralisation Equations
When to use: When asked for the ionic equation of neutralisation.
Why accepted: Examiners look for the core reaction between hydrogen and hydroxide ions, ignoring spectator ions (like Na^+ or Cl^-).
Correct Usage: Write H^+(aq) + OH^-(aq) \rightarrow H_2O(l). Do not include the salt ions unless asked for the full molecular equation.
Distinguishing Strong and Weak Acids in Equations
When to use: When writing equations for strong vs weak acids.
Why accepted: The arrow type is the key differentiator. A single arrow implies 100% dissociation; an equilibrium arrow implies partial dissociation.
Correct Usage: Use \rightarrow for HCl, HNO_3, H_2SO_4. Use \rightleftharpoons for CH_3COOH.
Past Paper Style Questions
Q:
Describe the observation when magnesium ribbon is added to dilute hydrochloric acid. [1]
A:
Effervescence / bubbles / fizzing.
Q:
What colour does methyl orange turn in an aqueous solution of pH 2? [1]
A:
Red.
Q:
State the difference between a strong acid and a weak acid. [1]
A:
Strong acids are completely dissociated; weak acids are partially dissociated.
Q:
Write the ionic equation for the reaction between hydrochloric acid and sodium hydroxide. [1]
A:
H^+(aq) + OH^-(aq) \rightarrow H_2O(l)
Q:
Explain why ethanoic acid is a weak acid, using the term 'dissociation'. [1]
A:
Ethanoic acid is partially dissociated in water.