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This lesson covers the definitions and properties of acids, bases and alkalis as required by the Edexcel GCSE Chemistry specification (1CH0). You need to know the key definitions, be able to identify common acids and alkalis by formula, understand the role of ions in solution, and describe how indicators are used to classify substances.
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. These hydrogen ions are responsible for the characteristic properties of acidic solutions, including sour taste (though you should never taste chemicals in a lab), corrosiveness, and the ability to react with metals and bases.
The hydrogen ions are always present in aqueous solution and can be written as H⁺(aq).
| Acid | Formula | Where It Is Found / Used |
|---|---|---|
| Hydrochloric acid | HCl | Stomach acid; used in the lab |
| Sulfuric acid | H₂SO₄ | Car batteries; industrial manufacturing |
| Nitric acid | HNO₃ | Making fertilisers and explosives |
| Ethanoic acid | CH₃COOH | Vinegar |
| Citric acid | C₆H₈O₇ | Citrus fruits (lemons, oranges) |
| Carbonic acid | H₂CO₃ | Fizzy drinks |
When these acids dissolve in water, they dissociate (split up) to release H⁺ ions. For example:
HCl(aq) → H⁺(aq) + Cl⁻(aq)
H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)
HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)
Exam Tip: The key definition to remember is that an acid is a substance that produces H⁺ ions in aqueous solution. This is the definition Edexcel uses in mark schemes. Make sure you include the word "aqueous" or write (aq) to show the ions are in solution.
A base is a substance that neutralises an acid. Bases react with acids to form a salt and water. Most bases are metal oxides or metal hydroxides.
| Base | Formula | Type |
|---|---|---|
| Copper oxide | CuO | Metal oxide (insoluble) |
| Zinc oxide | ZnO | Metal oxide (insoluble) |
| Magnesium oxide | MgO | Metal oxide (slightly soluble) |
| Iron(III) oxide | Fe₂O₃ | Metal oxide (insoluble) |
| Sodium hydroxide | NaOH | Metal hydroxide (soluble — an alkali) |
| Calcium hydroxide | Ca(OH)₂ | Metal hydroxide (slightly soluble) |
Not all bases dissolve in water. Those that do dissolve are called alkalis. Those that do not dissolve are simply called bases. Therefore:
All alkalis are bases, but not all bases are alkalis.
An alkali is a soluble base that produces hydroxide ions (OH⁻) when dissolved in water.
| Alkali | Formula | Notes |
|---|---|---|
| Sodium hydroxide | NaOH | Strong alkali; used in soap making and the lab |
| Potassium hydroxide | KOH | Strong alkali; used in alkaline batteries |
| Calcium hydroxide | Ca(OH)₂ | Slightly soluble; used in agriculture to neutralise acidic soil (limewater) |
| Ammonia solution | NH₃(aq) | Weak alkali; used in cleaning products |
When alkalis dissolve, they release OH⁻ ions:
NaOH(aq) → Na⁺(aq) + OH⁻(aq)
KOH(aq) → K⁺(aq) + OH⁻(aq)
Ca(OH)₂(aq) → Ca²⁺(aq) + 2OH⁻(aq)
Exam Tip: A very common exam question asks you to explain the difference between a base and an alkali. A base neutralises an acid. An alkali is a base that is soluble in water and releases OH⁻ ions in solution. Give an example of each: CuO is a base (insoluble), NaOH is an alkali (soluble base).
An indicator is a substance that changes colour depending on whether a solution is acidic, neutral or alkaline. Indicators allow us to classify substances without needing to know their chemical composition.
| Indicator | Colour in Acid | Colour When Neutral | Colour in Alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Phenolphthalein | Colourless | Colourless | Pink |
| Methyl orange | Red | Orange | Yellow |
| Universal indicator | Red/orange/yellow | Green | Blue/purple |
Litmus tells you whether something is acidic or alkaline, but it does not tell you the strength of the acid or alkali.
Universal indicator is a mixture of several indicators that produces a range of colours corresponding to the pH of the solution. It gives a more precise indication than litmus because it shows a continuous colour change across the pH range.
A pH meter (or pH probe) gives a numerical reading of pH, which is more accurate and precise than universal indicator. It can measure pH to one or two decimal places, making it useful for quantitative work.
Exam Tip: When asked to compare methods for measuring pH, state that a pH meter is more accurate and precise than universal indicator, because it gives a numerical value rather than a colour that must be matched by eye to a colour chart. Universal indicator only gives a whole number estimate.
Acidic solutions share a common set of properties because they all contain H⁺ ions:
Alkaline solutions share common properties because they all contain OH⁻ ions:
When working with acids and alkalis in the laboratory:
Exam Tip: If asked about safety precautions for an experiment involving acids or alkalis, always mention wearing safety goggles (to protect eyes) and washing any splashes with water. For concentrated reagents, mention the corrosive hazard and the need for additional care.
Question: Classify each of the following as an acid, base or alkali: (a) H₂SO₄, (b) CuO, (c) KOH, (d) Mg(OH)₂, (e) HNO₃.
Approach: Look for the diagnostic feature:
Answer:
| Substance | Classification | Reasoning |
|---|---|---|
| (a) H₂SO₄ | Acid (strong) | Releases 2H⁺(aq) + SO₄²⁻(aq) in water |
| (b) CuO | Base (insoluble) | Metal oxide; neutralises acid but does not dissolve |
| (c) KOH | Alkali (strong) | Soluble metal hydroxide; releases OH⁻(aq) |
| (d) Mg(OH)₂ | Base (slightly soluble) | Metal hydroxide but only sparingly soluble; borderline alkali |
| (e) HNO₃ | Acid (strong) | Releases H⁺(aq) + NO₃⁻(aq) in water |
Question: A solution turns red litmus paper blue and universal indicator deep purple. What can you deduce?
Answer: Red → blue on litmus means the solution is alkaline. A deep purple colour on universal indicator corresponds to a pH of about 13 or 14, so the solution is a strong alkali such as 1.0 mol/dm³ sodium hydroxide solution. A weak alkali such as ammonia solution would give blue, not deep purple, because [OH⁻] is much lower.
Question: Write balanced equations showing the ions released when (a) nitric acid and (b) calcium hydroxide dissolve in water.
Answer:
(a) HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)
(b) Ca(OH)₂(aq) → Ca²⁺(aq) + 2OH⁻(aq)
Note the 2 in front of OH⁻: calcium hydroxide contains two hydroxide ions per formula unit, so two OH⁻ ions are released.
Question: Solution A has a pH of 2 and Solution B has a pH of 5. Which solution has the higher concentration of hydrogen ions, and by what factor?
Answer: The pH scale is logarithmic: a decrease in pH of 1 unit corresponds to a 10-fold increase in [H⁺]. Solution A has a pH 3 units lower than Solution B, so [H⁺] in A is 10³ = 1000 times greater than in B.
| Feature | Acid | Base | Alkali |
|---|---|---|---|
| Key ion released in water | H⁺(aq) | — (reacts with H⁺) | OH⁻(aq) |
| pH range | < 7 | > 7 if soluble | 8–14 |
| Red litmus paper | Stays red | Turns blue (if alkali) | Turns blue |
| Blue litmus paper | Turns red | Stays blue | Stays blue |
| Soluble in water? | Yes (most common ones) | Not always | Yes (by definition) |
| Typical example | HCl, H₂SO₄ | CuO, Fe₂O₃ | NaOH, KOH |
| Reacts with metal carbonate? | Yes, gives CO₂ | — | No direct reaction |
| Reacts with acid? | — | Yes | Yes |
Common mistake: Writing "acids produce hydrogen" instead of "acids produce hydrogen ions in aqueous solution." Hydrogen gas (H₂) is produced only when an acid reacts with a metal. Dissolving an acid in water produces H⁺ ions, not H₂ gas.
Common mistake: Assuming every base is an alkali. Copper oxide and iron(III) oxide are bases because they neutralise acid, but they do not dissolve in water, so they are not alkalis. Only soluble bases that release OH⁻(aq) qualify as alkalis.
Common mistake: Saying a neutral solution "has no ions". Pure water contains tiny equal concentrations of H⁺(aq) and OH⁻(aq) (about 10⁻⁷ mol/dm³ each). Neutral means the concentrations of H⁺ and OH⁻ are equal, not zero.
Common mistake: Confusing "strong" with "concentrated". A strong acid is fully ionised; a concentrated acid simply has lots of moles of acid per dm³. You can have a dilute strong acid and a concentrated weak acid.
graph TD
Start["Given substance"] --> Q1{"Does formula start with H<br/>and release H⁺ in water?"}
Q1 -- "Yes" --> Acid["ACID (pH < 7)<br/>e.g. HCl, H₂SO₄, HNO₃"]
Q1 -- "No" --> Q2{"Is it a metal oxide<br/>or metal hydroxide?"}
Q2 -- "No" --> Other["Not an acid/base/alkali"]
Q2 -- "Yes" --> Q3{"Does it dissolve in water<br/>to give OH⁻(aq)?"}
Q3 -- "Yes" --> Alkali["ALKALI (pH > 7)<br/>e.g. NaOH, KOH"]
Q3 -- "No" --> Base["BASE (insoluble)<br/>e.g. CuO, Fe₂O₃"]
style Acid fill:#e74c3c,color:#fff
style Base fill:#8e44ad,color:#fff
style Alkali fill:#27ae60,color:#fff
The same question — "Explain the difference between an acid and an alkali" — can be answered in very different ways, and marks depend on the precision of the language.
Grade 3–4 (foundation response): "An acid has a pH less than 7 and an alkali has a pH greater than 7. Acids are sour and alkalis feel soapy." This mentions the pH scale but misses the key chemistry idea.
Grade 5–6 (stronger response): "An acid produces hydrogen ions when dissolved in water and has a pH below 7. An alkali is a soluble base that produces hydroxide ions in water and has a pH above 7." This uses correct ion names and distinguishes alkalis from bases.
Grade 7–9 (top-band response): "An acid releases H⁺(aq) ions in aqueous solution, for example HCl(aq) → H⁺(aq) + Cl⁻(aq). An alkali is a soluble base that releases OH⁻(aq) ions, for example NaOH(aq) → Na⁺(aq) + OH⁻(aq). The pH scale is logarithmic, so acidic solutions ([H⁺] > [OH⁻]) have a pH below 7 and alkaline solutions ([OH⁻] > [H⁺]) have a pH above 7. All alkalis are bases, but not all bases are alkalis: CuO is a base because it neutralises acid, but it is not an alkali because it does not dissolve in water." Top-band answers use precise terms such as pH scale, strong vs concentrated acid, and include state symbols and an ionic equation.
Edexcel alignment: This content is aligned with Edexcel GCSE Chemistry (1CH0) specification Topic 3 Chemical changes / Topic 4 Extracting metals and equilibria — specifically 3.1 Acids (definitions of acid, base and alkali; hydrogen and hydroxide ions in solution; use of indicators; introduction to the pH scale). Assessed on Paper 1.