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Physical & Chemical Properties of Alkalis

Alkali properties and reactions: pH > 7 and red litmus → blue, neutralisation, ammonium salts test (ammonia), and precipitate formation.

  • SEC G3 Pure Chemistry 2027

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An alkali is more specific than a base: it is a soluble base that produces hydroxide ions in water. Keeping that distinction clear makes the reactions in this lesson much easier.

1. Definition

An alkali is a base that is soluble in water and produces hydroxide ions, OH⁻(aq), in solution.

2. Key Ideas

  • Alkalis are bases, but not all bases are alkalis (many bases are insoluble).
  • Alkalis:
    • have pH > 7
    • turn red litmus blue
    • conduct electricity because they contain mobile ions
  • Core alkali reactions you must know:
    • neutralisation (hydroxide ions + hydrogen ions → water)
    • ammonium salt + alkali (warm) → ammonia gas
    • metal salt + alkali → metal hydroxide precipitate (often)

3. Detailed Explanations

Quick Recall (what alkalis do)
  • Alkalis have pH > 7 and turn red litmus blue.
  • Neutralisation ionic equation: H + (aq) + OH⁻(aq) → H₂O(l).
  • Warm ammonium salt + alkali → ammonia: NH₄ + + OH⁻ → NH₃ + H₂O.
  • Metal ions + alkali can form precipitates: Mⁿ⁺(aq) + nOH⁻(aq) → M(OH)ₙ(s).

A. Physical properties (and indicator behaviour)

Alkaline solutions:

  • can be identified using indicators rather than taste or touch,
  • have pH > 7 (revise: pH Scale & Indicators),
  • turn red litmus paper blue, and give a blue to purple colour with Universal Indicator.
Safety

Alkalis such as NaOH(aq) and KOH(aq) are corrosive. Avoid skin contact and rinse spills immediately with plenty of water.

B. Neutralisation (alkali + acid)

When hydroxide ions from an alkali react with hydrogen ions from an acid, they form water. This is neutralisation. Sodium and chloride ions in the example below remain in solution as spectator ions.

For a metal hydroxide such as sodium hydroxide: metal hydroxide + acid → salt + water

Net ionic equation for the hydrogen-ion/hydroxide-ion reaction: H + (aq) + OH-(aq) → H₂O(l)

Examples: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) 2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)

Aqueous ammonia is also an alkali, but its net reaction with hydrochloric acid forms ammonium chloride:

NH₃(aq) + HCl(aq) → NH₄Cl(aq)

The reacting base is ammonia, which accepts a proton. Water is the solvent, not a net product of this equation. Choose the equation for the actual base; do not apply the metal-hydroxide word equation to every alkali.

Explain the reacting particles

For a hydroxide alkali, hydrogen and hydroxide ions react to form water: H + (aq) + OH⁻(aq) → H₂O(l).

C. Alkali + ammonium salt (warm) → ammonia gas

When an alkali is warmed with an ammonium salt, ammonia gas, NH₃(g), is produced.

Ionic equation: NH₄ + (aq) + OH-(aq) → NH₃(g) + H₂O(l)

Example: NH₄Cl(s) + NaOH(aq) → NaCl(aq) + NH₃(g) + H₂O(l)

Test for ammonia:

  • hold damp red litmus paper in the gas under teacher-directed conditions,
  • it turns blue because ammonia dissolves in the moisture to form an alkaline solution.
Safety

Ammonia is irritating. Use the litmus test rather than smelling the gas. Use small amounts and work in a well-ventilated area.

D. Alkalis form metal hydroxide precipitates (from metal salt solutions)

Hydroxide ions can combine with some dissolved metal ions to form an insoluble metal hydroxide. A solid forming from solutions is called a precipitate. Not every metal salt gives one: sodium and potassium hydroxides are soluble.

Precipitation is different from neutralisation. In copper(II) hydroxide formation, copper ions and hydroxide ions become a solid; hydrogen ions are not the reacting partner.

General ionic equation: Mⁿ⁺(aq) + nOH⁻(aq) → M(OH)ₙ(s)

Examples: Cu²⁺(aq) + 2OH-(aq) → Cu(OH)₂(s) Mg²⁺(aq) + 2OH-(aq) → Mg(OH)₂(s)

Common observations with aqueous sodium hydroxide:

Ion in solutionAlkali addedObservation
Cu²⁺NaOH(aq)blue precipitate of Cu(OH)₂
Fe²⁺NaOH(aq)green precipitate of Fe(OH)₂ (turns brown on standing)
Fe³⁺NaOH(aq)brown precipitate of Fe(OH)₃
Mg²⁺NaOH(aq)white precipitate of Mg(OH)₂
Where this shows up

Use the Qualitative Analysis topic for complete tests, including behaviour in excess reagent. A colour alone does not replace the reagent, observation and conditions.

4. Common Mistakes

  • Defining alkali as “a base” and stopping (incomplete): alkali is a base soluble in water that produces OH⁻(aq).
  • Writing the wrong litmus direction: alkalis turn red → blue.
  • Writing the neutralisation ionic equation wrongly (it is H⁺ + OH⁻ → H₂O).
  • Writing precipitates as aqueous (wrong state): hydroxide precipitates are solids, (s).
  • Saying “ammonium salts + alkali produce hydrogen” (wrong gas): they produce ammonia, NH₃.

5. Exam Tips

What to write in tests questions

Reagent + observation + conclusion. Example: “Warm with NaOH(aq): ammonia produced; damp red litmus turns blue → ammonium ions present.”

  • Use the correct state symbols, especially (aq) for ions in solution and (s) for precipitates.
  • For precipitate questions, state colour and solid formation. A blue solution is different from a blue precipitate; include behaviour in excess reagent when asked.

6. Worked Examples

Modelled example 1

Definition

Core

Problem

What is an alkali?
Study the worked solution

  1. Name the broader class

    Method

    State that an alkali is a base.

    Reason

    This places alkalis within the substances that neutralise acids.

    Working

    An alkali is a base…

  2. Add both distinguishing features

    Method

    State soluble in water and produces OH⁻(aq).

    Reason

    These features distinguish alkalis from insoluble bases.

    Working

    An alkali is a base soluble in water that produces hydroxide ions, OH⁻(aq).

Guided practice 2

Neutralisation ionic equation

About 4 min

Problem

Write the ionic equation for neutralisation between HCl(aq) and NaOH(aq).

Remove spectator ions

Net ionic equation

Hints

Hint 1: spectator ions
Remove Na⁺ and Cl⁻ because they remain aqueous and unchanged.
View solution step by step

  1. Keep the reacting ions

    Method

    Retain H + (aq) and OH⁻(aq).

    Reason

    They undergo the neutralisation change.

    Working

    H⁺ + OH⁻.

  2. Form water

    Method

    Combine them in a 1:1 ratio.

    Reason

    Atoms and charge balance to form neutral liquid water.

    Working

    H + (aq) + OH-(aq) → H₂O(l).

Common misconception 3

Spot the wrong state symbol

Find and correct the mistake

Learner equation

A student writes Cu²⁺(aq) + 2OH-(aq) → Cu(OH)₂(aq). Locate and correct the error.

Connect solubility to state symbol

Correct product state

View solution step by step

  1. Identify the physical evidence

    Method

    Recognise copper(II) hydroxide as an insoluble precipitate.

    Reason

    A precipitate separates from solution as a solid.

    Working

    Cu(OH)₂ is not aqueous.

  2. Correct the equation

    Method

    Change only the product state to (s).

    Reason

    The formula and coefficients were already correct.

    Working

    Cu²⁺(aq) + 2OH-(aq) → Cu(OH)₂(s).

Examiner practice 4

Predict the gas (ammonium salt test)

3 marks

Examination question

Solid NH₄Cl is warmed with aqueous NaOH. A colourless gas turns damp red litmus paper blue. Identify the gas and justify the conclusion. [3 marks]

Use reagent, condition and observation

View solution step by step

  1. Use the ammonium reaction

    1 mark

    Method

    Recognise warming an ammonium salt with alkali.

    Reason

    This prescribed reaction releases ammonia.

    Working

    NH₄ + + OH⁻ → NH₃ + H₂O.

  2. Use the gas-test observation

    1 mark

    Method

    Interpret damp red litmus turning blue.

    Reason

    Ammonia dissolves in the moisture to give an alkaline solution.

    Working

    Damp red litmus → blue.

  3. Name the gas

    1 mark

    Working

    The gas is ammonia, NH₃(g).

Challenge 5

Precipitate colour

Minimal support

Observation-to-equation transfer

Aqueous NaOH is added to CuSO₄(aq). State the observation and write the ionic equation.

Link ion identity, colour and precipitation

Observation
Ionic equation

Hints

Hint 1: recall copper two
Copper(II) hydroxide is blue and has formula Cu(OH)₂.
Hint 2: balance charge
One Cu²⁺ requires two OH⁻ ions.
View solution step by step

  1. State the observation

    Method

    Report a blue precipitate.

    Reason

    Aqueous copper(II) ions form insoluble copper(II) hydroxide with hydroxide ions.

    Working

    Blue Cu(OH)₂(s) forms.

  2. Write the ionic equation

    Method

    Balance one copper(II) ion with two hydroxide ions.

    Reason

    This balances atoms and the + 2 charge while showing the solid product.

    Working

    Cu²⁺(aq) + 2OH-(aq) → Cu(OH)₂(s).

7. Mind Stretchers

Mind stretcher 1: “Base” vs “alkali”Extension

Question: Why is copper(II) oxide a base but not an alkali?

Show answer

Copper(II) oxide is a base because it neutralises acids, but it is not soluble in water, so it is not an alkali.

Mind stretcher 2: Improve the answerExtension

Question: Improve this sentence: “The solution is alkaline because it is blue.”

Show answer

“The solution turns red litmus paper blue, which supports that it is alkaline.” A blue solution alone could contain coloured ions, such as copper(II), and does not establish alkalinity. If a measured pH above 7 at room temperature is supplied, that provides additional evidence.

Try independently: Sodium hydroxide solution is added separately to dilute hydrochloric acid and copper(II) sulfate solution. Which reaction forms water, and which forms a solid? Write the net ionic equation for each.

Show answer and reasoning

With hydrochloric acid, hydroxide ions neutralise hydrogen ions: H + (aq) + OH⁻(aq) → H₂O(l). With copper(II) sulfate, a blue precipitate forms: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s). Both reactions use hydroxide ions, but their reacting partners and products differ.

8. Quiz

Practise and check

Use the Acid–Base Chemistry topic check to practise and check your understanding.

Syllabus and review details

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