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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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Learning objectives

  • describe the reaction between hydrogen ions and hydroxide ions to produce water, H+ + OH– → H2O, as neutralisation
  • describe the characteristic properties of bases in reactions with acids and with ammonium salts

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 (alkali + acid → salt + 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:

  • have a bitter taste and feel slippery/soapy (do not touch/taste chemicals in real life),
  • have pH > 7 (revise: pH Scale & Indicators),
  • turn red litmus paper blue.
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 an alkali reacts with an acid, the reaction is a neutralisation reaction.

Word equation: alkali + acid → salt + water

Ionic equation (the one you should write): H + (aq) + OH-(aq) → H₂O(l)

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

Mark-scheme language

Neutralisation is the reaction between H + (aq) and OH⁻(aq) to form water.

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:

  • ammonia has a pungent smell (do not inhale directly),
  • it turns damp red litmus paper blue.
Safety

Ammonia is irritating. Do not sniff gases directly; waft gently if instructed. Use small amounts and work in a well-ventilated area.

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

Alkalis are used to form insoluble metal hydroxides (precipitates) from solutions of metal salts.

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)

Typical precipitate colours (memorise the common ones):

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

These precipitate colours are heavily tested in qualitative analysis: Qualitative Analysis (QA).

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, colour is the mark. Write it clearly.

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 is alkaline because it turns red litmus paper blue and has pH greater than 7.”

8. Quiz

Quiz Time!

Test yourself on alkali properties, neutralisation, ammonium salt tests, and precipitate colours.

Go to Quiz Page