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Group 1 Elements: The Alkali Metals

Learn Group 1 alkali-metal properties, melting-point and reactivity trends, water-reaction observations, balanced equations and exam explanations.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • predict the properties of elements in Group 1 and Group 17 using the Periodic Table.
  • describe lithium, sodium and potassium in Group 1 (the alkali metals) as a collection of relatively soft, low density metals showing a trend in melting point and in their reaction with water

The Group 1 alkali metals occupy one column of the Periodic Table and have similar chemical properties because each atom has one outer-shell electron. O-Level questions compare their physical properties and their increasingly vigorous reactions with water.

Group 1 reaction with water becomes more vigorous down the groupLithium floats and fizzes steadily, sodium melts into a ball and moves rapidly, and potassium reacts very vigorously and may ignite. All three form an alkaline metal hydroxide solution and hydrogen gas. Comparing equal pieces under the same conditions shows that reaction vigour and reactivity increase down the group.Reaction with water: increasing vigour down Group 1Lithium, Lifloats and fizzes steadilymoves slowly on the surfaceSodium, Namelts into a ball and moves rapidlyfizzes more vigorouslyPotassium, Kvery vigorous; may ignitea lilac flame may be seenreactivity and reaction vigour increase2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)Fair comparison: use equal amounts under the same conditions, then extend the observed trend qualitatively.
Compare the same small amount of each metal under the same conditions. Increasing vigour is evidence that reactivity increases from lithium to potassium.

Predict a Group 1 trend with the interactive periodic table, then return here to connect the pattern to the explanation.

1. Definition

A. Alkali metal

Group 1 elements are called alkali metals because they form alkaline metal hydroxide solutions when they react with water.

2. Key Ideas

  • Group 1 metals have 1 outer-shell electron, so they form M⁺ ions.
  • Reactivity increases down the group (lithium < sodium < potassium).
  • This group trend is consistent with their positions near the top of the reactivity series.
  • They react with water to form a metal hydroxide and hydrogen.
  • Their softness and low density are typical physical properties; melting points decrease down the group.

3. Detailed Explanations

Quick Recall (Group 1 basics)
  • Group 1 metals have 1 outer-shell electron, so they form M⁺ ions.
  • Reactivity increases from lithium to sodium to potassium; use observations as evidence.
  • With water: metal hydroxide (alkaline solution) + hydrogen, H₂.
  • Hydrogen test: a lighted splint gives a squeaky pop.

A. Physical properties (what you can actually write)

PropertyTrend / detail (O-Level)
appearanceshiny when freshly cut; tarnish quickly in air
softnesssoft; can be cut with a knife
melting pointrelatively low; decreases down the group
densityrelatively low; lithium, sodium, potassium float on water; not a simple “increases down the group” trend (potassium is less dense than sodium)
conductivitygood conductors (metals)

The syllabus trend is qualitative: melting point decreases down Group 1. Do not invent numerical values if a question supplies only the order of the elements.

Laboratory Warning

Group 1 metals are stored under oil to limit contact with oxygen and water vapour. Their reactions with water must use very small pieces under teacher-controlled conditions; potassium must not be proposed for an unsupervised student experiment.

The syllabus expects qualitative trends and predictions. Compare evidence collected under the same conditions:

  • melting point decreases from lithium to sodium to potassium;
  • reaction with water becomes more vigorous from lithium to sodium to potassium;
  • an unfamiliar element below potassium should follow the same stated trends unless supplied data shows otherwise.
Describe before you predict

State the observed direction of the trend, then extend it. Do not invent an exact melting point or reaction time.

C. Reaction with water

General equation: 2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)

Example: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Observations you can use as evidence:

MetalTypical observation with water (small piece)
lithiumfloats, fizzes gently, moves slowly
sodiummelts into a silvery ball, darts around, vigorous fizzing
potassiumvery vigorous, may ignite with lilac flame

Why the solution becomes alkaline: MOH(aq) is an alkali because it produces OH⁻(aq).

Gas test: hydrogen gives a squeaky pop with a lighted splint.

This links three representations:

  • macroscopic: the metal moves and effervescence is seen; the resulting solution is alkaline
  • particle: metal atoms lose electrons to form M⁺ ions, while water ultimately forms H₂ and OH⁻
  • symbolic: the balanced equation represents the same change

2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)

4. Common Mistakes

  • Forgetting the products of water reaction: hydroxide + hydrogen.
  • Missing the hydrogen gas test (squeaky pop).
  • Writing vague observations instead of specific ones (“gas produced” is not enough; say “effervescence”).
  • Giving the trend without evidence from water-reaction vigour or supplied melting-point data.
  • Treating flame colours or reactions with oxygen as required Group 1 content.

5. Exam Tips

Two sentences that score quickly
  • “Reaction with water becomes more vigorous from lithium to potassium, so reactivity increases down Group 1.”
  • “With water: metal hydroxide (alkaline solution) and hydrogen gas are formed.”
  • Always include state symbols if the question expects them.
  • For a prediction, state the trend first and avoid an unsupported exact value.

6. Worked Examples

Modelled example 1

Predict products (water reaction)

Core

Problem

Write the balanced equation, including state symbols, for potassium reacting with water.
Study the worked solution

  1. Predict the products

    Method

    Use the Group 1 water-reaction pattern.

    Reason

    A Group 1 metal reacts with water to form its soluble hydroxide and hydrogen.

    Working

    K + H₂O → KOH + H₂.

  2. Balance atoms

    Method

    Use coefficients of 2 for potassium, water and potassium hydroxide.

    Reason

    This gives two K, four H and two O atoms on each side.

    Working

    2K + 2H₂O → 2KOH + H₂.

  3. Add physical states

    Method

    Label the metal, water, dissolved hydroxide and gas.

    Reason

    Potassium is solid, water is liquid, potassium hydroxide is aqueous and hydrogen is gaseous under the stated conditions.

    Working

    2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

Guided practice 2

Trend from evidence

About 5 min

Problem

Lithium fizzes gently with water, sodium reacts vigorously and potassium reacts very vigorously. State the trend supported by these observations.

Turn observations into an ordered claim

Increasing vigour
Reactivity down Group 1

Hints

Hint 1: order the evidence
Arrange gentle, vigorous and very vigorous from least to most.
Hint 2: connect vigour to reactivity
Under comparable conditions, a more vigorous reaction supports greater reactivity.
View solution step by step

  1. Order the observations

    Method

    Compare reaction vigour under the stated conditions.

    Reason

    Vigour provides the observable evidence for relative reactivity.

    Working

    Lithium < sodium < potassium in reaction vigour.

  2. State the group trend

    Method

    Map the order onto positions down Group 1.

    Reason

    Lithium, sodium and potassium occur successively lower in the group.

    Working

    Reactivity increases down Group 1.

Common misconception 3

Link outer electrons to ion charge

Find and correct the mistake

Learner response

A student says: “Sodium gains one electron to form Na⁺ because it is in Group 1.” Locate the direction error and explain the + 1 charge.

Track the negative electron

Electron change
Resulting charge

View solution step by step

  1. Locate the direction error

    Method

    Replace “gains” with “loses.”

    Reason

    Sodium has one outer electron and reaches a stable outer-shell arrangement by removing it.

    Working

    Na → Na⁺ + e⁻.

  2. Explain the sign

    Method

    Compare proton and electron charges after the loss.

    Reason

    Losing one negative charge leaves one net positive charge.

    Working

    Sodium forms Na⁺.

Examiner practice 4

Identify the gas

2 marks

Examination question

A colourless gas is produced when sodium reacts with water. State the test and positive result used to identify the gas. [2 marks]

Name the procedure and observation

View solution step by step

  1. State the test

    1 mark

    Method

    Bring a lighted splint to the gas.

    Reason

    This is the standard test for hydrogen.

    Working

    Test: lighted splint.

  2. State the positive result

    1 mark

    Method

    Listen for a squeaky pop.

    Reason

    Hydrogen ignites rapidly in the presence of oxygen.

    Working

    Positive result: squeaky pop → hydrogen.

Challenge 5

Predict a melting-point trend

Minimal support

Trend extrapolation

The melting points of lithium, sodium and potassium decrease in that order. Predict how the next Group 1 element’s melting point is likely to compare with potassium, and state the limit of the prediction.

Extend direction without inventing a number

Likely comparison
Defensible claim

Hints

Hint 1: continue the supplied order
Lithium to sodium to potassium is both downward in the group and downward in melting point.
Hint 2: separate direction from magnitude
A trend direction does not supply the size of the next change.
View solution step by step

  1. State the supplied trend

    Method

    Identify decreasing melting point down Group 1.

    Reason

    The three named elements establish a qualitative direction.

    Working

    Tₘ(Li) > Tₘ(Na) > Tₘ(K).

  2. Extend and bound the claim

    Method

    Predict a lower melting point for the next element.

    Reason

    This follows the given trend, but no spacing or equation supports an exact value.

    Working

    Likely lower than potassium; exact value cannot be claimed from these data.

7. Mind Stretchers

Mind stretcher 1: Predict an unknown elementExtension

Question: Element X is below potassium in Group 1. Predict how its reaction with water would compare with potassium and explain your prediction.

Show Answer

X should react more vigorously than potassium because the observed reactivity trend increases down Group 1.

Mind stretcher 2: Evaluate a fair comparisonExtension

Question: A student compares a large piece of lithium with a tiny piece of sodium in water and concludes lithium is more reactive. Why is the conclusion unreliable?

Show Answer

The metal amount was not controlled, so reaction vigour cannot be attributed only to metal identity. Use equal-sized or equal-mass pieces under the same conditions.

8. Quiz

Quiz Time!

Test physical properties, water-reaction observations, balanced equations, ion formation and trend explanations.

Go to Quiz Page