Group 1 (alkali metals) - lithium, sodium and potassium

Q: What products form when Group 1 metals react with water?

All Group 1 metals react with water to produce a soluble metal hydroxide (forming an alkaline solution) and hydrogen gas, following the general equation: where M is the Group 1 metal.

Q: Does reactivity increase or decrease down Group 1?

Reactivity increases down Group 1. Potassium is more reactive than sodium, which is more reactive than lithium.

Edexcel International GCSE Chemistry· 2.1–2.4 (sub-topic 2(a))· 12 min read

1. Similarities in Group 1 Reactions with Water★★☆☆☆⏱ 3 min

📘 Definition

Group 1 Water Reaction

All Group 1 metals undergo exothermic reaction with water to produce a soluble metal hydroxide and hydrogen gas, forming an alkaline (pH>7) solution.

Example:

$2 L i (s) + 2 H_{2} O (l) \to 2 L i O H (a q) + H_{2} (g)$

Identical reaction products for lithium, sodium and potassium are key evidence they belong to the same chemical family. The resulting hydroxide solution turns universal indicator blue or purple, confirming its alkalinity.

📐 Worked Example

Lithium is added to distilled water. Identify the products formed and describe a test to confirm the gaseous product.

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Step 1: Recall the general Group 1 water reaction: metal + water → metal hydroxide + hydrogen
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Step 2: Identify products: aqueous lithium hydroxide (LiOH) and hydrogen gas ( $H_{2}$ )
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Step 3: Test for hydrogen: hold a lit splint near the gas, a squeaky pop confirms hydrogen is present.

2. Reactivity Trends for Li, Na, K with Air and Water★★☆☆☆⏱ 3 min

While all Group 1 metals react similarly with air and water, reaction vigour increases down the group: lithium fizzes gently, sodium reacts vigorously and melts into a silver ball, potassium reacts violently with a lilac flame.

📘 Definition

Group 1 Reactivity Trend

Reactivity increases as you move down Group 1 of the periodic table, with order of reactivity: potassium > sodium > lithium.

📐 Worked Example

Equal sized pieces of Li, Na, K are added to identical beakers of water at the same temperature. Rank the metals from least to most vigorous reaction, justifying your answer.

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Step 1: Recall reactivity increases down Group 1.
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Step 2: Rank by group position: lithium is highest in the group, followed by sodium, then lowest is potassium.
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Step 3: Final order (least to most vigorous): Lithium < Sodium < Potassium, as reactivity increases down Group 1.

Exam tip:

You may be asked to list observation differences between the three reactions, so memorise key descriptive details for each metal.

3. Predicting Properties of Other Group 1 Metals★★★☆☆⏱ 3 min

The consistent reactivity trend down Group 1 allows you to predict properties of less familiar alkali metals like rubidium and caesium, which sit below potassium in the group. You can predict reaction products, reaction vigour and other trend-following properties without memorising specific values for these metals.

📐 Worked Example

Predict the products formed when rubidium reacts with water, and compare the vigour of its reaction to potassium.

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Step 1: All Group 1 metals form the same products with water: rubidium hydroxide and hydrogen gas. The balanced equation is shown below:
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$2Rb(s) + 2H_2O(l) \rightarrow 2RbOH(aq) + H_2(g)$
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Step 2: Rubidium is below potassium in Group 1, so its reactivity is higher.
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Step 3: Rubidium will react significantly more vigorously than potassium, potentially exploding on contact with water.

4. Higher Tier: Reactivity Trend Explanation via Electronic Configuration★★★★☆Higher only⏱ 3 min

📘 Definition

Electronic Configuration Explanation

All Group 1 elements have 1 electron in their outer shell. Down the group, the outer electron is further from the nucleus, with more inner shells providing shielding, reducing attraction between the nucleus and outer electron, so it is lost more easily, increasing reactivity.

This explanation is strictly qualitative for IGCSE: you do not need to quote ionisation energy values, atomic radius measurements or enthalpy data, only describe the three key factors: distance from nucleus, shielding, and ease of outer electron loss.

📐 Worked Example

Explain why potassium is more reactive than sodium, in terms of electronic configuration.

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Step 1: Both sodium and potassium have 1 electron in their outer shell.
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Step 2: Potassium is lower in Group 1 than sodium, so it has more inner electron shells, meaning its outer electron is further from the nucleus and experiences more shielding from inner shells.
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Step 3: Attraction between the positive nucleus and negative outer electron is weaker in potassium, so the outer electron is lost more easily, making potassium more reactive than sodium.

Exam tip:

Link each factor (distance, shielding) directly to ease of outer electron loss to secure all marks for higher tier explanation questions.

5. Common Pitfalls

Wrong move:

Stating reactivity decreases down Group 1

Why:

Confusing Group 1 trend with Group 7 (halogens) where reactivity decreases down the group

Correct move:

Remember Group 1 reactivity increases down the group, Group 7 reactivity decreases down the group

Wrong move:

Writing the product of water reaction as metal oxide instead of metal hydroxide

Why:

Confusing reaction with water with reaction with oxygen in air

Correct move:

Recall the general equation: $2 M + 2 H_{2} O \to 2 M O H + H_{2}$ , forming a hydroxide, not oxide

Wrong move:

Forgetting state symbols in balanced equations

Why:

Examiners require state symbols for full marks on equation questions

Correct move:

Always add (s) for the metal, (l) for water, (aq) for dissolved hydroxide, (g) for hydrogen

Wrong move:

Higher tier: Stating the nucleus gets less positive down the group as the reason for easier electron loss

Why:

Nuclear charge actually increases down the group, but shielding and distance outweigh this effect

Correct move:

Focus on the outer electron being further from the nucleus with more shielding, reducing attraction, making loss easier

Wrong move:

Predicting rubidium reacts less vigorously than potassium

Why:

Forgetting the direction of the reactivity trend down Group 1

Correct move:

Elements lower in Group 1 are more reactive, so rubidium reacts more vigorously than potassium

6. Quick Reference Cheatsheet

Metal	Reaction with water observations	Balanced equation	Relative reactivity
Lithium	Gentle fizzing, moves on surface, no flame	$2 L i + 2 H_{2} O (l) \to 2 L i O H (a q) + H_{2} (g)$	Lowest
Sodium	Vigorous fizzing, melts to silver ball, may have yellow flame	$2 N a + 2 H_{2} O (l) \to 2 N a O H (a q) + H_{2} (g)$	Medium
Potassium	Violent reaction, lilac flame, may spit/explode	$2 K + 2 H_{2} O (l) \to 2 K O H (a q) + H_{2} (g)$	Highest (of the three)
General Group 1 Rule	All form alkaline solution, release hydrogen	$2 M + 2 H_{2} O (l) \to 2 M O H (a q) + H_{2} (g)$ (M = Group 1 metal)	Increases down group

7. Frequently Asked

What products form when Group 1 metals react with water?

All Group 1 metals react with water to produce a soluble metal hydroxide (forming an alkaline solution) and hydrogen gas, following the general equation: $2 M + 2 H_{2} O \to 2 M O H + H_{2}$ where M is the Group 1 metal.

Does reactivity increase or decrease down Group 1?

Reactivity increases down Group 1. Potassium is more reactive than sodium, which is more reactive than lithium.

What's Next

Now that you have mastered Group 1 alkali metal properties and trends, you are ready to move on to other periodic table groups, starting with Group 7 halogens, which follow the opposite reactivity trend. You will also apply these trend prediction skills to other parts of the periodic table in later inorganic chemistry topics. For higher tier students, make sure you practice writing the electronic configuration explanation for reactivity trends, as this is a common 3-4 mark question in Paper 2C. Don’t forget to link this knowledge to Group 1 flame test identification, which is covered in a separate topic on chemical tests. Past paper practice for this topic will help you get comfortable with describing reaction observations and applying trend rules to unfamiliar Group 1 metals like rubidium and caesium.