How do alkali metals react with water?

In this activity, teacher demonstrations are used to help learners learn about how alkali metals react with cold water.

The reactions of lithium and sodium are demonstrated and explained; the reaction of potassium is demonstrated without comment and learners write an appropriate ‘voice over’ to accompany the demonstration.

Finally, learners predict how rubidium and caesium will react with cold water before watching the reactions on a video clip.

Learning objectives

learners will be able to:

• Describe and explain how the alkali metals react with cold water.

Sequence of activities

Introduction

Issue traffic light cards and mini whiteboards.

Ask learners to draw diagrams to show the difference in electron arrangements in atoms of lithium, sodium and potassium and ask some of them to show their ideas. Use questions to draw out similarities and differences between electron arrangements.

Ask learners to:

1. Predict which of the three elements will react most vigorously with water.
2. Use their cards to indicate:
   • Lithium (red)
   • Sodium (yellow)
   • Potassium (green)
3. Explain their choices (select a few learners).
4. Note the learning objectives.

Explain that they are going to see you demonstrate the reaction of some alkali metals with water. Stress that they need to watch and listen carefully because they are going to write a commentary about one of the reactions that they see you demonstrate.

Demonstration 1: lithium

1. Show the experimental set up for the reaction of lithium with cold water.
2. Explain that you are going to drop a piece of lithium into a trough half full of cold water.
3. Carry out the demonstration.
4. Add some universal indicator to the solution.
5. Repeat the demonstration but, this time, drop the lithium onto a piece of filter paper floating on the surface of the water.
6. Drop a small piece of lithium into a boiling tube half full of cold water (ensure that the boiling tube is not pointing at the learners or yourself).
7. Test for hydrogen with a lighted splint.
8. Use open questions which focus on the change in indicator colour and positive test for hydrogen to reinforce what is happening as lithium reacts with water.

Demonstration 2: sodium

1. Show the experimental set up for the reaction of sodium with cold water.
2. Explain that you are going to drop a piece of sodium into a trough half full of cold water.
3. Carry out the demonstration.
4. Add some Universal indicator to the solution.
5. Repeat the demonstration but, this time, drop the sodium onto a piece of filter paper floating on the surface of the water.
6. Drop a small piece of sodium into a boiling tube half full of cold water (ensure that the boiling tube is not pointing at the learners or yourself).
7. Test for hydrogen with a lighted splint.

As you carry out the demonstration, describe what is happening and explain how the change in indicator colour and positive test for hydrogen help us interpret what is happening as sodium reacts with water.

Use questions to make explicit the similarities and differences between the reactions of lithium and sodium.

Demonstration 3: potassium

1. Show the experimental set up for the reaction of potassium with cold water.
2. Say that you are going to repeat the same procedure as before, but using potassium.
3. Explain that you are going to do this without making any comments and they therefore need to watch what happens very carefully.
4. Carry out the demonstration.
5. Add some Universal indicator to the solution.
6. Drop a small piece of potassium into a boiling tube half full of cold water (ensure that the boiling tube is not pointing at the learners or yourself).
7. Test for hydrogen with a lighted splint.

Demonstration 3: draft commentary and repeat

1. Give each learner a copy of ’How do alkali metals react with water? learner sheet’.
2. Ask them to write a commentary that could be used as a ‘voice over’ to accompany the reactions as potassium is added to water. This should both describe and explain what happens.
3. Demonstrate the reactions of potassium with water again, but this time describe and explain what is happening.
4. Ask learners to reread their ‘voice over’ and:
   • Compare it with the commentary they have just heard.
   • Identify its good features or where it could have been developed further.
   • Complete the relevant sections on their ’How do alkali metals react with water? learner sheet’.

Video demonstrations: stage 1

1. Tell the learners that they are going to see a video of rubidium and caesium being added to cold water.
2. Ask them to write, on their ’How do alkali metals react with water? learner sheet’, how they think these elements will react with cold water.
3. Select some learners to describe what they think the reactions will be and why.
4. Show a video clip of rubidium and caesium reacting with water.
5. Make sure that all learners record that there is an increase in reactivity moving down group 1 because the outer electrons are further from the nucleus.

Video demonstrations: stage 2

Ask learners to:

1. Compare what they had predicted with what they saw in the video.
2. Write their comparison on their ’How do alkali metals react with water? learner sheet’.

Commentary

The initial predictions engage learners with the learning objectives.

They assess their own work, both for describing the reaction of potassium with water and for their predictions about rubidium and caesium.

Well written feedback confirms achievement, promotes confidence and indicates possible improvements.

Practical notes

Equipment

For the demonstrations

• Video clips showing the addition of rubidium and caesium to water. You may wish to use the slow motion videos Rubidium, water and indicator and Caesium in water, both featuring commentary, from ’The Periodic Table of Videos’ by The University of Nottingham.

For each learner

• Set of ’traffic light’ cards
• Mini whiteboard

Health, safety and technical notes

• Read our standard health and safety guidance.
• For the demonstrations, consult the safety instructions included with the procedure for exploring the reactivity trends of the alkali metals.
• It is the responsibility of the teacher to carry out an appropriate risk assessment for the demonstration.

Principal hazard

• Using corrosive and flammable materials.