29 How do alkali metals react with water? 14-16 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

In this activity, teacher demonstrations are used to help students 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 students write an appropriate ‘voice over’ to accompany the demonstration. Finally, students predict how rubidium and caesium will react with cold water before watching the reactions on a video clip.

Learning objectives

Students will be able to:

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

Sequence of activities

Issue traffic light cards and mini whiteboards.

Ask students 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 students to:

  • predict which of the three elements will react most vigorously with water
  • use their cards to indicate:
    • lithium (red)
    • sodium (yellow)
    • potassium (green)
  • explain their choices (select a few students)
  • 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.

  • Show the experimental set up for the reaction of lithium with cold water.
  • Explain that you are going to drop a piece of lithium into a trough half full of cold water.
  • Carry out the demonstration.
  • Add some Universal indicator to the solution.
  • Repeat the demonstration but, this time, drop the lithium onto a piece of filter paper floating on the surface of the water.
  • 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 students or yourself).
  • Test for hydrogen with a lighted splint.
  • 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.
  • Show the experimental set up for the reaction of sodium with cold water.
  • Explain that you are going to drop a piece of sodium into a trough half full of cold water.
  • Carry out the demonstration.
  • Add some Universal indicator to the solution.
  • Repeat the demonstration but, this time, drop the sodium onto a piece of filter paper floating on the surface of the water.
  • 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 students or yourself).
  • 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.
  • Show the experimental set up for the reaction of potassium with cold water.
  • Say that you are going to repeat the same procedure as before, but using potassium.
  • Explain that you are going to do this without making any comments and they therefore need to watch what happens very carefully.
  • Carry out the demonstration.
  • Add some Universal indicator to the solution.
  • 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 students or yourself).
  • Test for hydrogen with a lighted splint.
Give each student a copy of How do alkali metals react with water? Student sheet.

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.

Demonstrate the reactions of potassium with water again, but this time describe and explain what is happening.
Ask students to re-read 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? Student sheet .
Tell the students that they are going to see a video of rubidium and caesium being added to cold water.
  • Ask them to write, on their How do alkali metals react with water? Student sheet, how they think these elements will react with cold water.
  • Select some students to describe what they think the reactions will be and why.
  • Show a video clip of rubidium and caesium reacting with water.
  • Make sure that all students record that there is an increase in reactivity moving down group 1 because the outer electrons are further from the nucleus.
Ask students to:
  • compare what they had predicted with what they saw in the video
  • write their comparison on their How do alkali metals react with water? Student sheet.
Take in the How do alkali metals react with water? Student sheets and comment on positive achievements illustrated by ‘voice over’ accounts and predictions and draw attention to the ways in which the descriptions can be developed even further.

Assessment for learning commentary

The initial predictions engage students 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.

Resources

Props

For each student

Download Word Download PDF How do alkali metals react with water? Student sheet
  • Set of traffic light cards
  • Mini whiteboard

Equipment

  • Apparatus and materials to demonstrate the addition of lithium (Corrosive and Flammable), sodium (Corrosive and Flammable) and potassium (Corrosive and Flammable), to cold water
  • Video clip showing the addition of rubidium and caesium to water.

Safety notes

It is the responsibility of the teacher to carry out an appropriate risk assessment for the demonstration.

Principal hazard

  • Using corrosive and flammable materials.

Acknowledgment

T. Lister, Classic chemistry demonstrations. London: Royal Society of Chemistry, 1995.