This experiment shows the differences in the properties of various alkali metals. Solutions are prepared from these alkali metals with the pH and temperature changes recorded. Students also get to test their obervational skills by completing flame tests from these solutions.

Class practical

This experiment shows the differences in the properties of various alkali metals. Solutions are prepared from these alkali metals with the pH and temperature changes recorded. Students also get to test their obervational skills by completing flame tests from these solutions.

Lesson organisation

This experiment is suitable for pre-16 students. Most of the experiment can be done as a class practical. It takes about 20 minutes for students to investigate around four compounds.

Students should not be allowed to handle solid alkali metal hydroxides. You should demonstrate their properties.

You should decide which method you want to use for the flame tests, and if using an alternative method to that which is given here, carry out an appropriate risk assessment.

You need to decide in advance what strategy to use in allocating compounds to pairs of students.

A student worksheet with instructions and questions is available with this resource.

Apparatus Chemicals

Eye protection

Each pair of group of students will require:

Test-tubes in a rack, 2 or 3

Rubber bung to fit test-tube

Glass rod

White tile

Universal Indicator paper and colour chart

Wooden splints pre-soaked in distilled water, for flame tests (Note 1)

Bunsen burner

Heat resistant mat

A pH chart

Purified water

A range of solid alkali metal compounds such as:

Sodium and potassium nitrates (OXIDISING)

Sodium and potassium chlorides

Sodium and potassium sulfates

Lithium, sodium and potassium carbonates (IRRIANT)

Sodium and potassium hydroxides (CORROSIVE) - for teacher demonstration only

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection. 

Sodium nitrate, NaNO3(s), (OXIDISING, HARMFUL) and potassium nitrate, KNO3(s), (OXIDISING) - see CLEAPSS Hazcard. Alkali metal nitrates are deliquescent (absorb moisture from the air) and must be kept in stoppered bottles until shortly before use.

Sodium chloride, NaCl(s), and potassium chloride, KCl(s) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.  

Sodium sulfate, Na2SO4(s), and potassium sulfate, K2SO4(s) - see CLEAPSS Hazcard. 

Lithium carbonate, Li2CO3(s), (HARMFUL), sodium carbonate, Na2CO3(s), (IRRITANT) and potassium carbonate, K2CO3(s), (IRRITANT) - see CLEAPSS Hazcards and CLEAPSS Recipe Book. 

Sodium hydroxide, NaOH(s), (CORROSIVE) and potassium hydroxide, KOH(s), (CORROSIVE) - see CLEAPSS Hazcard. Alkali metal hydroxides react with the carbon dioxide in the air and must be kept in stoppered bottles until shortly before use. 

1 In advance of the lesson, a supply of wooden splints need to be thoroughly soaked in distilled water. 

Procedure

a Students should not dissolve solid sodium hydroxide or potassium hydroxide themselves. However, it is useful for students to see the precautions necessary when dissolving a compound with a very large negative enthalpy of solution. Wearing goggles (not safety spectacles), demonstrate the behaviour of the solid hydroxides by adding a single pellet to water, in a test-tube or small beaker. Point out the large rise in temperature. Small samples of the solution can then be used for the pH test. Sodium hydroxide solutions are IRRITANT at or above 0.05 M and CORROSIVE at or above 0.5 M.

Students can then carry out the following activities:
b Note the name of the compound and find out whether any special precautions are necessary when handling it. (Students should be told which reference book to use and should check with their teacher before proceeding.)

c Note the colour of the compound.

d Shake a small amount of the compound (a few crystals) in a test-tube with about 2 cm depth of purified water. Note whether the solid dissolves, and whether the test-tube gets warm.

e Use a glass rod to put a drop of the solution on to a small piece of Universal indicator paper, and compare the colour with a chart to find the pH.

f Take a wooden splint, pre-soaked in distilled water, and place it in the solution in the test tube. Allow some of the solution to soak into the splint then hold the splint in the side of a roaring Bunsen flame. Record the flame colour. Look up the formula of the compound.

g  Record all the results from their group in a table with suitable headings, such as these:
Name of compound / Hazards / Colour / pH / Test for solubility observation / Flame test; colour / Formula.

Teaching notes

When testing pH the students should compare their solutions with the effect of the purified water which is unlikely to be neutral (distilled water will be weakly acidic, due to dissolved CO2). Only if there is a difference can they suggest that the alkali metal compound is affecting the pH.

This activity provides an opportunity to explore the common misconception among students that alkali metal compounds are alkaline because of the presence of the alkali metal ions. If this were so, all the compounds would be alkaline in solution and not just the hydroxides and carbonates.

There is a student sheet to download - see Lesson organisation above.

Notes on the results
All the compounds suggested are colourless or white. The compounds are soluble in water. The hydroxides and carbonates are alkaline. Flame colours: lithium – bright red; sodium – bright yellow/orange; potassium – mauve/lilac (often masked by traces of sodium compounds in the sample).

Health & Safety checked, July 2016 

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated July 2016

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