Chemistry Edexcel iGCSE - Chapter 18 - Chemical Tests

Test for hydrogen gas

Hold a lit splint above the gas produce. Hydrogen explodes with a squeaky pop

Test for oxygen gas

A glowing splint relights if oxygen is present

Test for carbon dioxide gas

Bubble carbon dioxide through limewater (calcium hydroxide). Carbon dioxide turns limewater from colorless to cloudy.

Test for chlorine gas

Damp blue litmus paper will turn red and then bleach it white

Test for ammonia gas

Damp red litmus paper will turn blue because ammonia is the only alkaline gas.

Chemical test for water

Water turns white anhydrous copper(II) sulfate blue.

Physical test for water

Measure the boiling or freezing point of the water. Pure water boils at exactly 100°C and freezes at exactly 0°C.

If water is impure, will it boil and a higher or lower temperature?

Higher

If water is impure, will it freeze and a higher or lower temperature?

Lower

Describe the flame test

1. Dip a platinum wire loop into concentrated hydrochloric acid

2. Then dip the wire loop into the ion you want to test

3. Hold the loop over a non-luminous Bunsen flame and record the color change.

Why is a non-luminous (blue) flame used?

So the color change is clearer

Why do we dip the loop in hydrochloric acid

To clean the loop and prevent the mixture of any irons that might affect the colour.

Why do we use a platinum wire loop?

It has a high melting point and it is unreactive and won’t cause the flame to change color.

Color of lithium ions in the flame test

Red

Color of sodium ions in the flame test

Yellow

Color of copper ions in the flame test

Blue-green

Color of calcium ions in the flame test

Orange-red

Color of potassium ions in the flame test

Lilac

How to test for metal ions with sodium hydroxide

Dissolve the salt in distilled water and then add the same volume of the solution and sodium hydroxide. Then record the color of the precipitate formed.

Color of aluminium and calcium ion precipitate

White

Color of copper(II) ion precipitate

Blue

Color of iron(II) precipitate

Green

Color of iron(III) precipitate

Brown

Test for ammonium ions

Warm a mixture of dilute sodium hydroxide and ammonium ions. Ammonia gas will be produce. Hold damp red litmus paper over the gas. Ammonia changes damp red litmus paper to blue.

Test for carbonate ions

Add dilute hydrochloric acid to the sample. Carbon dioxide gas will be produced. Bubble the gas through limewater, which will turn cloudy if CO₂ is present.

Test for sulfate ions

Add dilute hydrochloric acid to remove any carbonate ions. Then add barium chloride. A white precipitate of barium sulfate will form if sulfate ions are present.

Test for halide ions

Add dilute nitric acid to remove any carbonate ions. Then add silver nitrate.

What is the color of the precipitate if chloride ions are present

White

What is the color of the precipitate if bromide ions are present

Cream

What is the color of the precipitate if iodide ions are present

Yellow