Chemical Tests

test for hydrogen

holding a burning splint held at the open end of a test tube of gas. If the gas is hydrogen it burns with a loud ‘squeaky pop’

test for oxygen

If oxygen is present in a test tube, a glowing splint relights when it is held inside.

test for carbon dioxide

If carbon dioxide is bubbled through limewater, the limewater turns milky or cloudy white

test for chlorine

Damp blue litmus paper is bleached white when it is placed in chlorine.

test for ammonia

Hold the litmus paper near the mouth of the test tube, damp red litmus paper turns blue

how to carry out a flame test

• Dip a clean wire loop into concentrated hydrochloric acid and heat to clean it. Then dip it in the hydrochloric acid again before dipping it into the metal compound that is being tested.

• Hold the loop in the roaring bunsen burner flame

• Use the colour of the flame to identify the metal compound.

what color should the flame be and why

blue flame (open the collar so more oxygen can enter), so it will not contaminate the color of the flame

what material should the wire be and why

nichrome or platinum as they have high melting point and do not contaminate the color

lithium +

flame test red

sodium +

flame test yellow / bright orange

potassium +

flame test lilac

calcium 2+

flame test red

copper 2+ flame test

flame test blue-green

NH4 + test

sodium hydroxide
warm the mixture
test for ammonia gas

Cu 2+ non flame test

add sodium hydroxide Blue precipitate should form

Fe 2+

add sodium hydroxide. green precipitate should form

Fe 3+

Add sodium hydroxide brown precipitate should form

Cl -

add nitric acid, add silver nitrate solution, white precipitate should form

Br -

add nitric acid, add silver nitrate solution, cream precipitate should form

I -

add nitric acid, add silver nitrate solution, yellow precipitate should form

SO4 2-

add hydrochloric acid and a few drops of barium chloride solution should form a white precipitate

CO3 2-

add hydrochloric acid and bubble through limewater, should turn cloudy

water

add water to white anhydrous copper (II) sulfate and it turns blue

pure water

Using a thermometer check if the boiling point is exactly 100 ^oC

Any impurities present will usually tend to raise the boiling point and depress the melting point of pure substance

why do you add hydrochloric acid/ nitric acid in a negative ion test

To dissolve the compound and remove carbonate ions, as they would also form a precipitate with the silver ions and so interfere with the test.

what is limewater

calcium hydroxide

magnesium 2+

add sodium hydroxide, should form white precipitate

calcium 2+ non flame test

add sodium hydroxide, white precipitate should form

aluminium 3+

add sodium hydroxide, should form white precipitate, dissolves in excess sodium hydroxide