CHEMICAL TESTS [dchemlc]

When a glowing splint is put into a tube containing a colourless gas, it relights

What is this gas?

• hydrogen

• oxygen

• ammonia

• carbon dioxide

Answer explanation

TEST FOR OXYGEN GAS

A glowing splint is put into the tube containing the gas. Oxygen relights a glowing splint.

TEST FOR HYDROGEN GAS

A lighted splint is held to the mouth of the tube. The hydrogen explodes with a squeaky pop.

 When a piece of damp universal indicator paper or a red litmus paper was held at the mouth of a test-tube, it turned white. What is this gas?

• hydrogen

• ammonia

• chlorine

• oxygen

Answer explanation

Chlorine is a green gas that bleaches (turns white) the damp litmus paper or universal indicator paper.

If blue litmus paper or universal indicator paper are used for this test, they go red first (the chlorine dissolves in the water to form an acidic solution) and then white.

What is the colour change observed in the following reaction?

anhydrous copper sulfate + water -------> hydrated copper sulfate

                 CuS0_4(s)   + 5H₂0_(l) ------->       CuS0₄·5H₂0_(s)

• Blue to black

• Black to white

• White to blue

• Blue to white

Answer explanation

Water turns white anhydrous copper(II) sulfate blue. Anhydrous copper(II) sulfate lacks water of crystallization and is white. Dropping water onto it replaces the water of crystallization and turns it blue.

A flame test gave a orange-red flame. What was the metal that was tested?

• Calcium ions

• Copper (ll) ions

• Potassium ions

• Sodium ions

Answer explanation

If a given sample of water was impure, it will;

• Freeze at exactly 0 ⁰C

• Boil below 100⁰C

•  Turn white anhydrous copper sulfate blue

• Freeze below 0⁰C

Answer explanation

You can find out whether a sample of water is pure by measuring the freezing (melting) point or boiling point. Pure water freezes at exactly O •c and boils at exactly 100 °C at 1 atmosphere pressure. If the water is impure, it will usually freeze at a lower temperature and boil at a higher temperature.

When sodium hydroxide was added to a solution of copper (ll) sulfate, a blue precipitate was formed. Therefore what will be the products formed in this reaction?

• Copper (ll) hydroxide

• Sodium sulfate

• water

• Sulfuric acid

Answer explanation

Formation of a blue precipitate shows the presence of copper{II) ions. The precipitate is copper{II) hydroxide.

CuS0₄(aq) + 2NaOH(aq) --> Cu(OH)₂(s) + Na₂S0₄(aq)

When sodium hydroxide was added to a solution an orange-brown precipitate was formed.

What could this precipitate be?

• Fe(OH)₃

• Fe(OH)₂

• Cu(OH)₂

• NaOH

Answer explanation

Formation of an orange-brown precipitate shows the presence of iron(III) ions (Fe3⁺). The precipitate is iron(III) hydroxide.

FeCl₃aq) + 3NaOH(aq) ---->

Fe(OH)₃(s) + 3NaCl(aq)

When sodium hydroxide was added to a solution containing unknown ions, no precipitate was formed. But a gas was evolved which turned damp litmus paper blue. What could this gas be?

• NH₃

• CO₂

• O₂

• H₂

Answer explanation

Sodium hydroxide solution reacts with ammonium salts to produce ammonia gas. In the cold, there is just enough ammonia gas produced for you to be able to smell it. If you warm it, you can test the gas coming off with a piece of damp red litmus paper or universal indicator paper. Ammonia is alkaline and turns the paper/universal indicator blue

When a few drops of dilute hydrochloric acid was added to a sample of salt, a gas was evolved which turned limewater milky when passed through it. What could this salt be?

• A hydroxide

• A carbonate

• A nitrate

• A chloride

Answer explanation

Add a little dilute hydrochloric acid to your sample of salt and look for fizzing/ bubbles of gas. Fizzing indicates that a gas is given off. You can test the gas by bubbling it through limewater to show that it is carbon dioxide.

To a solution of unknown ions, enough dilute nitric acid and some silver nitrate was added. A cream coloured precipitate was formed. What could this precipitate be?

• AgCI

• AgBr

• Agl

• AgOH

Answer explanation

A white precipitate (of silver chloride) shows the presence of chloride ions:

Ag⁺(aq) + Cl^-(aq) --> AgCl(s)

A cream precipitate (of silver bromide) shows the presence of bromide ions:

Ag⁺(aq) + Br-(aq) --> AgBr(s) A yellow precipitate (of silver iodide) shows the presence of iodide ions:

Ag⁺(aq) + I^-(aq) ---> Agl(s)