All Tests :

Gas Tests

Oyxgen

How is the test carried out?

What change can be observed for a positive test?

Why do we test for this gas?

Hydrogen

What change can be observed for a positive test?

How is the test carried out?

Why do we test for this gas?

Carbon dioxide

Ammonia

How is the test carried out?

What change can be observed for a positive test?

Why do we test for this gas?

Chlorine

How is the test carried out?

What change can be observed for a positive test?

Why do we test for this gas?

Put a lit splint in a test tube with an unknown gas in it.

The lit splint will be extinguished once it is placed inside the test tube if hydrogen is present. It will also make a squeaky pop if the test is positive.

Place a glowing splint (a split that was recently extinguished but is still glowing hot) inside a test tube with the unknown gas in it.

If oxygen is present, the glowing splint will be relit

Damp UI paper should be inserted into the gas.

If the UI paper turns red and then bleaches, chlorine is present. The gas also has a characteristic sharp smell that is quite dangerous.

Damp red litmus paper should be inserted into the gas.

If the paper turns blue then ammonia is present. The gas also has a characteristic pungent smell that is quite dangerous.

Splint Method

Lime Water Method

How is the test carried out?

What change can be observed for a positive test?

Why do we test for this gas?

Connect two test tubes (one containing your unknown gas and one containing the clear limewater) together via an enclosed tube with a bung on top, and allow the gas to bubble into the limewater.

The lime water will turn cloudy if there is CO2 present

Cations

How is the test carried out?

What change can be observed for a positive test?

Why do we test for this gas?

A lit splint is added to the test tube containing the unknown gas

If carbon dioxide is present, the flame will be extinguished

Carbon dioxide can be poisonous in high concentrations, but it also has lots of uses, e.g. fire extinguishers and fizzy drinks.

Oxygen is vital for life but in high concentrations it will cause things to ignite.

Hydrogen can be highly explosive but it's also a great fuel.

Chlorine has lots of industrial uses (e.g. bleaching), but it can be poisonous.

Ammonia is very important in the manufacture of fertiliser but it's also poisonous.

Anions

Wet Tests

Flame Tests

What is the general method (with explanations for each step)?

What are the cations that can be found and how are they identified?

Potassium (K+)

Calcium (2+)

Sodium (Na+)

Copper (2+)

Lithium (Li+)

If lithium is present, the flame will turn red

If sodium is present, the flame will turn yellow

If potassium is present, the flame will turn lilac

If calcium is present, the flame will turn orange-red

If copper is present, the flame will turn green-blue

Step 3) Dip the clean wire into the hydrochloric acid again, then dip it into the compound.

Step 4) Place the wire into the blue flame of a bunsen burner.

Step 2) Place the wire into a blue flame, and ensure that the flame remains colourless

Step 5) Record the colour and then figure out which cation the compound contained.

Step 1) Dip the wire into hydrochloric acid.

We do this because we want to clean the wire and remove any impurities.

If the wire is placed into the blue safety flame and there is no colour, this means that there are no impurities left on the wire, and that the colour change (if any) later on when the cation is added isn't obstructed or ruined by the wire adding extra colour that could change the results and make them inaccurate.

This can be done by looking at the list below

Since the blue flame is "colourless" (or at least less so than the orange of the safety flame), the resulting colour will be more accurate as we will be able to tell that the colour came from the cation and not from the bunsen burner. For example, with lithium, if we were to burn it while the flame is on safety, we wouldn't be able to tell if the red colour was just the bunsen burner's colour, or if it was caused by the cation.)

Iron III (Fe3+)

Copper II (Cu2+)

Iron II (Fe2+)

Ammonium (NH4+)

Method

Result of a positive test

A green precipitate forms after sodium hydroxide is added

Step 3) Wait a few minutes and then record the colour of the precipitate formed

Step 2) Next, using a different pipette, add 5cm3 of sodium hydroxide solution to the test tube

Step 1) Using a clean pipette, fill a test tube with 5cm3 of your unknown sample

If ammonia is present, the red litmus paper should turn blue

A brown precipitate forms after sodium hydroxide is added

A blue precipitate forms after sodium hydroxide is added

Method

Step 3) Wait a few minutes and then record the colour of the precipitate formed

Step 2) Next, using a different pipette, add 5cm3 of sodium hydroxide solution to the test tube

Step 1) Using a clean pipette, fill a test tube with 5cm3 of your unknown sample

Method

Step 3) Wait a few minutes and then record the colour of the precipitate formed

Step 2) Next, using a different pipette, add 5cm3 of sodium hydroxide solution to the test tube

Step 1) Using a clean pipette, fill a test tube with 5cm3 of your unknown sample

Method

Step 4) Put a piece of damp red litmus paper into the sample

Step 2) Next, using a different pipette, add 5cm3 of sodium hydroxide solution to the test tube

Step 1) Using a clean pipette, fill a test tube with 5cm3 of your unknown sample

Step 3) Warm the sample (at this point, if it was sodium hydroxide was present it should be giving off ammonia gas)

SO42-

Bromine (Br-)

CO32-

Chlorine (Cl-)

Method

Result of a positive test

Step 2) Next, a few drops of silver nitrate solution are added into the unknown solution

Step 3) Finally, the colour of the precipitate formed should be recorded

Step 1) The unknown sample is first acidified using a few drops of dilute nitric acid

Result of a positive test

Method

Result of a positive test

Method

Result of a positive test

If bromine is present, a cream precipitate should have formed

If chlorine is present, a white precipitate should have formed

Step 2) Add a few drops of the now acidified barium chloride solution to your unknown solution

Step 3) Record the colour of the precipitate (if any) formed

Step 1) Mix some hydrochloric acid with barium chloride solution

If a white precipitate has formed, then barium sulfate is present

Add a few drops of hydrochloric acid to your unknown solution

If bubbles form, then carbonate is present

The acidified part is important as this means it reacts with any other ions which could also give a precipitate and mess with the final conclusion of what anion is present in the solution

Method

Step 2) Next, a few drops of silver nitrate solution are added into the unknown solution

Step 3) Finally, the colour of the precipitate formed should be recorded

Step 1) The unknown sample is first acidified using a few drops of dilute nitric acid

The acidified part is important as this means it reacts with any other ions which could also give a precipitate and mess with the final conclusion of what anion is present in the solution

Iodine (I-)

Result of a positive test

If iodine is present, a yellow precipitate should have formed

Method

Step 2) Next, a few drops of silver nitrate solution are added into the unknown solution

Step 3) Finally, the colour of the precipitate formed should be recorded

Step 1) The unknown sample is first acidified using a few drops of dilute nitric acid

The acidified part is important as this means it reacts with any other ions which could also give a precipitate and mess with the final conclusion of what anion is present in the solution