Analysing Substances

Question 1

What are flame tests used for?

Answer 1

To detect the presence of a particular metal ion in a compound.

Question 2

What do metal ions do to the flame when they are hated in it?

Answer 2

Metal ions change the colour of a flame when they are heated in it.

Question 3

How is a flame test carried out?

Answer 3

1. Dip a clean flame test loop in the sample solution.
2. Hold the flame test loop at the edge of a Bunsen burner flame.
3. Observe the changed colour of the flame, and decide which metal it indicates.
4. Clean the loop in acid and rinse with water, then repeat steps 1 to 3 with a new sample.

Question 4

What colour is the flame when Lithium is heated?

Answer 4

Crimson.

Question 5

What colour is the flame when Sodium is heated?

Answer 5

Yellow.

Question 6

What colour is the flame when Potassium is heated?

Answer 6

Lilac.

Question 7

What colour is the flame when Calcium is heated?

Answer 7

Red.

Question 8

What colour is the flame when Barium is heated?

Answer 8

Green.

Question 9

What is a precipitate?

Answer 9

This is an insoluble solid.

Question 10

When may a precipitate be formed?

Answer 10

When a few drops of sodium hydroxide are added to a solution of a metal compound.

Question 11

Which metals form a white precipitate when reacted with sodium hydroxide?

Answer 11

Calcium, Magnesium and Aluminium.

Question 12

What colour is the precipitate formed when sodium hydroxide is added to copper?

Answer 12

Blue.

Question 13

What colour is the precipitate formed when sodium hydroxide is added to iron?

Answer 13

Green.

Question 14

How can the presence of carbonate ions be detected?

Answer 14

Step 1: Carbonates react with dilute acids to produce carbon dioxide and water.
Step 2: Collect the gas given off and bubble it through limewater,

Question 15

How can you test to see if a solution contains chloride ions, bromide ions or iodide ions?

Answer 15

Using a silver nitrate solution.

Question 16

How do you use a silver nitrate solution to test for chloride ions, bromide ions and iodide ions?

Answer 16

1. A few drops of dilute nitric acid are added to the solution.
2. A few drops of silver nitrate solution are then added.
3. The colour of any precipitate formed is recorded.

Question 17

How can you test to see if a contains sulphate ions?

Answer 17

By adding barium chloride solution.

Question 18

How do you use a barium chloride solution to test for sulphate ions?

Answer 18

1. A few drops of dilute hydrochloric acid are added to the solution.
2. A few drops of barium chloride solution are then added.

Question 19

What happens when sulphate ions are detected?

Answer 19

A white precipitate of barium sulphate forms.

Question 20

What are titrations used for?

Answer 20

To determine the volumes of acid and alkali needed to react together to produce a neutral solution.

Question 21

How are titrations carried out?

Answer 21

1. A pipette is used to accurately measure a volume of an alkali, often 25 cm3. A pipette filler is used to draw solution into the pipette safely. The alkali is emptied into a conical flask.
2. A few drops of a suitable indicator are then added to the conical flask. This will show a change of colour when the acid and alkali have neutralised one another and the titration is complete.
3. The acid is placed in a burette and the starting volume of acid is read against the scale marked on the burette.
4. The acid from the burette is added to the conical flask, and the flask is swirled to mix its contents. When the acid in the burette has almost run in, it is added one drop at a time. Eventually, a colour change shows that the correct amount has been added to react completely with the alkali in the conical flask.
5. The volume of acid added from the burette is noted. The titration results can then be used to calculate the concentration of the acid or alkali (if the concentration of the other is known).