C4.2 Identifying The Products Of Chemical Reactions

Question 1

How can you detect carbon dioxide?

Answer 1

• By bubbling gas through limewater

- Limewater turns cloudy white when carbon dioxide is present

Question 2

How do you detect chlorine?

Answer 2

• By dampening a piece of litmus paper
• Holding litmus paper to container that holds substance
• If chlorine is present, paper turns red, then white

Question 3

How do you smell substances in the lab?

Answer 3

• With container well away from nose, breathe in enough air to almost fill your lungs
• Hold container a few centimetres from nose, and waft any smell towards you
• Take a cautious sniff

Question 4

How do you detect hydrogen?

Answer 4

• Place a lighted splint near mouth of gas container

- If hydrogen is present, splint should ignite with a squeaky pop

Question 5

How do you detect oxygen?

Answer 5

• Place glowing splint near mouth of gas container

- If oxygen is present, splint should relight

Question 6

What is the basis of a flame test?

Answer 6

• When metal ions are heated, energy is transferred to electrons
• This makes electrons move to higher electron shells
• When they move back to normal shells, energy is transferred to surroundings as radiation, which we see as light
• Different metals produce different colours of light

Question 7

State metals and flame test colours

Answer 7

• Lithium is red
• Sodium is yellow
• Potassium is lilac
• Calcium is orange-red
• Copper is green-blue

Question 8

Why is sodium hydroxide solution used in experiments?

Answer 8

Because Group 1 metal hydroxides are soluble in water but most other metal hydroxides are insoluble

Question 9

Name metal ions and colours produced in hydroxide precipitate test

Answer 9

• Iron(II) is green
• Iron(III) is orange-brown
• Copper(II) is blue
• Calcium is white (doesn’t dissolve in excess sodium hydroxide solution)
• Zinc is white (dissolves in excess sodium hydroxide solution)

Question 10

How do you detect sulfate ions?

Answer 10

• Add a few drops of dilute hydrochloric acid
• Add a few drops of barium chloride solution
• If sulfate ions are present, you get a white precipitate

Question 11

How do you detect carbonate ions?

Answer 11

• Add a few drops of dilute hydrochloric acid
• If carbonate ions are present, bubbles of gas are produced, which can be bubbled through limewater to prove it’s carbon dioxide

Question 12

How do you detect halide ions?

Answer 12

• Add a few drops of dilute nitric acid
• Add a few drops of silver nitrate solution
• If halide ions are present, precipitate forms

Question 13

Give names of halide ions and colours of silver halide precipitate formed

Answer 13

• Chloride is white
• Bromide is cream
• Iodide is yellow

Question 14

What are the advantages of instrumental methods of analysis?

Answer 14

• Sensitivity: Can analyse very small amounts of substances
• Accuracy: Instruments are very accurate
• Speed: Can carry out analyses quickly and can run all the time

Question 15

How do you interpret gas chromatograph?

Answer 15

• Each peak represents a substance present in mixture
• Areas under each peak show relative amount of each substance in mixture
• Retention time is time taken for substance to travel through chromatography column (different for different substances)

Question 16

How does a mass spectrometer work?

Answer 16

• Sample molecules are ionised by machine to form molecular ions
• These may break up to form fragments, which the machine can separate and detect

Question 17

How do you interpret a mass spectrometer?

Answer 17

• Each peak represents a fragment of the molecule
• Peak on far right represents molecular ion
• Mass to charge ratio is equal to relative formula mass of molecule