c4.2 identifying the products of chemical reactions

Question 1

how can you detect carbon dioxide

Answer 1

you can use limewater (calcium hydroxide)

it turns cloudy white in the presence of carbon dioxide

Question 2

how can you detect chlorine

Answer 2

it dissolves in water to form an acidic solution.

1. use some water to dampen a blue litmus paper
2. hold it near to the to the container
3. if chlorine is present, the paper turns red first and then white

Question 3

how should you smell in laboratories

Answer 3

1. with the container away from you breathe in enough air to fill your lungs
2. hold the container a few cm away from you and take a cautious sniff.

Question 4

detect hydrogen

Answer 4

place a lighted splint near the mouth. it should ignite with a squeaky pop.

Question 5

detect oxygen

Answer 5

place a glowing splint near the mouth. it should relight.

Question 6

what is the flame test colour for lithium

Answer 6

red

Question 7

what is the flame test colour for sodium

Answer 7

yellow

Question 8

what is the flame test colour for potassium

Answer 8

lilac

Question 9

what is the flame test colour for calcium

Answer 9

orange-red

Question 10

what is the flame test colour for copper

Answer 10

green-blue

Question 11

why do elements change colour when heated

Answer 11

energy is transferred to their electrons, which makes them move to higher electron shells. When they move back to their normal shell, the energy is transferred to their surroundings by radiation, which we see as light.

Question 12

what are hydroxide precipitate tests

Answer 12

you have to add a few drops of sodium hydroxide solution to a solution containing metal ions.

Question 13

what is the hydroxide precipitate tests change of colour for iron (II)

Answer 13

green

Question 14

what is the hydroxide precipitate tests change of colour for iron (III)

Answer 14

orange-brown

Question 15

what is the hydroxide precipitate tests change of colour for copper (II)

Answer 15

blue

Question 16

what is the hydroxide precipitate tests change of colour for calcium

Answer 16

white

Question 17

what is the hydroxide precipitate tests change of colour for zinc

Answer 17

white

Question 18

detect sulfate ions

Answer 18

add some drops of dilute hydrochloric acid
then some drops of barium chloride solution
you should get a white precipitate

Question 19

detect carbonate ions

Answer 19

add some drops of dilute hydrochloric acid

bubble of gas should be produced if carbonate is there

Question 20

detect halide ions

Answer 20

add a few drops of dilute nitric acid
add a few drops of silver nitrate solution. 
chloride: white
bromide: cream
iodide: yellow

Question 21

instrumental methods of analysis advantages

Answer 21

sensitivity- they can analyse small amounts of the substance (useful if it is expensive)
accuracy- instruments are very accurate
speed- can carry out analyses very quickly and can be run all the time

Question 22

how to interpret a gas chromatogram

Answer 22

• each peak represents one substance
• the areas under each peak show the relative amount of that substance in the mixture
• the retention time is different for each substance

Question 23

how to interpret a mass spectra

Answer 23

• each peak represents a fragement of a molecule
• the peak on the far right represents a molecular ion
  (the mass to charge ratio of the molecular ion peak is equal to the relative formula mass. )

Question 24

what does a mass spectra show you

Answer 24

it measures the masses of atoms and molecules. it is used to analyse the relative amounts of different isotopes of an element, and the structure of molecules.
The mass spectrometer is particularly useful for analysing molecules. The sample molecules are ionised by the machine to form molecular ins. These may break up to form fragments, which the machine can separate and detect.