required practicals paper 2

Question 1

chromatography, required practical 6

Answer 1

• draw a pencil line 2cm from the bottom of the chromatography paper
• draw five pencil spots 1cm apart
• use a capillary tube to place a small spot of A-D and U
• pour water into a beaker at a volume of 1cm
• attach paper to a glass rod using tape and lower it into the beaker, put a lid on.
• make sure the pencil line and spots are above the water’s surface and that the paper does not touch the sides of the beaker
• remove the paper when the water has travelled three quarters up
• use a pencil to mark where the water reached and let it dry
• to identify which of A-D are in U, we can see which ones it has and hasn’t lined up with.

Question 2

why do the spots have to be small?

Answer 2

so they cannot bleed into each other

Question 3

why do you have to make sure the pencil line and spots are above the water’s surface and that the paper does not touch the sides of the beaker?

Answer 3

so that this does not interfere with the way the water moves

Question 4

why do we put a lid on?

Answer 4

to reduce the evaporation of the solvent

Question 5

what do we do to identify the chemicals in colours A-D?

Answer 5

calculate the Rf values

Question 6

required practical 5: rates of reaction

Answer 6

1. use a measuring cylinder to put 10cm^3 of sodium thiosulfate solution in a conical flask
2. then place it on a printed black cross
3. next add 10cm^3 of hydrochloric acid into the conical flask
4. swirl the solution and start a stopwatch
5. stop the clock when you can no longer see the cross looking down into the flask.
6. carry out again using lower concentrations of sodium thiosulfate solution
7. then repeat experiment and calculate the mean for each value of sodium thiosulfate solution

Question 7

what is the problem with the rate of reactions required practical and its solution?

Answer 7

problem = people have different eyesights, makes it less repeatable
solution = the students all use the same size printed cross so the results may not be too varying

Question 8

required practical 5: rate of reactions using a gas

Answer 8

1. place 50cm^3 of hydrochloric acid in a conical flask
2. attach it to a bung and delivery tube
3. now place the delivery tube in a container filled with water and put a measuring cylinder on top of this
4. now add a 3cm strip of magnesium to the flask and start a stopwatch
5. measure the volume of hydrogen gas produced in the measuring cylinder every 10 seconds until no more is produced
6. now repeat using different concentrations of hydrochloric acid

Question 9

required practical 8: water

Answer 9

1. we are given a sample of water and must find out if it is pure
2. First test the pH, pure water has a pH of 7.
3. we can place a small amount of water onto universal indicator paper and see if it turns green
4. next we test for dissolved solids by using a balance to weigh an empty evaporating basin, then recording the mass and filling it with our water sample
5. after this we place it on a tripod and gauze with a bunsen burner to heat the water until it evaporates.
6. after this we allow the basin to cool and then weigh it again, if there were dissolved solids, the mass of the empty evaporating basin will have increased.
7. this is because the water has evaporated but the dissolved solids have formed crystals on the surface of the evaporating basin.

Question 10

how can water be purified by distillation?

Answer 10

• first, water on top of a tripod and gauze is heated gently using a bunsen burner
• the water will evaporate and form water vapour, it will then travel along the delivery tube, which is above a test tube bathing in a beaker of icy water.
• when the water gets to this point, it will condense back into liquid water
  this is distilled water. (pure water)