Practical skills

Question 1

A student investigated the effect of putting cylinders cut from a potato into sodium chloride solutions of different concentration. He cut cylinders from a potato and weighed each cylinder. He then placed each cylinder in a test tube. Each test tube contained a different concentration of sodium chloride solution. The tubes were left overnight. He then removed the cylinders from the solutions and reweighed them.
(a) Before reweighing, the student blotted dry the outside of each cylinder. Explain why.
(2 marks)

Answer 1

Water will affect the mass/only want to measure water taken up or lost;
Amount of water on cylinders varies/ensures same amount
of water on outside;
Neutral: removes water
Accept: ‘(sodium chloride) solution’ for water
Do not accept ‘sodium chloride’
Neutral: refs. to fair testing

Question 2

The student carried out several repeats at each concentration of sodium chloride solution. Explain why the repeats were important.
(2marks)

Answer 2

(Allows) anomalies to be identified/ignored/effect of anomalies to be
reduced/effect of variation in data to be minimised;
Makes the average/mean/line of best fit more reliable/allows concordant
results;
Accept: ‘outliers’ instead of anomalies
Q Reject: abnormalities
Reject: idea of not recording anomalies/preventing anomalies from occurring
Accept: ‘cancels out anomalies’ as bottom line response
Q Reject: makes the average/mean more accurate
Neutral: makes the average/mean more valid
Neutral: makes ‘it’/results/conclusion more reliable

Question 3

The student calculated the percentage change in mass rather than the change in mass. Explain the advantage of this. (2 marks)

Answer 3

Allows comparison/shows proportional change;
Idea that cylinders have different starting masses/weights;
Reject: if comparison is in context of the start and final mass of the same cylinder
Neutral: different masses
Neutral: different starting sizes

Question 4

A group of students investigated the effect of sucrose concentration on the length of cylinders cut from a carrot.
(a) The students used a cork borer to cut cylinders from the carrot. Describe how the students should cut these cylinders to make sure that this was a fair test and would produce reliable results. (2 marks)

Answer 4

Lengthways/down the root;
Through one tissue only / through same part / same
proportion of tissues;

Question 5

They measured the initial length of each cylinder then placed the cylinders into test tubes containing different concentrations of sucrose solution. Bungs were placed in the tubes and the tubes were left overnight. Explain why the bungs were placed in the tubes. (2 marks)

Answer 5

To prevent the water from evaporating / prevent evaporation;
Changing the concentrations / water potential (of solution);

Question 6

The student produced the sucrose solutions with different concentrations from a concentrated sucrose solution.
Name the method she would have used to produce these sucrose solutions
(1mark)

Answer 6

Serial dilution