potato practical

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.

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;

Question 2

The student calculated the percentage change in mass rather than the change in mass. Explain the advantage of this.

Answer 2

Allows comparison / shows proportional change;

Idea that cylinders have different starting masses / weights;

Question 3

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

Answer 3

(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;

Question 4

The students were advised that they could improve the reliability of their results by taking additional readings at the same concentrations of sodium chloride.

Explain how.

Answer 4

Anomalies to be identified / effect of anomalies to be reduced / effect of variation in data to be minimised;

Reject: idea of not recording anomalies / preventing anomalies from occurring

     A mean to be calculated;

Question 5

The students used a graph of their results to find the sodium chloride solution with the same water potential as the apple tissue. Describe how they did this.

Answer 5

Plot (sodium chloride) concentration against ratio / draw line of best fit;

Reject: if wrong axes or type of graph

Find (sodium chloride concentration from the graph) where the ratio is 1 / there is no change in mass;

Question 6

he students were advised that they could improve their graph by taking additional readings. Explain how.

_____________________

Answer 6

Line / curve of best fit is more reliable / precise;

Neutral: graph

     Intercept / point where line crosses axis is more reliable / precise;

Reject: references to ‘more accurate’

     OR

     Can plot SD values / error bars;

(To show) variability about the mean / how spread out the results are

Question 7

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.

Answer 7

Lengthways / down the root;

Through one tissue only / through same part / same proportion of tissues;

Question 8

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.

Answer 8

To prevent the water from evaporating / prevent evaporation;

Changing the concentrations / water potential (of solution);