To compare different solutes, a technician set up three series of tubes containing discs of potato tissue, as shown in Fig. 22.1. One series used sucrose, o...

This tests osmosis compared across solutes, with reading, calculation, plotting two lines and explanation of water potential.

(a) In 0.5 mol/dm3 sucrose the change is 0 %. [1]

(b) In 1.0 mol/dm3 salt the change is -22 %. [1]

(c) Mass is found with an (electronic / top-pan) balance. [1]

(d) For 1.0 mol/dm3 sucrose (5.00 g to 4.60 g):

\( \dfrac{4.60-5.00}{5.00}\times100=\dfrac{-0.40}{5.00}\times100=\mathbf{-8\%} \)

Working [1], answer [1] [2].

(e) Sucrose and salt plotted as two labelled lines on the same grid (note the negative region):

Marks: axes correct with labels and units [1]; suitable scale with the negative region shown [1]; both sets of points accurate [1]; two labelled best-fit lines [1] [4].

(f) The salt results differ because salt causes larger changes in mass at each concentration [1], and the salt discs start losing mass from a lower concentration than the sucrose discs do [1] [2].

(g) At the same concentration salt causes a greater change because salt (sodium chloride) dissociates into ions, giving two particles per unit [1], so it produces more dissolved particles than sucrose at the same concentration [1]; this lowers the water potential of the solution more, so more water leaves the discs by osmosis [1] [3].

(h) No change in mass occurs where the sucrose line crosses 0 %, which is at 0.5 mol/dm3 [1]; read where the sucrose line shows 0 % change [1] [2]. At this point the solution has the same water potential as the potato cells.

(i) Independent variable: type of solute / concentration of solution [1]; dependent variable: percentage change in mass of the discs [1] [2].

(j) Any two controlled variables: temperature, time in the solution, size of disc, volume of solution. [2]

(k) To improve reliability: repeat with several discs and take a mean [1], and standardise the blotting and timing [1] [2].

(l) The most effective preservative solute is salt [1], because it gives the lowest water potential at a given concentration, so it draws the most water out of the microbes by osmosis [1] [2], which prevents them growing.

(m) Glucose and sucrose gave almost the same results because both are non-dissociating molecules that do not split into ions [1], so at the same concentration they produce a similar water potential and similar osmosis [1] [2].