Exercise 2.8
A researcher carried out an experiment in which she took two carrots of similar size. She placed one carrot in plain water and the other carrot in concentrated salt solution (Fig. ). After hours she recorded her observations. (i) What hypothesis does she want to test through this experiment? (ii) What would you suggest for the improvement of this experiment? (iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
(i) The hypothesis being tested
That water moves into or out of a plant tissue by osmosis, in the direction set by the concentration of the solution around it.
Stated so it can be tested: a carrot in plain water will take in water and stay firm, while an identical carrot in a concentrated salt solution will lose water and go limp.
(ii) How to improve the experiment
Weigh both carrots before and after, as in Activity $\displaystyle 2.2$, so the result is a measured change \(\displaystyle = \text{final weight} - \text{initial weight} \) instead of an impression of how they look.
Match the carrots properly — same variety, same length and thickness, cut and trimmed at the same time.
Hold everything but the solution identical: same volume of liquid, same size of beaker, same temperature, same $\displaystyle 24$ hours.
State the salt concentration used, for example $\displaystyle 20$ per cent, so that someone else can repeat the experiment exactly.
Use two or three carrots in each beaker rather than one, so a single odd carrot cannot decide the result.
(iii) Why the two carrots end up different
Plain water is hypotonic to the cell sap inside the carrot cells, so water enters them by osmosis. The vacuoles fill, the pressure inside each cell rises and the cells press outwards against their cell walls — the carrot stays stiff and crunchy.
Concentrated salt solution is hypertonic, so water leaves the cells by osmosis. The vacuoles lose water, the cells go slack and the tissue loses its firmness — the carrot turns rubbery and limp.
It is the same thing that happens to a whole plant short of water: the vacuoles empty, the cells stop being firm, and the plant wilts.