Practical Skill: Agar Blocks

Question 1

The effect of changing surface area to volume ratio on diffusion can be investigated by

Answer 1

timing the diffusion of ions through cubes of agar of different sizes

Question 2

step 1 Coloured agar is made up and cut into ……………… (eg. 0.5cm x 0.5cm x 0.5cm, 1cm x 1cm x 1cm and 2cm x 2cm x 2cm)

Purple agar can be created if it is made up with ………………solution and Universal Indicator
Alternatively, the agar can be made up with ……….. only

Answer 2

cubes of the required dimensions

dilute sodium hydroxide

Universal Indicator

Question 3

step 2 ………………………….. of these cubes is calculated and recorded

Answer 3

The surface area, volume and surface area to volume ratio

Question 4

step 3 The cubes are then placed into boiling tubes containing a ………………….(such as dilute ……………..)

The same ………………d should be carefully measured out into each boiling tube

The acid should have higher molarity than the sodium hydroxide so that its ………….

Answer 4

diffusion solution

hydrochloric acid

volume of dilute hydrochloric acid

diffusion can be monitored by a change in colour of the indicator in the agar blocks

Question 5

Measurements can be taken of either:

Answer 5

The time taken for the acid to completely change the colour of the indicator in the agar blocks

The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time (eg. 5 minutes)

Question 6

Analysis
If the time taken for the acid to completely change the colour of the indicator in the agar blocks is recorded, these times can be converted to …..

Answer 6

rates

Question 7

A graph could be drawn showing how the rate of diffusion (rate of colour change) changes with the ……………………

Answer 7

surface area : volume ratio of the agar cubes

Question 8

When an agar cube (or for example a biological cell or organism) increases in size, the volume increases faster than the surface area, because the volume is cubed whereas the surface area is squared. When an agar cube (or biological cell/organism) has more volume but proportionately less surface area, diffusion takes longer and is less effective. In more precise scientific terms, the greater the surface area to volume ratio, the faster the rate of diffusion!