So, surface area to volume ratio is a common concept in Science…
In digestion, they learnt that food are broken into small pieces so that it has larger surface area to volume ratio for enzymes to work on.
But, it’s hard to student to grasp the concept. They don’t get it how food in smaller pieces have larger exposed surface area to volume ratio.
So, here’s a simple demonstration.
First, make 2 jelly using red cabbage (reason being it’s a natural pH indicator) with identical moulds. Cut only 1 of the jelly into quarters. Then, add them into container of equal volume of lye water (alkaline solution).
Due to red cabbage being natural pH indicator, the jelly will change colour in alkaline solution (yellow in strong alkaline). As seen, the jellies cut in quarter turned yellow faster than the larger jelly because it has larger surface area to volume ratio.
My favourite demo for the chapter on acid and alkali is…
red cabbage indicator
Add a pieces few leaves of red cabbage into a beaker of hot water. The leaves will decolourise and the solution will eventually turn purple. We will use this solution as the indicator. Add 2 drops into different samples and observe for colour change.
Colourful results from red cabbage indicator
Here’s my colourful result using these samples (from yellow, clockwise)
Lye water, bicarbonate of soda, sprite, detergent, lemon
Best used with samples that are colourless.
You can encourage students’ participation by allowing them to bring their own samples. Students generally are more excited to work on stuff that they contributed.
You can also extend this activity by giving them time to make their own indicator paper which they can bring home. Simply cut strips of filter paper, soak it in red cabbage juice and let it air dry. Students can then use them by dipping it into samples.