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.
One of my favourite lesson package to carry out for lesson on density is “Gold Ingot Scam”. It’s in the form of a case study in which students role-play as police officers. This is based on a real crime in Singapore in 2011.
I like this activity because there are videos from Crimewatch that can be used for pre and post lesson. In fact, in the part 2 of the video, it shows the detective carrying out the experiment.
I did this as a group work with my students.
Each group receive an electronic balance, a pendulum bob wrapped with Ferero Roche gold foil and a measuring cylinder.
The pendulum bob is kinda tricky though. The small one will not show much increase in water level, but the medium bob might get stuck in the cylinder. They kinda vary slightly in size, so you have to check beforehand. I was blessed with a lab tech who was willing to check that the medium bobs used did not get stuck in the cylinder. Luckily it was only 10 sets needed for him to check *phew*
I provided for the students a mock police report and a template for them to complete their report.
I did not provide instructions as I wanted the students to figure out on their own. Most generally got it that they have to measure mass and volume. But, while they’re were carrying out the experiment, I did spot some mistakes though.. such as
Measuring volume, then mass (with water droplets still on sample!!)
Not releasing the sample completely in the measuring cylinder. Some held on to the string, with the sample in the middle of the cylinder filled with water.
In the next lesson, I discussed these with them and elicit response from them on how those steps can affect the results.
For those keen for the lesson materials, you can get the materials here.
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.
Conducted a simple titration for a lesson on Acid Rain. Parts of lesson is scenario-based to provide solution for acidic soil.
Level: Sec 2 Normal Technical
Topic: Air Pollution (Effects of Air Pollution – Acid Rain)
Students should be able to:
– test for pH of solution using universal indicator
– test for acidity or alkalinity of solution using pH paper
– conduct simple titration and determine the end-point for neutralisation