Making it visible

Yasmin Jayathirtha

The Greeks have a word called ‘hubris’ which means excessive/foolish pride or self-confidence. I certainly suffered from it when I thought it would be easy to design activities and outlines to teach atomic structure/bonding and equations to students in classes 6 and 7. I do not do it, starting from the 8^th/9^th, but State syllabi, wanting to put in some chemistry in the science textbooks start with it. As I began to consider what would be the best way to start, I got a messy diagram, because the concepts branch off, merge and demand a level of abstraction that younger students might find difficult. Then any learning becomes a matter of rote and pictures that other people draw become a reality for the students and they do not tend to figure out why these models work. But even established scientists did not believe in the models of atoms for a long time so it is a difficult idea to visualize.

The lesson plan or order of topics began to look more like a difficult maze than a plan;

Starting with atoms, what led to the idea of atoms i.e., the particular nature of matter? What are the activities that can give an idea that there are particles and give some idea of the sizes. This is important because we are talking about deducing the existence of things we cannot see. It can be pointed out that through instruments we can now manipulate atoms.

Over the next few columns I will share activities that can be done fairly simply to illustrate the concepts that need to be understood beyond being merely descriptive.

The existence of matter as particles
There is a story told about the coming of the Parsis to India. They came to Gujarat and asked the ruler if they could settle in his kingdom. The ruler offered them a brimming glass of milk to show that there was no space for them. The leader of the Parsis carefully dissolved a spoonful of sugar in the milk to show that they would assimilate and contribute. But mixing cannot occur if matter is continuous! Let us assume that matter is made up of particles – how do we know it’s true and how can we estimate the sizes? A lot of the experiments depend on phenomena that we know (like sugar dissolving) and thinking about it. One phenomenon is that of diffusion, both in liquids and in gases. In fact, the existence of the different states of matter is simply explained by particles.

1. Brownian motion was the earliest described movement of large particles (pollen grains in water) and was not explained till 1905 by Einstein as bombardment by invisible particles. To see Brownian motion in the classroom, shake the blackboard duster in a shaft of sunlight and watch the dust particles move randomly. YouTube also has some nice videos.
2. Diffusion: Open about 5cm³ of scent, or light an agarbathi, or put some strong smelling flowers in the corner of a room. How long does it take for the students scattered around the classroom to smell it? Estimate the amount of substance given off, ~5cm³ for the scent, probably less for the flowers. You can catch the smoke of the agarbathi and measure it. What is the volume of the room in cm³? What is the volume of one breath (~100cm³). It can be measured by holding your breath and exhaling into a measuring cylinder filled with water. Use a straw that can be bent and exhale through your mouth.

Now the calculations, aimed to show the size of particles – how much did the scent get diluted: volume of the scent/the volume of the room. What fraction did you breathe in: volume of your breath/ the volume of the room. How much of the scent was in your lungs? This shows the smallness of the particles.

Take two small crystals of potassium permanganate ~ 1mm^st3. Add one crystal to about 100cm³ of water and keep aside to watch the diffusion and check how long it takes to become uniformly coloured. Add the other crystal to another 100cm³ of water and stir to dissolve. Take 10cm³ of this solution and dilute to a 100cm³. Repeat the dilutions till you get a solution in which the colour is barely seen. How many times did you dilute? What fraction of the permanganate is present in the last solution? Does it contain permanganate?

Size of particles: This experiment is easy to do but the calculations are a little finicky!

1. Take oil in a burette or syringe. Add the oil drop-wise into a measuring cylinder (or medicine cup) to determine how many drops there are in 1cm³ of oil.
2. Take one drop of oil on a plate or watch glass. Dip the point of a glass rod in the oil and wipe it on the edge of a filter paper circle. Repeat till no more is picked up. Put down how many pointfuls there are in a drop of oil.
3. Take a very clean tray, fill it with water and shake some chalk from a blackboard duster or some talcum powder on the surface. Dip the rod in oil and touch the centre of the dusty patch with it. You have put one pointful of oil on the water. The oil will spread and push the powder away forming a clear round patch. Measure the diameter of the patch and calculate the area in cm².
4. You know the volume of the oil you have put and the area of the oil patch. You can find out the thickness of the patch. The particles cannot be bigger than the thickness. So you have an estimation of the size of the oil particles.

These experiments suggest that there are many particles in small volumes and that they are small. Nothing here is proof that they exist, of course, but they suggest it. The next time we will look at activities that will be quantitative, considering how particles differ from each other.

Reference
Practical Chemistry resources from the Nuffield Foundation. www.rsc.org>resource>listing

The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.