The notion of purity

Avneesh Shukla

In our everyday life we interact with a variety of substances, which we classify on the basis of their shape, size, colour, conductivity, state of matter, purity, and so on. In this article I share my experiences of demonstrating this theme to 6^th-8^th grade learners of the Government Upper Primary School in Uttarkashi.

As I entered the classroom, the children greeted me in unison, “Good Morning Sir!” I returned their greeting. After a brief introduction to what we were going to do that day, I placed one beaker containing tap water, one conical flask containing soil + water, and one conical flask containing ash + water on the table. I then asked the students if they had seen the Tata salt and RO water advertisements on TV. Some of them had seen the ads. I asked them to give me the taglines of these ads. A student got up and said, “Tata namak desh ka shudh namak” (Tata salt, country’s pure salt). Another student got up and gave the tagline of the second ad, “R.O. deta hai shudh paani” (R.O. gives pure water). I then asked the students what the opposite of pure is. Impure, came the answer. I wrote the words pure and impure on the blackboard and told them that we were going to explore pure and impure substances. I asked the students whether they could classify everyday objects as pure and impure. After some thinking, the students shared this list with me.

I asked the children to give me reasons for their classification. And this is what I got.

Then I asked the students to look at the water samples I had set up on the table earlier (water in beaker, conical flask containing soil + water, and conical flask containing ash + water) and asked them to decide which was pure and which impure. The children were so excited that they not only observed my samples but created some of their own. Ink + water, soap + water, chalk + water.

Following were the observations of the children.
• We saw coal and stone in the sample of ash + water.
• Some small particles could be observed in the beaker containing water.
• Small and big soil particles could be observed in the soil + water sample.
• Additional impurity in the form of stone, coal, etc., has been added to the water.
• The colour of water changed to black/brown when we added soil and sand to water.
• Initially we added more ink to the water and saw that the water had turned dark blue and appeared to be dense. Then we added 1-2 drops of ink in 250 ml of water and observed that the water was now light blue in colour and contained small particles.
• Maybe some impurities are present in the samples.
• Water in the beaker seemed clean and pure.
• Ink + water solution is clear and therefore pure.
• Other samples have stones, coal, sand, etc., and therefore are impure.

I appreciated these observations and explained that the social and scientific basis of what is pure and what is impure are different. Socially, something that is useful to us, clear in appearance (for example, water or milk without any visible impurity) is pure and something that is harmful, not directly or indirectly useful to us, and in which impurity is clearly visible is impure. Scientifically, everything that is made of only one kind of substance is known as pure and if something is added or mixed with the substance then it becomes impure. This does not mean that scientifically impure substances are harmful for us.

I then asked the learners what the chemical composition of water is. Instantly some children replied H₂O. Then I explained that water has H₂O (H₂O molecule) if something (doesn’t matter whether useful or harmful) is added or mixed with H₂O, then scientifically it will fall under the category of impure substance. I then asked the children how table salt is represented in science. The children said NaCl. I told them that just like water, if something is added to NaCl, then it will become impure. NaCl by itself is a pure substance. After the lesson was done, the students were asked to fill a worksheet in which they had to categorize the substances given as pure or impure.

Here I concluded this wonderful day, highlighting the difference between the social beliefs of what is pure and what is impure and the scientific reasons behind the concept of pure and impure substances.

The author completed his BSc (2012) and MSc (2014) in chemistry from Hemvati Nandan Bahuguna Garhwal University (UK, India). He joined Azim Premji Foundation, Uttarkashi District Institute as the science resource person. His interest is in science popularization and making science easy and interesting. He can be reached at avneeshshukla98@gmail.com.