In part one of this article (published in November 2017), I talked about doing experiments with the students of Government Girls Upper Primary School, Uttarkashi to help them understand what solute, solvent and solution were. In this article I discuss a second set of experiments that I did with the same students.
I asked a group of students to take four plastic cups with equal amounts of water. The students had to add one, two, three and four drops of blue liquid fabric whitener to cup 1, 2, 3 and 4 respectively. These were their observations after performing the activity.
- In all the cases the whitener is the solute since it is less in quantity.
- Water is the solvent since it is more in quantity.
- The colour difference was easy to see. Cup 1 had a light colour but cup 4 had a dark blue colour.
- Among the four cups, cup 4, to which four drops were added, had the concentrated solution and cup 1 to which only one drop was added had the diluted solution.
I took cups 1 and 2 and asked them now which had the diluted solution and which the concentrated. All replied in unison that since the first one has a lesser amount of solute cup 1 had the diluted solution and cup 2 the concentrated solution. Then I took cup 2 (having two drops) and cup 3 (having three drops) and asked the same question. Without losing a second they replied cup 2 had the diluted solution now and cup 3 the concentrated solution. Then I asked…
In the first instance, cup 2 had the concentrated solution but in the second, the same cup 2 had the diluted solution, how could that be?
The students were confused for a second but then a small voice said, “sir pehele humne 1st aur 2nd solution liya phir 2nd aur 3rd solution liye to yahan jisse tulna karni thi woh solution change ho gaye isliye 2nd solution jo pehle concentrated tha 1st ki tulna main woh ab 3rd ki tulna maih diluted ho gaya (sir, in the first instance we took solutions from cups 1 and 2 and in the second instance solutions from cups 2 and 3 so in the second instance the solution we were comparing cup 2 with changed and that is why the same solution which was concentrated when compared to cup 1 became diluted in comparison to cup 3.) I appreciated this wonderful explanation of the learners and told them that from this we can infer that we can call a solution concentrated or diluted only if we have another solution to compare it with.
Then I asked the students what actually happened when we added sugar/salt to water in the first set of experiments (see aticle, Solute, solvent and solution – I carried in November 2017).
The students replied that as sugar and salt dissolved in water, the colour of the water changed a little bit as well. I asked what actually happens here, where do the sugar and salt go? One of the students tried to explain it by saying that water is made of hydrogen and oxygen molecules, When we add sugar or salt to water they perhaps mix with the oxygen and hydrogen molecules. I was amazed at this marvelous explanation. So that the class understood this better I asked two students to carry out a small demonstration. They were to take a plastic cup with many chalk pieces in it and then add a small amount of salt to it and observe the process. The students observed that there were gaps between the chalk pieces and when they added salt, it occupied those empty spaces. Then I asked the students about sugar and salt in water. They replied that a similar phenomenon perhaps occurs when sugar or salt are mixed in water. The student who first suggested the answer now confidently said that the formula for water is H2O and that there are empty spaces between the H2O molecules in water which are occupied by the salt or sugar. I tried explaining the same phenomenon now with air. I asked whether there was air in the room. They all replied ‘yes’. Then I asked why is it that we (humans) don’t mix or dissolve in air? All the students laughed and one of them replied, “Sir, size bhi matter karta hai, aur humara size itna hai hi nahi ki hum air ke beech ke space main fit ho paaye (the size of the particles also matter and substance will mix or dissolve only if the size is appropriate). I was again surprised at this explanation. I further built on this understanding by telling them that apart from size, the nature of solvent (generally we use water but if some substance does not dissolve in water we use other solvents) is also responsible for solubility.
The author is with the Azim Premji Foundation. He can be reached at email@example.com.