Solute, solvent and solution – 2

Avneesh Shukla

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 1^st aur 2^nd solution liya phir 2^nd aur 3^rd solution liye to yahan jisse tulna karni thi woh solution change ho gaye isliye 2^nd solution jo pehle concentrated tha 1^st ki tulna main woh ab 3^rd 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.

After this I wrote the opinions of the students which they gave on the first day as:

Sugar in water, Juice in water, Shampoo in water, Salt in water, Chalk in water, Sugarcane with water, Milk in water, Fabric whitener in water, Mango shake with water, Sand in water, Cold drink in water, Honey in water, Soil in water, Tea leaves in water, ORS in water, Ink in water, Detergent in water, Blood in water

And then again I asked them to categorize the above list according to whether they were solutes or solvents. Without losing a second they all said water was the solvent and discussed about the solute in groups. After discussion in groups the consolidated responses were as follows:
Solute: Sugar Solvent: Water Solution: Sugar solution
Solute:Salt Solvent:Water Solution:Salt solution
Solute:Ink Solvent:Water Solution:Ink solution
Solute:Chalk Solvent:Water Solution:Chalk solution
Solute:Sand Solvent:Water Solution:Sand solution
Solute:Soil Solvent:Water Solution:Soil solution
Solute:Fabric whitener Solvent:Water Solution:Fabric whitener solution
Solute:Tea leaves Solvent:Water Solution:Tea leaves solution
Solute:Juice + sugar + flavour + colouring agent Solvent:Water Solution:Juice solution
Solute:Cold drink + sugar + flavour + colouring agent Solvent:Water Solution:Cold drink solution
Solute:Blood + water + hemoglobin Solvent:Water Solution:Blood in water

Solution having more than one solute
After this I asked the students whether they knew of examples where the number of solutes was more than one. Some of the responses of the students were as follows:
• Sugar + salt in water (sugar + salt)
• Juice in water (juice + sugar + flavour + colouring agent)
• Cold drink solution in water (cold drink + sugar + flavour + colouring agent)
• Blood in water (blood + water + hemoglobin)
• Tea (tea leaves + sugar + milk)

Then I asked the students what water and air were? The students were puzzled. So I probed them to think about the composition of water and air. After thinking for a while they replied that air has different gases such as oxygen, carbon dioxide, nitrogen, methane and others and water has only water or H2O molecule. In air, nitrogen is present in the highest percentage (solvent) and others in smaller percentages (solute), so air is a solution. And water has only water molecule so it is not a solution. I took a bottle of KINLEY water and asked the students to see the composition of water mentioned on the bottle. They searched and found that the label on the bottle mentions that this is mineral added water. Suddenly one of the students replied that water contains minerals (sodium, potassium in less amount), so water is also a solution.

We concluded by summarizing the activities and observations made over the two days and describing the whole process and highlighting the key and core concepts of this important topic (solute, solvent and solution) in chemistry.

Precautions to take during the experiments

• All the activities should be done in the presence of a guide or facilitator.
• Don’t taste any substances it can be harmful for health.
• Before the experiment ensure that all the apparatus are clean.
• Don’t stir the solutions with pens (which the students did in this activity); an action like that can change your results. If necessary use a glass rod.
• Don’t mix the materials by hand directly, use paper for this purpose.
• After performing one activity clean your hands and apparatus for minimizing errors.
• After performing experiments clean your hands with soap or detergent.

The author is with the Azim Premji Foundation. He can be reached at avneesh.shukla@azimpremjifoundation.org.