Solute, solvent and solution – I

Avneesh Shukla

Chemistry is an integral part of our lives. Consciously or unconsciously we interact with a lot of chemicals and processes (chemical or physical) every day. To help a group of young students (of class 7 and 8 from the Government Girls Upper Primary School, Damta, Uttarkashi) understand this importance of chemistry in our lives I got them to do a couple of experiments which I would like to share with you through this article.

Before carrying out the experiment, I arranged for the following materials and reading resources:
• Soil
• Wooden stick
• Sand
• Water
• Sugar
• Indigo blue (Ujala)
• Salt
• Chalk pieces
• 10 plastic disposable cups

Reading resources to enhance my understanding of what the students are learning:
• Uttarakhand State textbook
• NCERT textbook
• YOUTUBE videos
(https://www.youtube.com/watch?v=hsyU4iqoTUM)
(https://www.youtube.com/watch?v=e-2EoyDYamg)

I divided the students into four groups and asked them to list the substances that they thought would dissolve in another substance, say water. This was their list:
• Sugar
• Salt
• Milk
• Sand
• Soil
• Ink
• Juice
• Chalk
• Alcohol
• Cold drink
• Tea leaves
• Detergent
• Shampoo
• Sugarcane
• Mango shake
• Honey
• ORS
• Blood
• Ujala
• Oil

Their list told me that these students were unsure of the solubility of some substances. For instance, while some believed that oil would dissolve in water some did not. Similarly detergent and honey too. The best way for them to learn about solubility was for them to experiment and find out so I gave each group some materials for the group activity.

Groups 1 and 2: Sand, soil, water and two plastic disposable cups
Groups 3 and 4: Sugar, salt, water and two plastic disposable cups

I asked the groups to mix these substances and note down their observations. Groups 1 and 2 mixed sand in water and soil in water. Groups 3 and 4 mixed sugar in water and salt in water. Their observations were as follows:

• We took some amount of water in a plastic cup and added some soil, suddenly the water turned brown.
• When we added sand in water, after some time it settled down. The colour of water changed to gray.
• We took a little sugar in a cup and then poured water on it. The colour changed a little bit.
• We took water and then added salt in it; on doing this the large amount of salt got dissolved in water but some amount didn’t. We stirred this with a pen and saw that all the salt dissolved. The colour was much the same as that of ordinary water.

From this experiment the students learnt that they were able to see soil and sand in water but not sugar and salt so would it be right to say…

Sugar and salt disappear when added to water?
Some believed yes, others no and some others were confused. So I gave a clue. Suppose a blind person was outside the classroom or any one of the students was standing outside with her eyes closed, would she be right in saying that the rest of the class was invisible? Most of the students replied no she wouldn’t be because that is a problem with her vision. A few students were of the opinion that since this student couldn’t see the rest of them, she would be right in saying that the rest of the class was invisible to her. Immediately one student replied if the “blind” student can constantly hear the noise the class is making then she will know by her sense of hearing that the class is there and so the class can’t be invisible.

Now I asked the students to think about the salt and sugar solutions similarly – can we say salt and sugar disappeared when added to water? The students replied “No, Sir! The fact that we are unable to see is the limitation of our eyes. It is perhaps the very small size of the sugar and salt particles in the sample (solution) that made them invisible. If we use an instrument (advanced microscope) then perhaps they will become visible.”

After this wonderful discussion I asked them about the quantity of sugar, salt, soil, sand and water they used. All the groups said that they took small amounts of sugar, salt, soil and sand and added them to a large amount of water. My next question, what will happen if we add more amounts of sugar, salt, soil and sand to the water? Some said, “Ghol gaada ho jaayega” (the solution will become dense). We decided to experiment again with more amounts of salt, sand, sugar and soil. The groups found that the solutions had in fact become dense and seemed darker than before and also that the level of solution in the cup had gone up. I noted down their observations on the display board and asked them to focus and share their views on the same. The observations were written as:

Case 1
Less: Soil More: Water Final sample: Soil + water
Less: Sand More: Water Final sample:Sand + water
Less:Sugar More:Water Final sample:Sugar + water
Less:Salt More:Water Final sample:Salt + water

Case 2: After increasing the amount
Less:Soil More: Water Final sample: Soil + water
Less:Sand More:Water Final sample:Sand + water
Less:Sugar More:Water Final sample:Sugar + water
Less:Salt More:Water Final sample:Salt + water

Note: The amount of soil, sand, sugar and salt was more than in the first case but overall it was less than the amount of water taken.

The responses of the students were as follows:

• In the first case we took soil, sand, sugar and salt all in less amount and water in more amount.
• In the second case we increased the amount of soil, sand, sugar and salt.
• We used water for mixing.
• When we added soil, sugar, sand and salt in water it became ghol (solution).
• All the things we used in less quantity were solids and that which we used in large quantity, water, was liquid.

After performing the activity I asked them to think about the following question …

“Can we separate out sugar, salt, soil and sand from their respective ghols (solution) in water?”
Again some said yes, some no, and some were confused. I then asked them what substances were needed to make the ghol. As they had already performed the experiments, they told me that for a sugar ghol we would need sugar and water; for a salt ghol we would need salt and water. After getting their answers I explained to them that to make our ghols we took one substance in a small quantity and another in large quantity. The matter or substances that were in smaller quantity were known as Solutes, and the substance in large quantity was the Solvent, and the mixture we got was the Solution.

After this explanation I asked them to identify the solute, solvent and solution in the above performed activity. Without losing a second they replied in the first case we took sand, soil, sugar and salt in small quantities, hence these were the solutes and water in large quantity was the solvent and the final product was the solution. In the second case although there was an increase in the quantities of the substances, it was still less than the quantity of water, so solute and solvent were the same. From the above experiments we inferred also that solute and solvent can change depending on the quantities we use in the solution.

Solute: Sand Solvent: Water Solution:Sand solution
Solute:Soil Solvent:Water Solution:Soil solution
Solute:Sugar Solvent:Water Solution:Sugar solution
Solute:Salt Solvent:Water Solution:Salt solution

The question now was since we used only solid solutes in the above experiment, would it be possible to have liquid/gas solutes. After thinking a while they replied juice in water, cold drink in water, ink in water and alcohol in water are examples of solutions in which the solutes are liquid. What about gases then? Students thought about this but couldn’t think of any examples. So I asked, “Is there air in the room? They replied, “YES”. What would happen if incense sticks were lit in the room? They replied the fragrance would spread throughout the room. So was this gas solution? They replied after some thought, “Yes, it is – smoke, fog and hydrogen, oxygen, carbon dioxide gases in air,” are the examples of solution in which solutes are gases. From our discussions we learnt that solutes and solvents could either be solids, liquids, or gases.

In the first case we took less quantity of solute and added a large quantity of solvent (water). In the second case we increased the amount of solute and the solution became dense. The solution in which solute is more is known as Concentrated Solution (gaada, Sandra vilyan) and the solution in which solute is less is known as Diluted Solution (tanu vilyan).

Before I wrapped up my class I asked them one last question. Why did we use water as the solvent in both the cases? They replied that as water is easily available, present in excess amount and is inexpensive we used water. I appreciated their answers but added that since most substances dissolve in water we used water as the solvent, but sometimes we also use other solvents if we have solutes that don’t dissolve in water.

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