Activity 9.1 — Saturated, Unsaturated and Concentration — "The Amazing World of Solutes, Solvents, and Solutions"

1. 13 marksCuriosity Grade 8, Chapter 9, page 136

   Describe Activity 9.1 and the table you record. What happens as you keep adding salt?

   Hint. The design is deliberately repetitive — one spoon at a time, always stirring.

   The activity. Half-fill a clean glass tumbler with water. Add one spoon of salt and stir well until it dissolves completely. Then gradually add a spoonful of salt into the glass tumbler and stir, watching how many spoons go in before the salt stops dissolving completely. Record each attempt in Table 9.1 as salt dissolves or salt does not dissolve.

   What happens.

   Initially, the salt completely dissolves in the water, forming a solution. After adding a few more spoons of salt, a stage comes when the added salt does not dissolve completely and the undissolved salt settles at the bottom. This indicates that the water can no longer dissolve any more salt because it has reached its limit.

   ✦ Two features of the design carry the whole conclusion. The salt goes in one spoon at a time, so you can say exactly which spoonful was the first to fail. And you stir well every time, so when a spoonful finally refuses to dissolve, poor mixing cannot be blamed. Take away the stirring and the result would prove nothing at all.

   A fixed amount of water is the other control. The limit is a limit for that amount of water. Add more water and more salt will dissolve, which is why the definitions that follow all say 'a fixed quantity'.

2. 23 marksCuriosity Grade 8, Chapter 9, page 137

   Define saturated and unsaturated solutions. Why must the temperature be mentioned in both definitions?

   Hint. Activity 9.2 shows what happens to a saturated solution when you heat it.

   The solution in which more solute can be dissolved at a given temperature, is called an unsaturated solution. However, when the solute stops dissolving and begins to settle at the bottom, the solution is called a saturated solution at that particular temperature.

   Why temperature must be stated. Because the limit itself changes with temperature. Activity 9.2 shows a beaker of baking soda solution that is saturated at 20 °C dissolving more once heated to 50 °C, and more again at 70 °C.

   A saturated solution at a particular temperature behaves as an unsaturated solution if the temperature is increased.

   ✦ So 'this solution is saturated' is an incomplete statement. The same liquid in the same glass can be saturated and unsaturated within a minute of each other, with nothing added or removed — only the temperature changed. Since the word means nothing without a temperature attached, examiners expect saturated at that temperature every time.

   The test for saturation is visible: undissolved solute settling at the bottom after thorough stirring. If it all dissolves, the solution was unsaturated.

3. 33 marksCuriosity Grade 8, Chapter 9, page 137

   What is concentration? What do dilute and concentrated mean, and why does the chapter call them relative terms?

   Hint. Ask: dilute compared with what?

   The amount of solute present in a fixed quantity of solution (or solvent) is termed as its concentration. Depending upon the amount of solute present in a fixed quantity of solution, it can be called a dilute solution (less amount of solute) or a concentrated solution (more amount of solute). Dilute and concentrated are relative terms.

   Why relative. There is no fixed amount of salt above which a solution becomes 'concentrated'. The words only make sense in a comparison.

   So, one can say in Activity 9.1, the solution obtained by dissolving one spoon of salt is dilute as compared to that obtained by dissolving two or more spoons of salt.

   ✦ The same solution can be both, depending on what you set it beside. Two spoons in a tumbler is concentrated compared with one spoon and dilute compared with four. Since neither word is an absolute measurement, an answer that says simply 'this solution is concentrated' has not said anything checkable.

   Do not confuse concentration with saturation. Concentration is how much solute is in there; saturation is whether any more could go in. A saturated solution of a barely soluble substance can be very dilute indeed.

4. 44 marksCuriosity Grade 8, Chapter 9, page 137

   Which solution is more concentrated — 2 spoons of salt in 100 mL of water, or 4 spoons in 50 mL?

   Hint. Concentration is solute per fixed quantity of solvent, so put both on the same footing.

   The second one — 4 spoons in 50 mL — and by a factor of four.

   Working, in spoons per 100 mL:

   Solution	Salt	Water	Per 100 mL
   First	2 spoons	100 mL	2 spoons
   Second	4 spoons	50 mL	4 × 2 = 8 spoons

   8 spoons per 100 mL against 2 — so the second solution is four times as concentrated.

   ✦ The trap is to compare only one number. Looking at the salt alone, 4 beats 2 and the answer happens to come out right. Looking at the water alone gives the opposite impression. Concentration is a comparison of the two together, which is why the definition says in a fixed quantity — you have to bring both solutions to the same quantity of solvent before the numbers can be compared at all.

   A quicker route to the same answer: the second has twice the salt in half the water, and 2 × 2 = 4.

5. 53 marksCuriosity Grade 8, Chapter 9, page 137

   Define solubility. How is it different from concentration?

   Hint. One is a limit; the other is a measurement of what happens to be there.

   The maximum amount of solute that dissolves in a fixed quantity of the solvent is called its solubility.

   And the Snapshot adds the quantity and the condition: Solubility is the maximum amount of solute that can be dissolved in a fixed quantity (100 mL) of a solution or a solvent at a particular temperature.

   	Concentration	Solubility
   Tells you	How much solute is actually present	How much could be present, at most
   Depends on	How much you happened to add	The solute, the solvent and the temperature
   Can be changed by	Adding more solute or more solvent	Only by changing the temperature (or the substances)
   Is it a limit?	No	Yes

   ✦ A solution is saturated exactly when its concentration has reached the solubility. That single sentence links all three words, and it is worth memorising as the connection they are usually tested on.

   And solubility is a property of the pair. Salt has one solubility in water and, effectively, none in oil — so 'the solubility of salt' is an incomplete phrase. You must name the solvent and the temperature, which is exactly why the chapter's projects ask you to compare salt in water, vinegar and oil.

6. 63 marksCuriosity Grade 8, Chapter 9, page 136

   Answer the two discussion points of Activity 9.1: how many spoons dissolved before some remained, and what that indicates about water's capacity.

   Hint. The first answer is yours to measure; the second is the same for everybody.

   On the number of spoons. This is a genuine measurement and your answer will be your own — it depends on how much water you took, how big your spoon is, and the temperature of the room. Record what you actually observed in Table 9.1 rather than copying a number from anywhere.

   What it indicates. This indicates that the water can no longer dissolve any more salt because it has reached its limit. Water does not have an unlimited capacity to dissolve salt. There is a definite maximum for a given amount of water at a given temperature, and beyond it the extra salt simply settles at the bottom, still salt and still undissolved.

   ✦ The two questions are doing different jobs, which is why both are asked. The first is a measurement, and different students getting different numbers is not a problem — it is a signal that the answer depends on conditions you can identify and control. Since the second question asks what the observation means, everyone should reach the same conclusion even from different numbers. Separating a measurement from its interpretation is the habit this pair of questions is training.

7. 73 marksCuriosity Grade 8, Chapter 9, pages 136-137

   A student stirs a saturated salt solution for ten more minutes and the salt at the bottom still does not dissolve. Explain, and say what stirring actually does.

   Hint. Stirring changes how fast something happens, not whether it can happen.

   The solution has reached its limit, and stirring does not raise a limit. The water can no longer dissolve any more salt because it has reached its limit. Every space the salt particles could occupy among the water particles is already taken, so there is nowhere for more to go however vigorously the liquid is moved.

   What stirring does do. It brings fresh solvent into contact with the undissolved solid and carries dissolved particles away from its surface, so the solute dissolves considerably faster. In Activity 9.1 you stir at every stage for exactly this reason — otherwise you would be waiting a long time to find out whether a spoonful was going to dissolve or not.

   ✦ Rate and limit are two different things, and this is the clearest place in the chapter to fix the difference. Stirring, grinding the solid finer and warming the liquid all make dissolving faster. Only warming changes how much can ultimately dissolve. A student who confuses the two will predict that patient stirring must eventually work, and Activity 9.1 is designed to show that it never does.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 9 (hecu109.pdf), Reprint 2026-27, pages 134-151. Questions are referenced from the NCERT textbook for identification.