Activity 9.2 — How Temperature Affects Solubility — "The Amazing World of Solutes, Solvents, and Solutions"

1. 14 marksCuriosity Grade 8, Chapter 9, pages 137-138

   Describe Activity 9.2 step by step, including the three temperatures and what is observed at each.

   Hint. The pattern repeats: saturate, heat, watch it dissolve, saturate again.

   The activity.

   1. Take about 50 mL of water in a glass beaker and measure its temperature with a laboratory thermometer — say 20 °C.
   2. Add baking soda (sodium hydrogen carbonate) a spoonful at a time, stirring, till some solid baking soda is left undissolved at the bottom of the beaker — the solution is now saturated at 20 °C.
   3. Heat the contents to 50 °C while stirring. You will observe that it has dissolved.
   4. Keep adding more baking soda at this temperature until some again remains undissolved.
   5. Heat further to 70 °C, still stirring. The undissolved baking soda dissolves.

   Safety first: Be careful while using the heating device.

   ✦ The cycle is repeated twice on purpose. One heating could be a fluke or a mistake; going 20 → 50 → 70 °C and getting the same result each time turns a single observation into a pattern. Because each stage begins from a genuinely saturated solution, every extra spoonful that dissolves after heating is new capacity that was not there before.

   It is a demonstration activity, performed by the teacher — hot liquids and an open flame.

2. 23 marksCuriosity Grade 8, Chapter 9, page 138

   What is the inference from Activity 9.2? State it as a general rule.

   Hint. Compare the three temperatures in order.

   Water at 70 °C dissolves more baking soda than water at 50 °C. The amount of baking soda dissolved in water at 20 °C is even lesser.

   It has been found that for most of the substances, the solubility increases with an increase in temperature.

   Temperature	Baking soda dissolved
   20 °C	Least
   50 °C	More
   70 °C	Most

   ✦ Note the words for most of the substances. The chapter does not claim this holds for everything, and you should not either. It is a general tendency with exceptions, and stating it as an absolute law would be overclaiming. The Snapshot is equally careful: generally, in liquids, the solubility of solids increases and that of gases decreases with an increase in temperature.

   And the consequence the chapter draws is the useful one: a saturated solution at a particular temperature behaves as an unsaturated solution if the temperature is increased. Nothing was added to the beaker, because heating alone turned a saturated solution into one that could take more.

3. 33 marksCuriosity Grade 8, Chapter 9, page 138

   Why must the mixture be stirred continuously throughout Activity 9.2, and why is the temperature measured with a thermometer rather than judged by feel?

   Hint. Both are about making the result trustworthy rather than about making it happen.

   Continuous stirring. It makes the baking soda dissolve as fast as it is able to, and it keeps the temperature even throughout the beaker. Without stirring the liquid near the flame would be much hotter than the liquid at the top, so you could not say what temperature the solution was at when a spoonful dissolved — and the whole activity is about connecting a temperature to an amount.

   Measuring with a thermometer. The conclusion is a comparison between 20 °C, 50 °C and 70 °C. Judging by feel could not distinguish those reliably, and above about 50 °C you could not touch the beaker at all. Since the numbers are the result, they have to be read from an instrument.

   ✦ This is the difference between a demonstration and an experiment. Warming a solution and seeing more dissolve is a demonstration. Warming it to a measured temperature, from a known starting point, with the contents kept uniform, is an experiment — and only the second lets you say that solubility increases with temperature rather than merely with heating.

4. 43 marksCuriosity Grade 8, Chapter 9, page 138

   What does the chapter tell you about Asima Chatterjee, and how does her work connect to this chapter?

   Hint. Read the 'Be a scientist' box, and note which of this chapter's ideas her method depended on.

   Asima Chatterjee is renowned for her work in developing anti-epileptic and anti-malarial drugs. She used solvents and solutions extensively to extract and isolate important compounds from medicinal plants. She earned a Doctorate of Science, becoming the second Indian woman to do so after Janaki Ammal. She became the first woman to receive the Shanti Swarup Bhatnagar Award in the field of chemical science and was also honoured with the Padma Bhushan.

   The connection to this chapter is direct. Getting a useful compound out of a plant means finding a solvent that will dissolve the compound you want and leave behind the ones you do not — a question about solubility, and about which solvent suits which solute. Every choice of solvent, temperature and concentration in that work is an application of section 9.2.

   ✦ The heritage box on the same page shows the same idea at work much earlier. Water has primarily been used as a solvent for the preparation of medicinal formulations in Ayurveda, Siddha, and other traditional systems of medicine in India ... The Indian systems of medicine have also referred to the use of oils, ghee, milk, and other substances as solvents. Because different substances dissolve in different solvents, using several solvents is a way of extracting several different things — a principle the traditional formulations relied on and modern extraction still does.

   (The chapter asks what inspired her to work on medicinal plants. It does not answer, so that is a genuine research question — find out, and say where you checked.)

5. 53 marksCuriosity Grade 8, Chapter 9, pages 137-138

   A saturated sugar solution is prepared at 70 °C and then left to cool to room temperature. Predict what you would see, and explain.

   Hint. Run the chapter's rule backwards.

   Prediction: solid sugar will appear — crystals settling out at the bottom of the container.

   The reasoning. For most of the substances, the solubility increases with an increase in temperature. Run that in reverse: as the solution cools, the amount of sugar the water can hold decreases. But the sugar already dissolved has nowhere to go, and the solution now contains more sugar than water at the lower temperature can hold. So the excess must come out of solution as solid.

   ✦ The chapter states the forward half of this and lets you infer the backward half — a saturated solution at a particular temperature behaves as an unsaturated solution if the temperature is increased. Increasing the temperature creates spare capacity; decreasing it removes capacity that was being used. Being able to run a rule in both directions is a large part of what these questions test.

   This is the same thing that happens to over-sweetened tea as it cools, and it is why a sugar syrup that was clear on the stove can turn grainy in the jar. Nothing has been added or taken away; only the limit has moved.

6. 63 marksCuriosity Grade 8, Chapter 9, page 137

   Why does Activity 9.2 use baking soda rather than the salt of Activity 9.1?

   Hint. Think about what you need to be able to see clearly, twice over.

   A reasonable answer. The activity needs a solid whose solubility changes noticeably over the range 20 °C to 70 °C — enough that a beakerful of undissolved solid visibly disappears on heating, twice. If the solubility barely shifted with temperature, you would heat the beaker and see almost nothing happen, and the activity would fail to show what it is for.

   ✦ Be honest about what the chapter does and does not say. It simply instructs you to use baking soda and gives no reason for the choice, so the explanation above is a sensible inference and should be offered as one, not as the book's own statement. Saying the chapter does not explain the choice, but a substance whose solubility responds clearly to temperature is what the activity needs is a stronger answer than inventing an authority for it.

   A related thought worth adding. Baking soda is safe to handle, cheap, available in any kitchen, and white — so undissolved grains are easy to see against a glass beaker. Because you have to judge exactly when solid stops remaining at the bottom, visibility matters as much as chemistry here.

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.