NCERT Solutions Curiosity Chapter 9 .2.1 How does temperature affect the solubility of a solute? — Activity 9.2: Let us experiment (Demonstration activity)

Book page 1379 Updated on2026-09-05

Q1.

Now, heat the contents to 50 °C while stirring (Fig. 9.7). What happens to the undissolved baking soda?

Answer

It dissolves. The layer of solid baking soda lying at the bottom of the beaker disappears as the water warms from 20 °C to 50 °C, and the liquid becomes clear again.

Why it happens: At 20 °C the water had already reached its limit — it was a saturated solution, and the extra baking soda had nowhere to go. Heating raises that limit. The hotter water particles move faster, knock particles off the solid more effectively and hold more of them apart, so the solution that was saturated at 20 °C is unsaturated at 50 °C and the settled solid goes into solution.

Safety first: Be careful while using the heating device. Keep the thermometer bulb in the liquid but not touching the bottom of the beaker, stir with the glass rod and never with the thermometer, and switch off the spirit lamp before removing the beaker.

Q2.

Again, heat the contents further to 70 °C while continuing to stir. What do you observe?

Answer

The same thing happens again — the undissolved baking soda dissolves. After the solution was made saturated once more at 50 °C by adding fresh baking soda, heating it to 70 °C clears that solid too.

Put the three stages side by side and the pattern is unmistakable:

Temperature of water	State of the solution before heating	Amount of baking soda dissolved
20 °C	Saturated; solid at the bottom	Least
50 °C	Becomes unsaturated, then saturated again after more is added	More
70 °C	Becomes unsaturated again	Most

The same beaker at three temperatures. Blue dots are dissolved baking soda particles spread through the water; the orange layer is baking soda that has not dissolved. As the water is heated, the layer shrinks and finally disappears.

Why it happens: Every temperature has its own solubility value for baking soda. Raising the temperature raises that value, so each time you heat the beaker the same water can hold more dissolved solid than before.

Q3.

What do you infer from this experiment?

Answer

For most solids, solubility in a liquid increases as the temperature rises. Water at 70 °C dissolves more baking soda than water at 50 °C, and water at 20 °C dissolves the least of the three.

Two statements follow, and both are worth writing down:

A saturated solution at one temperature behaves as an unsaturated solution if the temperature is raised — the leftover solid then dissolves.
Any figure quoted for solubility must carry a temperature with it, otherwise it does not say anything definite.

Why it happens: Two opposite processes go on at the crystal's surface — particles leaving it and going into the water, and dissolved particles returning and sticking back on. A saturated solution is simply the state where these two happen at the same rate. Heating speeds up the leaving far more than the returning, so the balance shifts and more solid ends up dissolved before the two rates match again.

Check it yourself: Let the 70 °C solution cool back to room temperature without disturbing it. Solid baking soda will reappear at the bottom — proof that you crossed the limit going up and are crossing it again coming down.

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