Chapter 9: The Amazing World of Solutes, Solvents, and Solutions (Chemistry)
Answers to all "Keep the curiosity alive" questions of Chapter 9, The Amazing World of Solutes, Solvents, and Solutions (NCERT Class 8 Science, Curiosity, 2026-27): solutions, saturated and unsaturated solutions, solubility, volume by displacement, density = mass ÷ volume, sinking and floating, the orange with and without its peel, and how heating changes density. All 12 questions are answered, with the key answer highlighted.
A solution is a uniform mixture of a solute dissolved in a solvent. A saturated solution cannot dissolve any more solute at that temperature. Density = mass ÷ volume (g/cm³). Water's density is 1 g/cm³; anything denser sinks in water, anything less dense floats.
True or false? Correct the false statements. (i) Oxygen is more soluble in hot water than in cold water. (ii) A mixture of sand and water is a solution. (iii) The space occupied by an object is its mass. (iv) An unsaturated solution has more solute dissolved than a saturated solution. (v) The gases in the atmosphere form a uniform mixture.
Solution
False. Oxygen (like other gases) is more soluble in cold water. Warm water holds less dissolved oxygen.
False. Sand does not dissolve; it settles down. A mixture of sand and water is a non-uniform mixture, not a solution.
False. The space an object occupies is its volume. Mass is the amount of matter in it.
False. An unsaturated solution has less solute than a saturated one, and it can still dissolve more.
True. Air is a uniform mixture: a solution of gases.
(i) F: more soluble in cold water (ii) F: it is a non-uniform mixture (iii) F: that is volume (iv) F: it has less solute (v) T
Fill in the blanks. (i) In the displacement method, the solid is ____ in water and the ____ in water level is measured. (ii) The maximum amount of ____ dissolved in ____ at a given temperature is its solubility. (iii) Density generally ____ as temperature increases. (iv) A glucose solution in which no more glucose can dissolve at that temperature is a ____ solution.
Solution
completely immersed (submerged); rise
solute; a fixed amount (100 g) of solvent
decreases
saturated
(i) immersed; rise (ii) solute; a given amount (100 g) of solvent (iii) decreases (iv) saturated
Which statement is most appropriate, and why are the others not? (i) A saturated solution can dissolve more solute. (ii) An unsaturated solution has dissolved the maximum solute possible. (iii) No more solute can dissolve in a saturated solution at that temperature. (iv) A saturated solution forms only at high temperatures.
Solution
(iii) is correct: that is the definition of a saturated solution.
(i) describes an unsaturated solution.
(ii) describes a saturated solution, not an unsaturated one.
(iv) is wrong because a solution can become saturated at any temperature. Only the amount needed changes with temperature.
(iii); the others mix up saturated and unsaturated, or wrongly limit saturation to high temperatures.
Why does an unpeeled orange float in water, while a peeled one sinks?
Solution
The peel is spongy and full of tiny air pockets. With the peel on, the orange has a larger volume for little extra mass, so its overall density is less than water's and it floats. The peel acts like a life jacket.
Without the peel, the juicy fruit alone is denser than water, so it sinks.
The air-filled peel makes the whole orange less dense than water; the peeled fruit is denser than water and sinks.
A test tube of water with a glass tube in its stopper is placed in hot water (about 70 °C). The water level in the glass tube rises. How does this affect the density?
Solution
The water expands on heating, so its volume increases and it rises up the tube. Its mass stays the same, so the density (mass ÷ volume) decreases. That is why density generally decreases as temperature rises.
The water expands (more volume, same mass), so its density decreases.
