Solutes, Solvents & Solutions — Complete Answer Key
Every in-text question and every “Keep the Curiosity Alive” exercise from the chapter, answered directly with clear steps, formulas, and the chapter’s own diagrams.
In-text Questions (Probe, Ponder & Thought Bubbles)
What do you think is happening in the picture above?
The picture shows people at a coastline collecting salt from a salt pan. Seawater is allowed to evaporate in shallow pans under the sun, and as the water evaporates, dissolved salt is left behind as solid crystals. The workers are scraping and gathering this crystallised salt — a real-life example of separating a solute (salt) from its solvent (water) by evaporation.
What happens when you add too much sugar to your tea and it stops dissolving? How can you solve this problem?
When too much sugar is added, the tea can no longer dissolve any more of it — the tea has become a saturated solution, and the extra sugar settles at the bottom as undissolved solid.
How to solve it: Since solubility generally increases with temperature, gently heating the tea allows it to dissolve more sugar (the saturated solution behaves like an unsaturated one at the higher temperature). Stirring well also helps the existing sugar dissolve faster, though it cannot increase the total amount that can dissolve at a given temperature.
Why do sugar and salt dissolve in water but not in oil? Why is water considered a good solvent?
Sugar and salt are polar / ionic substances. Water molecules are also polar, so they can surround and pull apart the particles of sugar and salt, dissolving them — this follows the rule “like dissolves like.” Oil, on the other hand, is a non-polar substance, so it cannot attract or separate the polar/ionic particles of salt and sugar, and they remain undissolved in it.
Water is called a good (almost universal) solvent because its polar nature allows it to dissolve a very wide range of solids, liquids, and gases — far more substances than most other common liquids.
Why are water bottles usually tall and cylindrical in shape instead of spherical?
A tall, cylindrical shape is preferred over a spherical one because it:
We know air is a mixture. Would a mixture of gases also be considered a solution?
Yes. A solution is simply a uniform (homogeneous) mixture in which the components are evenly distributed and cannot be seen separately. Air is a uniform mixture of gases such as nitrogen, oxygen, carbon dioxide, and water vapour, evenly mixed at the molecular level — so air, and gas mixtures in general, are correctly classified as solutions (specifically, gas-in-gas solutions).
What will happen if we keep on adding more salt to a given amount of water?
Initially, each spoonful of salt dissolves completely and the solution remains unsaturated. As more salt is added, a point is reached where the water can no longer dissolve any additional salt — the solution becomes saturated. Beyond this point, any further salt added will not dissolve and will simply settle as solid, undissolved salt at the bottom of the container.
I observed that sawdust floats in water while sand sinks. I wonder why that happens?
This happens because of the difference in density between the two substances and water.
Rule: objects less dense than the liquid float; objects denser than the liquid sink.
A 1-litre pack of oil/ghee is labelled with a weight of only 910 g. What does this tell us about the density of the oil — is it less or more than that of water?
This is exactly why oil floats on top of water — it weighs less than an equal volume of water.
Why are measuring cylinders always designed narrow and tall instead of wide and short like a beaker?
In a narrow cylinder, even a small change in volume produces a noticeably larger rise or fall in the liquid level than the same volume change would in a wide container. This makes the scale markings more spread out and easier to read precisely, giving a smaller “smallest measurable volume” and therefore greater measurement accuracy. A wide, short beaker would show only a tiny, hard-to-read change in height for the same volume, making readings far less precise.
I wonder how the level of a coloured liquid is measured, since its meniscus bottom can’t be seen clearly?
For colourless liquids (like water), the reading is taken at the bottom of the meniscus, which is clearly visible. For coloured or opaque liquids, the bottom of the meniscus is hard to see through the liquid, so instead the reading is taken at the top of the meniscus, which remains clearly visible against the cylinder markings.
A raw egg sinks in a tumbler of tap water. What change can you make to the setup to make the egg float instead of sinking?
Keep the Curiosity Alive — Exercise Solutions
State whether the statements given below are True [T] or False [F]. Correct the false statement(s).
Correction: Oxygen gas is more soluble in cold water than in hot water — solubility of gases decreases as temperature increases.
Correction: A mixture of sand and water is a non-uniform mixture, not a solution, because sand does not dissolve and its particles are not evenly distributed.
Correction: The amount of space occupied by any object is called its volume (mass is the amount of matter present in it).
Correction: A saturated solution has the maximum solute dissolved at that temperature; an unsaturated solution has dissolved less solute than this maximum and can still dissolve more.
The gases of the atmosphere are evenly mixed throughout, so the atmosphere is correctly described as a uniform mixture (a solution of gases).
Fill in the blanks.
(i) The volume of a solid can be measured by the method of displacement, where the solid is immersed / dipped in water and the rise (increase) in water level is measured.
(ii) The maximum amount of solute dissolved in a fixed quantity of solvent / solution at a particular temperature is called solubility at that temperature.
(iii) Generally, the density decreases with increase in temperature.
(iv) The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a given temperature, is called a saturated solution of glucose.
You pour oil into a glass containing some water. The oil floats on top. What does this tell you?
Correct option: (ii) Water is denser than oil.
An object or liquid floats on another liquid only when it is less dense than that liquid. Since oil floats on water, oil must have a lower density than water — equivalently, water is denser than oil. (Options i, iii, iv are incorrect: oil is not denser than water, the two liquids clearly do not have equal density since they separate into layers, and oil floating — rather than mixing — shows that oil does not dissolve in water.)
A stone sculpture weighs 225 g and has a volume of 90 cm³. Calculate its density and predict whether it will float or sink in water.
Density = 2.5 g/cm³ → the stone sculpture will SINK in water.
Which one of the following is the most appropriate statement, and why are the other statements not appropriate?
Correct option: (iii) No more solute can be dissolved into the saturated solution at that temperature. This is the defining property of a saturated solution.
Why the others are wrong:
You have a bottle with a volume of 2 litres. You pour 500 mL of water into it. How much more water can the bottle hold?
The bottle can hold 1500 mL (1.5 litres) more water.
An object has a mass of 400 g and a volume of 40 cm³. What is its density?
Density = 10 g/cm³
Analyse the figures below. Why does the unpeeled orange float, while the peeled one sinks? Explain.
The peel’s trapped air lowers density (float); removing it raises the average density above water’s (sink).
Object A has a mass of 200 g and a volume of 40 cm³. Object B has a mass of 240 g and a volume of 60 cm³. Which object is denser?
| Object | Mass | Volume | Density |
|---|---|---|---|
| A | 200 g | 40 cm³ | 5 g/cm³ |
| B | 240 g | 60 cm³ | 4 g/cm³ |
Object A is denser than Object B.
Reema has a piece of modelling clay that weighs 120 g. She first moulds it into a compact cube that has a volume of 60 cm³. Later, she flattens it into a thin sheet. Predict what will happen to its density.
Density remains unchanged at 2 g/cm³, regardless of the clay’s new flat shape.
A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?
Volume ≈ 75.9 cm³ (about 76 cm³)
An experimental setup is shown below. On keeping the test tube (b) in a beaker containing hot water (~70 °C), the water level in the glass tube rises. How does it affect the density?
Heating → volume increases, mass unchanged → density of the water decreases.
Discover, Design & Debate
Why is there no aquatic life in the Dead Sea? Are there other similar water bodies?
How well does common salt dissolve in different solvents — water, vinegar, and oil?
Order of solubility of salt: Water ≈ Vinegar >> Oil (almost insoluble) — confirming “like dissolves like.”
Is water truly the most versatile solvent?
Arguments FOR (water is the most versatile solvent): Water’s polarity and ability to form hydrogen bonds let it dissolve an unusually wide range of substances — salts, sugars, acids, bases, many gases (like oxygen and carbon dioxide), and countless biological molecules. It is essential to all known life, cheap, safe, and abundant, earning it the title “universal solvent.”
Arguments AGAINST (water has limits): Water cannot dissolve non-polar substances such as oils, fats, waxes, and many plastics. For these, other solvents (like alcohols, acetone, or oils themselves) work far better. So while water is extremely versatile among polar substances, it is not truly “universal.”
Use both sides above to argue either position in class — there is no single “correct” side; the goal is to support your stance with these scientific reasons.