Where Mindful Learning Begins
Where Mindful Learning Begins
Class 8 Science Curiosity NCERT Solutions Chapter 9 — every Probe and Ponder prompt, all 7 Activities, all 12 "Keep the Curiosity Alive" exercise questions, and all 3 "Discover, Design, and Debate" project prompts, solved and explained on one page.
This chapter builds up solutes, solvents, and solutions from everyday examples like ORS and gulab jamun syrup, explains saturation, solubility, and how temperature affects both solids and gases dissolving — then shifts to why objects float or sink, introducing density, and how to actually measure mass and volume (including tricky irregular shapes like a stone).
Chapter 9 starts with a familiar puzzle — why does homemade ORS taste the same in every sip? — to introduce solutes, solvents, and solutions as a special, uniform kind of mixture. From there, it builds up saturation and solubility (how much salt or baking soda a fixed amount of water can actually dissolve, and how temperature changes that answer differently for solids versus gases), before pivoting to a second big idea: why some objects float while others sink. This leads into density — its formula, its units, how temperature and pressure affect it, and hands-on ways to actually measure the mass and volume of both regular and oddly-shaped objects. Every Activity and exercise is solved here exactly as the textbook presents it.
A uniform mixture where a solute dissolves completely into a solvent — including gases dissolving in liquids, and even air itself.
Every solvent has a limit to how much solute it can dissolve at a given temperature — and that limit usually shifts with heat.
Mass packed into a given volume — measured with a balance and a measuring cylinder, and central to whether an object floats.
Solute, solvent, and solution
A solution is a uniform mixture, like salt or sugar dissolved in water. When a solid dissolves in a liquid, the solid is the solute and the liquid is the solvent: Solute + Solvent → Solution. When two liquids form a solution, the substance present in the smaller amount is the solute, and the one in the larger amount is the solvent.
Salt or sugar mixed with water forms a uniform mixture (a solution), since the particles spread evenly and cannot be seen separately. Chalk powder, sand, or sawdust mixed with water forms a non-uniform mixture instead, since these solids don't dissolve and their particles remain visibly separate.
Gases as solutions: gases can dissolve to form solutions too — air is a gaseous solution, with nitrogen (the gas present in the largest amount) considered the solvent, and oxygen, argon, carbon dioxide, and other gases considered the solutes.
Saturation, solubility & concentration
| Term | Meaning |
|---|---|
| Unsaturated solution | A solution in which more solute can still be dissolved at that temperature |
| Saturated solution | A solution in which the solute has stopped dissolving and begins to settle at the bottom, at that temperature |
| Solubility | The maximum amount of solute that dissolves in a fixed quantity of solvent at a given temperature |
| Concentration | The amount of solute present in a fixed quantity of solution (or solvent) |
| Dilute solution | A solution with a relatively smaller amount of solute (a relative term) |
| Concentrated solution | A solution with a relatively larger amount of solute (a relative term) |
Effect of temperature on solubility: for most solids (like baking soda), solubility increases with an increase in temperature — a saturated solution at one temperature behaves like an unsaturated solution once heated further. For gases, it's the opposite: solubility decreases as temperature increases — this is why cold water holds more dissolved oxygen than warm water, which matters for aquatic life.
Why objects float or sink
Broadly, objects that float in a liquid are lighter (less dense) than that liquid, and objects that sink are heavier (denser) than it — though density is not the only factor that decides this.
Density
Density is the mass present in a unit volume of a substance: Density = Mass ÷ Volume. It is independent of an object's shape or size, but depends on temperature and pressure (pressure mainly affects gases; its effect on solids and liquids is negligible).
| Quantity | SI unit | Other common units |
|---|---|---|
| Mass | Kilogram (kg) | Gram (g) |
| Volume | Cubic metre (m³) | Litre (L), millilitre (mL), cm³ (1 mL = 1 cm³) |
| Density | kg/m³ | g/mL, g/cm³ (for liquids, by convenience) |
Conversion: 1 kg/m³ = 1000 g/m³ = 1 g/L = 1 g/1000 mL = 1 g/1000 cm³. The mass of 1 mL of water is close to 1 g at room temperature (so 10 mL ≈ 10 g, 100 mL ≈ 100 g).
Worked example: an aluminium block of mass 27 g and volume 10 cm³ has density = 27 ÷ 10 = 2.7 g/cm³, meaning aluminium is 2.7 times denser than water.
Relative density = Density of a substance ÷ Density of water at that temperature — a number with no units.
A 1-litre pack of oil weighing only 910 g tells us that oil's density = 910 g ÷ 1000 mL = 0.91 g/mL, which is less than water's density (1 g/mL) — confirming that oil is less dense than water, exactly why oil floats on top of water.
Mass vs weight: mass is the quantity of matter in an object (g, kg); weight is the force of Earth's gravity pulling on it (measured in newtons, N). Most balances actually measure weight but display it using mass units.
Effect of temperature on density: heating spreads a substance's particles further apart, increasing its volume while mass stays the same — so density decreases with heating (this is why hot air rises and hot air balloons work).
Effect of pressure on density: increasing pressure pushes gas particles closer together, decreasing volume and increasing density; liquids are barely affected (nearly incompressible); solids are affected even less, with changes usually negligible.
A raw egg normally sinks in plain tap water since it is denser than water. Adding salt to the water gradually increases the water's own density; once the salty water becomes denser than the egg, the egg will float — the same principle that makes floating so easy in extremely salty water bodies like the Dead Sea.
Sample answer: the opening picture shows people collecting salt from salt pans by the sea — seawater is allowed to evaporate in shallow beds, leaving behind solid salt crystals that people gather by hand, a traditional method of salt production.
When too much sugar is added to tea, it eventually stops dissolving because the tea has reached its solubility limit at that temperature — it has become a saturated solution, and any further sugar simply settles at the bottom instead of dissolving. This can usually be fixed by heating the tea further, since solubility of solids like sugar generally increases with temperature, allowing more sugar to dissolve.
Sugar and salt dissolve in water because water is a highly effective solvent for many solid substances, especially ionic and polar ones — but oil is a very different kind of liquid (non-polar), and substances like sugar and salt simply cannot mix into it the way they mix into water. Water is considered such a good solvent because it can dissolve an extraordinarily wide range of substances — salts, sugars, many gases, and more — making it central to biological and everyday chemical processes.
Water bottles are usually tall and cylindrical rather than spherical mainly for practical reasons: a cylindrical shape is easier and cheaper to manufacture, stacks and stores efficiently without wasted space, stands stably upright, and is comfortable to hold and drink from — a sphere would roll around, waste shelf space, and be awkward to grip.
This is an open reflection prompt meant to set up the chapter's central ideas — solutions, solubility, and (later) density — all explained in full through the Activities below.
Table 9.1 (illustrative pattern):
| Amount of salt taken (teaspoon) | Observation |
|---|---|
| One | Salt dissolves completely |
| Two | Salt dissolves completely |
| Three | Salt dissolves completely |
| Four | Some salt remains undissolved, settling at the bottom |
| … | No further salt dissolves — the solution has reached saturation |
Note: the exact number of spoons that dissolve will vary depending on the amount of water used, the size of the spoon, and the water's temperature — the pattern above (dissolving completely for a while, then stopping) is what matters.
Discussion points answered: after a certain number of spoons, the added salt no longer dissolves completely and starts settling at the bottom. This indicates that water has a limited capacity to dissolve salt at a given temperature — once this limit is reached, the solution is said to be saturated, and adding more solute beyond this point simply leaves it undissolved.
Going further — which is more concentrated: 2 spoons of salt in 100 mL of water, or 4 spoons of salt in 50 mL of water? The second one (4 spoons in 50 mL) is more concentrated. Comparing solute per unit volume: 2 spoons ÷ 100 mL = 0.02 spoons/mL, while 4 spoons ÷ 50 mL = 0.08 spoons/mL — four times as concentrated, since it has both more solute and less solvent.
Observation: at 20 °C, the water dissolves baking soda only up to a certain point, after which some remains undissolved at the bottom. On heating this mixture to 50 °C (while stirring), the previously undissolved baking soda dissolves completely. Adding more baking soda at 50 °C again reaches a new saturation point — but heating further to 70 °C dissolves this undissolved portion too.
Answer: water at 70 °C can dissolve more baking soda than water at 50 °C, which in turn dissolves more than water at 20 °C. This shows that for most substances, solubility increases as temperature increases — and a solution that is saturated at one temperature becomes unsaturated (able to dissolve more) if the temperature is raised.
Method: switch on the digital balance and check it reads zero (using the tare/reset button if it doesn't). Place a clean, dry watch glass or butter paper on the pan, note its reading, then reset the balance to zero again using the tare button — this way, the weight of the watch glass itself is not counted. Carefully place the solid object (such as a stone) on the watch glass, and read the mass displayed directly, for example, 16.400 g.
Note: to measure the mass of a liquid instead, simply replace the watch glass with a beaker, tare the balance with the empty beaker on the pan, then pour in the desired amount of liquid and read its mass directly.
Maximum volume: a measuring cylinder marked up to 100 mL can measure a maximum volume of 100 mL.
Finding the smallest readable volume: if the difference between two bigger marks (e.g. between 10 mL and 20 mL) is 10 mL, and there are 10 smaller divisions between them, then each small division reads 10 ÷ 10 = 1 mL — the smallest volume this particular cylinder can measure.
Embedded question — why are measuring cylinders narrow and tall, not wide and short like a beaker? A narrow, tall shape means the same change in volume causes a much bigger, more easily visible change in the liquid's height along the marked scale. In a wide, short container, the same volume change would barely shift the liquid level, making it far harder to read accurately — so the narrow, tall design improves precision.
Going further — choosing the right size cylinder: to measure exactly 70 mL of water, a 50 mL cylinder can't do it in a single step (you'd need to measure 50 mL, then add 20 mL separately, which is inconvenient), while a 250 mL or 500 mL cylinder could measure it in one step but with less precision, since their smallest readable divisions are larger (2 mL and 5 mL respectively) than a 100 mL cylinder's (1 mL). A 100 mL measuring cylinder is therefore the best choice for measuring 70 mL — accurate in a single step.
Method: place a clean, dry measuring cylinder on a flat surface and pour water slowly up towards the 50 mL mark, using a dropper for fine adjustment near the end. On close observation, the water's surface inside the cylinder forms a curved shape called the meniscus.
Reading the meniscus correctly: for water and other colourless liquids, read the mark that lines up with the bottom of the meniscus, keeping your eyes level with it. For coloured liquids, where the bottom of the curve may be hard to see clearly, read the mark that lines up with the top of the meniscus instead.
Formula: Volume = length × width × height.
Worked example: for a notebook with length 25 cm, width 18 cm, and height 2 cm: Volume = 25 cm × 18 cm × 2 cm = 900 cm³.
Method: fill a measuring cylinder with water to a known initial volume (say 50 mL), then tie the object with a thread and slowly lower it fully into the water. The water level rises because the submerged object displaces (pushes aside) a volume of water exactly equal to its own volume. Record the new, final volume (say 55 mL); the object's volume equals the final volume minus the initial volume.
Table 9.2 (illustrative pattern):
| Object | Initial volume (A) | Final volume (B) | Volume displaced (B−A) | Volume of object (cm³) |
|---|---|---|---|---|
| Stone | 50 mL | 55 mL | 5 mL | 5 cm³ |
| Metal key | — | — | — | (record your own reading) |
| Any other object | — | — | — | (record your own reading) |
Note: since mL and cm³ are equivalent for solids (1 mL = 1 cm³), the displaced volume in mL can be written directly as the object's volume in cm³.
Going further — calculating the stone's density: combining this activity's volume for the stone (5 cm³) with Activity 9.3's measured mass for the same stone (16.400 g), the stone's density = Mass ÷ Volume = 16.400 g ÷ 5 cm³ = 3.28 g/cm³.
Asima Chatterjee is celebrated for her pioneering work developing anti-epileptic and anti-malarial drugs, work that depended directly on the very ideas this chapter covers — she used solvents and solutions extensively to extract and isolate important therapeutic compounds from medicinal plants.
She earned a Doctorate of Science, becoming only the second Indian woman ever to do so, following Janaki Ammal. She went on to become the first woman to receive the prestigious Shanti Swarup Bhatnagar Award in the field of chemical science, and was further honoured with the Padma Bhushan — a career built on the practical, patient chemistry of dissolving, separating, and isolating substances from nature.
In Ningel village, in Manipur's Thoubal district, salt is still produced today using centuries-old traditional methods. The village has a handful of salt wells, one of which is lined with a tree trunk over a hundred years old, placed deep into the ground to help draw up naturally salty water — a real-world solution of dissolved salt.
A few families, mostly women, continue this sacred practice: they collect the salty solution and boil it in large metal pans over firewood kilns until the water evaporates away and salt crystals form. These crystals are shaped by hand into round "salt cakes" using banana leaves and simple handmade tools, then wrapped in a traditional cloth called a phanek to protect them — cakes believed to carry some medicinal value of their own. In Ningel, salt is far more than food: it is a living thread of history, culture, and belief.
The chashni (sugar syrup) of gulab jamun is made of a large amount of sugar (solid) dissolved in only a small amount of water (liquid). Even though sugar is present in a much greater amount, water is still considered the solvent and sugar the solute — since for a solid dissolved in a liquid, the solid is always the solute, regardless of the relative quantities involved.
Water has long been used as the primary solvent for preparing medicinal formulations in Ayurveda, Siddha, and other traditional Indian medicine systems. Alongside water, hydro-alcoholic herbal extracts, and substances like oils, ghee, and milk have also been used as solvents, chosen specifically to help achieve the therapeutic benefits of a given drug formulation.
Mass is the quantity of matter in an object, measured in grams or kilograms. Weight is the force with which Earth's gravity pulls on that object, measured in newtons. Most everyday balances actually measure weight, but their scales are marked in mass units (g or kg) for convenience — true two-pan balances are an exception.
Earth is made up of several layers — crust, upper mantle, lower mantle, outer core, and inner core — each with its own characteristic density. The outermost crust is the lightest layer, and density steadily increases moving towards the centre, as rising pressure and temperature make deeper materials heavier and more compact.
Long before large ships existed, people used bamboo and wooden logs to cross rivers and seas. Bamboo was ideal for this because it is light, hollow, and floats easily; tying bamboo poles together made simple rafts and boats for fishing, trade, and travel — a tradition that continues in some regions today, both practically and as a tourist attraction.
Water is at its densest at 4 °C. As it cools further and freezes into ice at 0 °C, its particles rearrange into a structure that takes up more space (expansion), making ice less dense than liquid water — which is why ice floats. This matters for aquatic life, since floating ice forms an insulating layer that keeps the water underneath warm enough for fish to survive even in extreme cold.
(i) False. Oxygen gas is actually more soluble in cold water than in hot water, since gas solubility decreases as temperature rises.
(ii) False. A mixture of sand and water is a non-uniform mixture, not a solution — sand does not dissolve, and its particles remain visibly separate, settling at the bottom.
(iii) False. The amount of space occupied by any object is called its volume, not its mass.
(iv) False. It is actually the saturated solution that holds the maximum amount of solute possible at that temperature; an unsaturated solution has less solute dissolved than a saturated one could hold (and can still dissolve more).
(v) True. The mixture of different gases in the atmosphere (air) is indeed a solution — specifically, a uniform gaseous solution.
(i) The volume of a solid can be measured by the method of displacement, where the solid is immersed/submerged in water and the rise (increase) in water level is measured.
(ii) The maximum amount of solute dissolved in a fixed quantity of solvent 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.
Answer: (ii) Water is denser than oil.
Reason: since oil floats on top of the water, oil must be less dense than water — equivalently, water is denser than oil. (Oil floating also tells us the two liquids are immiscible, i.e. oil doesn't dissolve in water, ruling out option (iv); options (i) and (iii) are the opposite of, or contradict, what floating actually shows.)
Density = Mass ÷ Volume = 225 g ÷ 90 cm³ = 2.5 g/cm³.
Prediction: since the stone's density (2.5 g/cm³) is greater than the density of water (1 g/cm³), the stone sculpture will sink in water.
Answer: (iii) No more solute can be dissolved into the saturated solution at that temperature. This is exactly the definition of a saturated solution.
Why the others are wrong: (i) is incorrect because a saturated solution, by definition, cannot dissolve any more solute at that temperature — that describes an unsaturated solution instead. (ii) is incorrect because an unsaturated solution has not yet reached the maximum amount possible — that describes a saturated solution instead. (iv) is incorrect because saturation can happen at any temperature, not only high ones — a solution simply reaches its solubility limit for whatever temperature it is currently at.
Total capacity = 2 litres = 2000 mL. Water already poured in = 500 mL.
Remaining space = 2000 mL − 500 mL = 1500 mL — the bottle can hold 1500 mL (1.5 litres) more water.
Density = Mass ÷ Volume = 400 g ÷ 40 cm³ = 10 g/cm³.
Answer: an orange's peel is porous and contains many tiny pockets of trapped air. This trapped air lowers the orange's overall average density (peel + flesh + air pockets together) to below that of water, allowing the whole unpeeled orange to float.
Once the peel (and its trapped air pockets) is removed, only the denser flesh remains, and this flesh alone has a higher average density than water — so the peeled orange sinks. This nicely shows that an object's overall density (not just the density of one part of it) determines whether it floats or sinks.
Object A: Density = 200 g ÷ 40 cm³ = 5 g/cm³.
Object B: Density = 240 g ÷ 60 cm³ = 4 g/cm³.
Answer: Object A is denser (5 g/cm³ > 4 g/cm³).
Density of the cube = 120 g ÷ 60 cm³ = 2 g/cm³.
Prediction after flattening: the density stays exactly the same, at 2 g/cm³. Flattening the clay into a thin sheet reshapes it, but since no clay is added or removed, the mass stays at 120 g; and since the clay is not being compressed (just spread into a different shape), its total volume also stays at 60 cm³. As this chapter states, density is independent of an object's shape or size — only a genuine change in mass or volume (such as compressing the material or removing some of it) would change the density.
Volume = Mass ÷ Density = 600 g ÷ 7.9 g/cm³ ≈ 75.95 cm³ (approximately 76 cm³).
Answer: the test tube contains a fixed, unchanging mass of water throughout the experiment — no water is added or removed. When placed in hot water, the water inside the test tube is heated, and its particles gain energy and spread further apart, causing the water to expand in volume — this is exactly why its level rises visibly in the narrow glass tube.
Since Density = Mass ÷ Volume, and the mass stays constant while the volume increases due to heating, the density of the water decreases as it is heated — a direct demonstration of the chapter's principle that density generally decreases with an increase in temperature.
These three prompts are research-based investigations and a class discussion rather than fixed-answer questions. Here's guidance on how to approach each one.
Guidance: the Dead Sea is famous for having an extremely high concentration of dissolved salts — far saltier than ordinary seawater — making it one of the most hypersaline (extremely salty) water bodies on Earth. This extreme salt concentration creates conditions too harsh for virtually all fish, plants, and most other aquatic organisms to survive in, since they cannot cope with such intense osmotic stress — hence the name "Dead" Sea.
Connecting back to this chapter: this extreme saltiness also makes the Dead Sea's water unusually dense — which is exactly why people famously float so easily on its surface, in the same way the "Think Like a Scientist" activity in this chapter shows that adding enough salt to water can make even a sinking egg float.
Other similar water bodies to research: look into other well-known hypersaline lakes, such as the Great Salt Lake (USA) or Lake Assal (Djibouti), using an up-to-date geography or environmental science source, since exact salinity figures and details are best verified from current references rather than assumed.
What to expect and why:
Try this practically at home (with adult supervision) by adding equal small amounts of salt to equal volumes of each liquid and observing how much dissolves, recording your results in a simple comparison table.
Case for water being the most versatile solvent: water dissolves an extraordinarily broad range of substances — salts, sugars, many acids and bases, and even gases like oxygen and carbon dioxide. It is also cheap, safe, non-toxic, and essential to every known biological process, making its combination of breadth, safety, and abundance genuinely unmatched.
Case against, or for other solvents: water cannot dissolve non-polar substances like oils, fats, and many organic compounds — solvents such as oils, alcohols, or hydro-alcoholic mixtures (as seen in traditional Indian medicine, covered earlier in this chapter) are deliberately used specifically because water fails to dissolve certain important compounds.
A balanced conclusion for the debate: water's real "versatility" lies in how useful it is across an enormous range of everyday and biological substances, even though it is not universally effective for every possible substance — arguably making it the most broadly important solvent, even if not the only one ever needed.
Now that solutions, solubility, and density are covered, move on to how light behaves with mirrors and lenses, revisit Chapter 8, or book a free demo class for personalised coaching.
Expert CBSE Coaching · Class 9–12