NCERT Solutions: Chapter9 The Amazing World of Solutes, Solvents, and Solutions (class 8 Science)
All questions and detailed solutions from Chapter 9 are fully pre-rendered below, covering Saturated & Unsaturated Solutions, Universal Solvents, Density Calculations, Buoyancy Phenomena, and the Dead Sea.
The picture shows Mahatma Gandhi obtaining salt from the sea during the Salt March, accompanied by his followers.
This illustrates the historical process of extracting salt from seawater through evaporation. Seawater is a natural solution with salt as the solute and water as the solvent. As the water evaporates under the heat of the Sun, the dissolved salt comes out of the solution and forms solid salt crystals, which is the foundational principle behind traditional salt production.
When too much sugar is added, the tea becomes a saturated solution at that specific temperature; the excess sugar can no longer dissolve and settles down at the bottom of the cup.
How to solve this problem:
- Warm the tea: Increasing the temperature increases the solubility of solid solutes in liquid solvents, allowing the extra sugar to dissolve completely.
- Stirring: Vigorous stirring distributes solute particles evenly and accelerates the dissolution process.
- Add more liquid: Adding more hot water or milk increases the quantity of solvent available to dissolve the remaining sugar.
Sugar and salt dissolve in water because water is a polar liquid. Water molecules attract and pull apart the charged ions of salt and polar molecules of sugar, allowing them to disperse uniformly throughout the liquid to form a homogeneous solution.
In contrast, cooking oil is non-polar. Non-polar oil molecules cannot break the strong ionic bonds of salt or interact effectively with sugar, so they do not dissolve in oil.
Why water is a good solvent: Water can dissolve a remarkably wide variety of substances (especially ionic compounds, acids, and polar organic compounds), which is why it is widely celebrated as the universal solvent.
Water bottles are designed in tall, cylindrical geometries for several ergonomic and practical reasons:
- Easy Grip and Handling: A narrow cylindrical profile fits comfortably inside the human hand, making it easy to hold, carry, and pour from.
- Stability: A flat-bottomed cylinder stands upright securely on tables and shelves without rolling away, unlike a sphere.
- Packing Efficiency: Cylindrical bottles fit neatly into backpack side pockets, cup holders, and refrigerators with minimal wasted space.
Potential investigative questions include:
- How does temperature affect the solubility of gases differently from solids in water?
- Why does ice float on water despite being a solid?
- What role does fluid density play in everyday phenomena like hot air balloons and submarines?
- How can we separate two miscible liquids that have different boiling points?
Yes, provided it is a homogeneous (uniform) mixture. Air is a gaseous solution because its component gases are completely and evenly distributed throughout. In air, nitrogen gas (constituting about 78%) is present in the largest quantity and acts as the solvent, while oxygen, carbon dioxide, argon, and water vapour act as solutes.
The added salt will dissolve readily at first. However, as more salt is added, a stage is reached where the water cannot dissolve any more salt at that temperature. The solution becomes a saturated solution, and any additional salt added beyond this saturation point remains completely undissolved and settles at the bottom of the beaker.
Yes, gases dissolve in water, although usually in small quantities. For example, dissolved oxygen ($O_2$) in ponds and oceans is essential for the respiration and survival of aquatic organisms (fish, plants). Carbon dioxide gas is dissolved under pressure in carbonated beverages.
The solubility of gases in water depends on temperature and pressure: colder water holds more dissolved gas than warm water, and increasing pressure increases gas solubility.
A measuring cylinder is designed tall and narrow to ensure high precision in volume measurement:
- In a narrow cylinder, a small volume of liquid produces a significant and easily noticeable rise in height, allowing finer graduation markings.
- In a wide beaker, a small volume change produces an almost imperceptible change in liquid height, resulting in poor measurement accuracy.
- The narrow diameter also minimizes surface curvature, making it easier to read the liquid meniscus accurately at eye level.
For clear and transparent liquids, the volume is measured at the bottom of the concave meniscus at eye level. However, for deeply coloured or opaque liquids (such as potassium permanganate solution), the bottom of the meniscus may not be clearly visible. In such cases, the reading is taken carefully at the upper meniscus level, keeping the eye strictly horizontal to the graduation mark to avoid parallax error.
(i) Oxygen gas is more soluble in hot water rather than in cold water.
(ii) A mixture of sand and water is a solution.
(iii) The amount of space occupied by any object is called its mass.
(iv) An unsaturated solution has more solute dissolved than a saturated solution.
(v) The mixture of different gases in the atmosphere is also a solution.
(i) False: Correction: Oxygen gas is more soluble in cold water than in hot water. As temperature rises, dissolved gas molecules gain kinetic energy and escape from the liquid into the air.
(ii) False: Correction: A mixture of sand and water is a heterogeneous suspension, not a solution. Sand particles do not dissolve in water; they remain suspended temporarily and settle to the bottom under gravity.
(iii) False: Correction: The amount of space occupied by an object is called its volume. Mass is the total quantity of matter contained within the object.
(iv) False: Correction: A saturated solution contains the maximum possible amount of dissolved solute at a given temperature. An unsaturated solution contains less solute than a saturated solution and can dissolve more.
(v) True: The atmosphere is a homogeneous mixture of gases (mainly nitrogen and oxygen), where nitrogen acts as the solvent and other gases act as solutes.
(ii) The maximum amount of _______ dissolved in _______ at a particular temperature is called solubility at that temperature.
(iii) Generally, the density _______ 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 _______ solution of glucose.
(i) The volume of a solid can be measured by the method of displacement, where the solid is immersed / placed in water and the rise in water level is measured.
(ii) The maximum amount of solute dissolved in 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.
(i) Oil is denser than water
(ii) Water is denser than oil
(iii) Oil and water have the same density
(iv) Oil dissolves in water
Correct Answer: (ii) Water is denser than oil
Explanation: Immiscible liquids arrange themselves according to their densities: the liquid with lower density floats on top of the liquid with higher density. Since oil floats on water, oil is less dense than water, meaning water is denser than oil.
Given Data:
• Mass of stone sculpture ($m$) = $225\text{ g}$
• Volume of stone sculpture ($V$) = $90\text{ cm}^3$
Formula:
$$\text{Density } (\rho) = \frac{\text{Mass}}{\text{Volume}} = \frac{225\text{ g}}{90\text{ cm}^3} = 2.5\text{ g/cm}^3$$
Floatation Prediction:
The density of pure water is $1.0\text{ g/cm}^3$. Since the density of the stone sculpture ($2.5\text{ g/cm}^3$) is significantly greater than the density of water, the sculpture will sink in water.
(i) A saturated solution can still dissolve more solute at a given temperature.
(ii) An unsaturated solution has dissolved the maximum amount of solute possible at a given temperature.
(iii) No more solute can be dissolved into the saturated solution at that temperature.
(iv) A saturated solution forms only at high temperatures.
Statement (iii) is the most appropriate statement.
Justification of All Statements:
- (i) is incorrect: By definition, a saturated solution has reached its limit and cannot dissolve any more solute at that specific temperature.
- (ii) is incorrect: An unsaturated solution has not reached its limit and can dissolve additional solute at that temperature.
- (iii) is correct: A saturated solution is defined as one in which no more solute can dissolve at that given temperature.
- (iv) is incorrect: A saturated solution can be prepared at any temperature (low, room, or high); the quantity of solute required to reach saturation simply changes with temperature.
Calculation:
• Total bottle capacity = $2\text{ litres} = 2000\text{ mL}$
• Volume of water already present = $500\text{ mL}$
• Remaining capacity = $2000\text{ mL} - 500\text{ mL} = 1500\text{ mL}$ (or $1.5\text{ litres}$)
Answer: The bottle can hold $1500\text{ mL}$ (1.5 litres) more water.
Formula:
$$\text{Density } (\rho) = \frac{\text{Mass}}{\text{Volume}} = \frac{400\text{ g}}{40\text{ cm}^3} = 10\text{ g/cm}^3$$
Answer: The density of the object is $10\text{ g/cm}^3$.
Why the unpeeled orange floats: The rind (peel) of an orange is highly porous and contains thousands of microscopic air pockets. This significantly increases the total volume of the fruit without adding much mass, lowering its average overall density below that of water ($1\text{ g/cm}^3$). As a result, the upward buoyant force exceeds its weight, allowing it to float.
Why the peeled orange sinks: When the peel is removed, the buoyant air pockets are lost. The volume decreases substantially while the fruit's dense, water-saturated pulp remains. This increases its average density above $1\text{ g/cm}^3$, causing the peeled orange to sink.
Density of Object A:
$$\rho_A = \frac{\text{Mass}}{\text{Volume}} = \frac{200\text{ g}}{40\text{ cm}^3} = 5\text{ g/cm}^3$$
Density of Object B:
$$\rho_B = \frac{\text{Mass}}{\text{Volume}} = \frac{240\text{ g}}{60\text{ cm}^3} = 4\text{ g/cm}^3$$
Conclusion: Object A is denser than Object B because its density ($5\text{ g/cm}^3$) is greater than that of Object B ($4\text{ g/cm}^3$).
The density of the modelling clay will not change.
Scientific Reason: Density is an intensive property defined by the ratio of mass to volume ($\rho = \frac{m}{V}$). Reshaping the clay changes its geometric appearance, but does not alter its total mass ($120\text{ g}$) or its total physical volume ($60\text{ cm}^3$). Therefore, its density remains constant at:
$$\rho = \frac{120\text{ g}}{60\text{ cm}^3} = 2\text{ g/cm}^3$$Formula:
$$\text{Volume } (V) = \frac{\text{Mass}}{\text{Density}} = \frac{600\text{ g}}{7.9\text{ g/cm}^3} \approx 75.95\text{ cm}^3$$
Answer: The volume of the iron block is approximately $75.95\text{ cm}^3$.
The density of the water in setup (b) will decrease.
Scientific Explanation: When the test tube is heated by the hot water bath, thermal energy causes the water molecules to move more vigorously, increasing the average distance between them. This causes thermal expansion, increasing the liquid's volume (reflected by the rise in water level in the narrow tube). Because the mass of the water remains constant while its volume expands, the density ($\rho = \frac{m}{V}$) decreases.
Why No Aquatic Life Exists: The Dead Sea has an extreme mineral salinity of approximately 34% (nearly 10 times saltier than typical ocean water). This hyper-saline environment exerts massive osmotic pressure on living cells, rapidly drawing water out of organisms through exosmosis and causing severe dehydration. Consequently, fish, aquatic plants, and amphibians cannot survive; only specialized halophilic bacteria and microscopic algae can tolerate these conditions.
Similar Hyper-Saline Water Bodies:
- Great Salt Lake (Utah, USA): Salinity ranges between 5% and 27%, supporting brine shrimp and halophilic microbes.
- Lake Assal (Djibouti, Africa): One of the saltiest lakes in the world, with salinity exceeding 35%.
- Don Juan Pond (Antarctica): A hyper-saline lake with over 40% salinity that does not freeze even at sub-zero temperatures.
| Solvent | Observed Solubility of Salt | Scientific Reason |
|---|---|---|
| Water | Completely soluble; forms a clear, uniform solution. | Water is a strong polar solvent whose dipole molecules separate and hydrate sodium ($Na^+$) and chloride ($Cl^-$) ions. |
| Vinegar | Moderately soluble; dissolves slightly less salt than pure water. | Vinegar consists primarily of water (~95%) with acetic acid (~5%), retaining good polar properties for dissolving salt. |
| Cooking Oil | Insoluble; salt crystals remain undissolved at the bottom. | Oil is non-polar and hydrophobic; its molecules cannot break ionic electrostatic bonds. |
| Affirmative: Water is the Most Versatile Solvent | Negative: Water Has Significant Limitations |
|---|---|
| Dissolves an extraordinary range of ionic salts, sugars, acids, bases, and atmospheric gases. | Completely fails to dissolve non-polar organic substances like oils, greases, fats, wax, and polymers. |
| Forms the essential liquid basis of blood plasma, plant sap, and cellular cytoplasm in living organisms. | Industrial chemical synthesis frequently requires organic solvents like ethanol, acetone, ether, or benzene. |
| High dielectric constant enables it to weaken electrostatic attraction between ions easily. | Dry cleaning and paint thinning cannot use water because oil-based stains do not dissolve in water. |