NCERT Solutions Class 8 Science Chapter 09 The Amazing World of Solutes, Solvents and Solutions

Question 1. State whether the statements given below are True [T] or False [F]. Correct the false statement(s).
(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.
Answer:
(i) [F] False. The correct statement is: Oxygen gas is more soluble in cold water rather than in hot water. When temperature goes up, most gases become less soluble in water.
(ii) [F] False. The correct statement is: A mixture of sand and water is a non-uniform mixture (a suspension), not a solution. For a true solution, all parts must spread out evenly.
(iii) [F] False. The correct statement is: The amount of space an object takes up is called its volume. Mass refers to how much matter is in the object.
(iv) [F] False. The correct statement is: A saturated solution holds more (or the same amount of) solute dissolved compared to an unsaturated solution. An unsaturated solution can take in even more solute.
(v) [T] True. Air is a good example of a gaseous solution - nitrogen acts as the solvent and other gases like oxygen work as the solutes.
In simple words: Check each statement carefully: gases dissolve better in cold water, not hot; sand and water don't mix evenly so it's not a solution; space is volume, not mass; saturated means "full," unsaturated means "has room for more"; and air is a solution of gases.

Exam Tip: When correcting false statements, always rewrite the full sentence with the correct concept clearly stated - examiners check that you understand what you got wrong, not just that you marked it wrong.

Question 2. Fill in the blanks.
(i) The volume of a solid can be measured by the method of displacement, where the solid is ______ in water and the ______ in water level is measured.
(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.
Answer:
(i) The volume of a solid can be measured by the method of displacement, where the solid is immersed in water and the rise in water level is measured.
(ii) The maximum amount of solute dissolved in a fixed quantity of a solvent (or 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.
In simple words: To find solid volume, push it into water and measure how much the water goes up. Solubility is the most solute that can dissolve in a certain amount of solvent at one temperature. When things heat up, they spread out more, so density gets smaller. A saturated solution is one that cannot take any more dissolved material.

Exam Tip: For fill-in-the-blank questions, ensure the word you write makes the sentence scientifically correct and grammatically sound - read it aloud after filling to confirm.

Question 3. You pour oil into a glass containing some water. The oil floats on top. What does this tell you?
(a) Oil is denser than water
(b) Water is denser than oil
(c) Oil and water have the same density
(d) Oil dissolves in water
Answer: (b) Water is denser than oil
When two liquids that do not mix come together, the liquid with lower density stays on top and the liquid with higher density sinks. Since oil floats on water, this tells us water has greater density than oil.
In simple words: If something floats on top of water, it must be lighter (less dense). Oil floats, so water is heavier (denser).

Exam Tip: Remember the key rule: less dense objects float on more dense liquids - this is why ice floats on water and oil floats on water.

Question 4. 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.
Answer: We use the formula: Density = Mass ÷ Volume
Density = 225 g ÷ 90 cm³ = 2.5 g/cm³
Water has a density of approximately 1 g/cm³.
Since the sculpture's density (2.5 g/cm³) is greater than water's density (1 g/cm³), the sculpture will sink in water.
In simple words: Calculate density by dividing mass by volume. If your answer is bigger than 1 g/cm³, the object will sink because it is heavier than water.

Exam Tip: Always compare the object's density with water's density (1 g/cm³) to predict whether it floats or sinks - greater than 1 means sink, less than 1 means float.

Question 5. Which one of the following is the most appropriate statement and why are the other statements not appropriate?
(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.
Answer: The most appropriate statement is (iii) No more solute can be dissolved into the saturated solution at that temperature. This statement captures the exact meaning of what "saturated" means.
Why the others are not appropriate:
(i) This describes what happens with an unsaturated solution, not a saturated one - a saturated solution cannot take in any more solute.
(ii) This description actually fits a saturated solution, not an unsaturated one - so the pairing of statement and label is backwards.
(iv) Saturated solutions can form at any temperature - what changes is the actual amount of solute needed to reach saturation, not the temperature requirement itself.
In simple words: "Saturated" means full - no room for more. Only statement (iii) gives this meaning correctly. The other statements either describe the wrong type of solution or have false conditions.

Exam Tip: Learn the definition of saturation clearly - a saturated solution is one where no additional solute can dissolve at a given temperature. This is the key concept examiners test repeatedly.

Question 6. 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?
Answer: First, convert litres to millilitres:
2 litres = 2 × 1000 mL = 2000 mL
The bottle's total capacity = 2000 mL
Water already poured = 500 mL
Remaining space = Total capacity - Water poured
Remaining space = 2000 mL - 500 mL = 1500 mL
The bottle can hold 1500 mL more water, which equals 1.5 litres.
In simple words: Change 2 litres into 2000 millilitres. You put in 500, so 2000 minus 500 leaves 1500 mL of space.

Exam Tip: Always convert units to the same system before doing arithmetic - mixing litres and millilitres leads to calculation errors.

Question 7. An object has a mass of 400 g and a volume of 40 cm³. What is its density?
Answer: Using the density formula:
Density = Mass ÷ Volume
Density = 400 g ÷ 40 cm³ = 10 g/cm³
The object's density is 10 g/cm³.
In simple words: Divide the mass by the volume - 400 divided by 40 gives you 10 g/cm³.

Exam Tip: This is a straightforward calculation - show your formula, substitute the numbers, and state the final answer with units (g/cm³).

Question 8. Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange float, while the peeled one sinks? Explain.
Answer: The unpeeled orange floats because the peel is packed with many tiny air pockets. These air spaces get trapped inside the peel. The trapped air makes the total volume of the unpeeled orange very large without adding much extra weight. This causes the overall density (mass divided by volume) to drop below water's density, so it floats.
When the peel is removed, all those air pockets disappear. The orange flesh that remains is heavier per unit volume than water - meaning it is denser than water - so the peeled orange sinks.
In simple words: The orange peel has lots of air trapped inside it. This air makes the whole orange less dense than water, so it floats. Without the peel and air, the orange fruit alone is heavier than water, so it sinks.

Exam Tip: This question tests understanding that changing shape or volume affects density calculations - focus on explaining how air pockets lower density in the floating case.

Question 9. 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?
Answer: Calculate the density of each object using the formula: Density = Mass ÷ Volume

Density of Object A = 200 g ÷ 40 cm³ = 5 g/cm³

Density of Object B = 240 g ÷ 60 cm³ = 4 g/cm³

Comparing the two results: Object A (5 g/cm³) has greater density than Object B (4 g/cm³). Therefore, Object A is the denser object.
In simple words: Divide mass by volume for each object. Object A gives 5, Object B gives 4. The bigger number means denser, so A is denser.

Exam Tip: When comparing density values, you must calculate for both objects first - do not just compare mass or volume alone, as density depends on both together.

Question 10. Reema has a piece of modeling 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.
Answer: The density of the modeling clay will not change. Density is a fixed property of a substance that does not depend on its shape. When Reema reshapes the clay from a cube into a flat sheet, the shape changes but the mass stays at 120 g and the volume remains at 60 cm³. Since both mass and volume stay the same, the density (120 g ÷ 60 cm³ = 2 g/cm³) also stays exactly the same.
In simple words: Changing the shape does not change the density. The density stays 2 g/cm³ whether the clay is a cube or a sheet, because the total amount of clay and space it takes up do not change.

Exam Tip: Understand that density is an intrinsic property - it does not change when you reshape or reposition an object; only the substance itself and the amount of it matter.

Question 11. A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?
Answer: We need to rearrange the density formula to find volume:
Density = Mass ÷ Volume, so Volume = Mass ÷ Density

Volume = 600 g ÷ 7.9 g/cm³ ≈ 75.95 cm³

The volume of the iron block is approximately 75.95 cm³.
In simple words: When you know density and mass but need volume, divide mass by density. Here, 600 divided by 7.9 gives roughly 76 cubic centimetres.

Exam Tip: Remember that the density formula can be rearranged three ways depending on which value you are solving for - practice all three forms so you can spot which rearrangement fits each question type.

Question 12. You are provided with an experimental setup as shown in Fig. 9.26a and 9.26b. On keeping the test tube (Fig 9.26b) in a beaker containing hot water (~70°C), the water level in the glass tube rises. How does it affect the density?
Answer: This experiment shows how temperature changes the density of water. When the water inside the test tube is heated, its particles gain energy and move around faster and more spread out. This causes the water to expand - its total volume increases, which is why you see the level rise in the narrow glass tube.
Looking at the density formula: Density = Mass ÷ Volume, the mass of water does not change during heating, but the volume increases. Therefore, the density of the water decreases when it is heated.
In simple words: Heat makes water spread out and take up more space. Since the density formula divides mass by volume, and volume goes up while mass stays the same, density must go down.

Exam Tip: This is a key concept - heating liquids and solids usually causes expansion and lower density; cooling causes contraction and higher density. Water is a notable exception near freezing, but for this chapter, assume heat lowers density.