Probe and Ponder (Page 1)
Q1. What happens when you add too much sugar to your tea and it stops dissolving? How can you solve this problem?
Answer: When you add too much sugar, the tea becomes saturated, and the extra sugar settles at the bottom. To solve this problem, you can heat the tea. Heating increases the solubility, allowing the extra sugar to dissolve.
Q2. Why do sugar and salt dissolve in water but not in oil? Why is water considered a good solvent?
Answer: Sugar and salt dissolve in water because water molecules can surround and pull apart the particles of sugar and salt. They do not dissolve in oil because oil is different chemically and cannot interact with them in the same way. Water is considered a good solvent because it can dissolve many different kinds of substances.
Q3. Why are water bottles usually tall and cylindrical in shape instead of spherical?
Answer: Water bottles are tall and cylindrical because this shape is easy to hold, pack, and store. A spherical (round) bottle would roll away and be hard to drink from. Also, cylinders are strong and can hold a good amount of water without using too much plastic.
In-text Questions (Page 2)
Q1. Can you predict whether this mixture is uniform or not (Fig. 9.1)? What happens when chalk powder is mixed with water—does it form a uniform mixture?
Answer: The mixture of sugar, salt, and water (Fig. 9.1) is a uniform mixture because the particles are evenly distributed. When chalk powder is mixed with water, it does not form a uniform mixture. The chalk powder does not dissolve and will settle down or float, making it a non-uniform mixture.
Q2. We know air is a mixture. Would a mixture of gases also be considered a solution?
Answer: Yes, a mixture of gases like air is considered a solution. It is a gaseous solution where nitrogen acts as the solvent (because there is more of it) and other gases like oxygen and argon act as solutes.
Activity 9.1: Let us investigate (Page 3)
Activity Explanation: Take a glass half-filled with water. Add one spoon of salt and stir until it dissolves. Keep adding salt one spoon at a time and stirring until the salt stops dissolving and settles at the bottom.
Q1. How many spoons of salt were you able to dissolve before some of it remained undissolved?
Answer: (This depends on your specific glass size, but typically) You might be able to dissolve about 4 to 6 spoons of salt before it stops dissolving.
Q2. What does this indicate about the capacity of water to dissolve salt?
Answer: This indicates that water has a fixed capacity to dissolve salt at a specific temperature. Once that limit is reached, it cannot dissolve any more, and the solution becomes saturated.
In-text Question (Page 4)
Q1. Can you now reflect – which solution is more concentrated; 2 spoons of salt in 100 mL of water or 4 spoons of salt in 50 mL of water?
Answer: The solution with 4 spoons of salt in 50 mL of water is more concentrated.
Solution A: 2 spoons in 100 mL.
Solution B: 4 spoons in 50 mL (which is equal to 8 spoons in 100 mL).
Since Solution B has more salt for the same amount of water, it is more concentrated.
Activity 9.2: Let us experiment (Page 4-5)
Activity Explanation: Take 50 mL of water at room temperature (20°C). Add baking soda until it stops dissolving (saturated solution). Then, heat the water to 50°C and stir. Add more baking soda until it stops dissolving again. Finally, heat it to 70°C and observe.
Q1. What happens to the undissolved baking soda? (After heating to 50°C)
Answer: The undissolved baking soda dissolves when the water is heated.
Q2. What do you infer from this experiment?
Answer: This experiment shows that the solubility of a solid (like baking soda) increases as the temperature increases. Hot water can dissolve more solute than cold water.
In-text Question (Page 6)
Q1. Does temperature affect the solubility of gases in liquids also? If so, how?
Answer: Yes, temperature affects the solubility of gases. However, unlike solids, the solubility of gases decreases as the temperature increases. This means warm water holds less gas (like oxygen) than cold water.
Think Like a Scientist (Page 8)
Q1. Have you noticed that some packets of ghee or oil are labelled with a volume of 1 litre but a weight of only say 910 grams (Fig. 9.11)? What does this tell us about the density of the oil, and is it less or more than that of water?
Answer: This tells us that the density of oil is less than the density of water.
1 litre (1000 mL) of water weighs 1000 grams.
1 litre of oil weighs only 910 grams.
Since the same volume of oil weighs less than water, oil is lighter (less dense) than water, which is why it floats.
Activity 9.3: Let us measure (Page 9)
Activity Explanation: Use a digital weighing balance. Place a watch glass on it and press the “tare” button to set the reading to zero. Place a stone on the watch glass and note down the reading to find the mass of the stone.
(No direct questions to answer in this activity section, it is a procedure).
Activity 9.4: Let us observe and calculate (Page 10-11)
Activity Explanation: Take a 100 mL measuring cylinder and look at the markings to understand how to read it.
Q1. What is the maximum volume it can measure?
Answer: The cylinder shown in Fig. 9.16 can measure a maximum volume of 100 mL.
Q2. How much is the volume difference indicated between the two bigger marks (for example, between 10 mL and 20 mL)?
Answer: The difference between two bigger marks (like 10 and 20) is 10 mL.
Q3. How many smaller divisions are there between the two bigger marks?
Answer: There are 10 smaller divisions between the bigger marks.
Q4. How much volume does one small division indicate?
Answer: One small division indicates 1 mL (10 mL divided by 10 divisions = 1 mL).
Q5. Why are measuring cylinders always designed narrow and tall instead of wider and short like a beaker?
Answer: They are designed to be narrow and tall to measure volume more accurately. In a narrow cylinder, even a small amount of liquid changes the height significantly, making it easier to read small differences.
Activity 9.5: Let us measure 50 mL of water (Page 11-12)
Activity Explanation: Place a dry measuring cylinder on a flat table. Pour water up to the 50 mL mark. Use a dropper to adjust the level perfectly.
Q1. How do you read the water level accurately?
Answer: You must look at the meniscus (the curved surface of the water). For water, read the mark that lines up with the bottom of the curve (meniscus). Keep your eyes at the same level as the water.
Activity 9.6: Let us calculate (Page 12)
Activity Explanation: Take a regular-shaped object like a notebook or shoe box. Measure its length (l), width (w), and height (h) with a ruler. Calculate the volume by multiplying them.
Formula: Volume = length × width × height (V = l × w × h).
(No direct questions, this is a calculation exercise).
Activity 9.7: Let us measure (Page 13)
Activity Explanation: Fill a measuring cylinder partially with water (e.g., to 50 mL). Tie a stone with a thread and gently lower it into the water until it is fully submerged. Note the new water level.
Q1. What do you notice?
Answer: You will notice that the water level rises when the stone is put inside.
Q2. How do you find the volume of the stone?
Answer: Subtract the initial water volume from the final water volume.
Volume of stone = Final Volume − Initial Volume.
(Example: 55 mL − 50 mL = 5 mL or 5 cm³)
Think Like a Scientist (Page 15)
Q1. Take a glass tumbler and fill it with tap water. Carefully place a raw whole egg into the water and observe what happens. You will notice that the egg sinks to the bottom. What change can you make to this setup to make the egg float in water instead of sinking?
Answer: You can dissolve a lot of salt in the water. Adding salt makes the water denser (heavier) than the egg. Once the salt water becomes denser than the egg, the egg will float.
Keep the curiosity alive (Exercises) (Page 16-18)
Q1. 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.
Answer: False. Oxygen gas is more soluble in cold water than in hot water.
(ii) A mixture of sand and water is a solution.
Answer: False. A mixture of sand and water is a non-uniform mixture, not a solution, because sand does not dissolve.
(iii) The amount of space occupied by any object is called its mass.
Answer: False. The amount of space occupied by an object is called its volume.
(iv) An unsaturated solution has more solute dissolved than a saturated solution.
Answer: False. An unsaturated solution has less solute dissolved than a saturated solution at the same temperature.
(v) The mixture of different gases in the atmosphere is also a solution.
Answer: True. Air is a gaseous solution.
Q2. Fill in the blanks.
(i) The volume of a solid can be measured by the method of displacement, where the solid is submerged/immersed in water and the rise in water level is measured.
(ii) The maximum amount of solute dissolved in 100 mL 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.
Q3. You pour oil into a glass containing some water. The oil floats on top. What does this tell you?
Answer: (ii) Water is denser than oil.
(Note: Option (i) “Oil is denser than water” is incorrect because lighter things float. So if oil floats, it is less dense than water, meaning water is denser).
Q4. 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: Given: Mass = 225 g, Volume = 90 cm³.
Formula: Density = Mass / Volume
Calculation: Density = 225 / 90 = 2.5 g/cm³.
Prediction: The density of water is roughly 1 g/cm³. Since the stone’s density (2.5 g/cm³) is greater than water, it will sink.
Q5. Which one of the following is the most appropriate statement, and why are the other statements not appropriate?
Answer: (iii) No more solute can be dissolved into the saturated solution at that temperature is the most appropriate statement.
(i) is incorrect because a saturated solution cannot dissolve more solute.
(ii) is incorrect because an unsaturated solution has not yet reached the maximum amount.
(iv) is incorrect because saturated solutions can form at any temperature.
Q6. 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: Total volume of bottle = 2 Litres = 2000 mL.
Water poured = 500 mL.
Remaining space = 2000 mL – 500 mL = 1500 mL (or 1.5 Litres).
Q7. An object has a mass of 400 g and a volume of 40 cm³. What is its density?
Answer: Given: Mass = 400 g, Volume = 40 cm³.
Formula: Density = Mass / Volume
Calculation: Density = 400 / 40 = 10 g/cm³.
Q8. 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 its skin (peel) is porous and contains tiny air pockets. These air pockets make the overall density of the unpeeled orange less than water. When you peel it, you remove these air pockets. The fruit inside is denser than water, so the peeled orange sinks.
Q9. 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: Object A Density: 200 / 40 = 5 g/cm³.
Object B Density: 240 / 60 = 4 g/cm³.
Conclusion: Object A is denser because 5 is greater than 4.
Q10. 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 will remain the same.
Density depends on the material (mass and volume of the substance itself), not its shape. Even if she flattens it, the mass (120 g) and the volume of the clay material (60 cm³) stay the same, so the density stays 2 g/cm³.
Q11. A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?
Answer: Formula: Volume = Mass / Density
Calculation: Volume = 600 / 7.9 = 75.95 cm³ (approximately).
Q12. 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: When the test tube is placed in hot water, the air inside the test tube gets heated. Hot air expands (volume increases). Since the mass of the air stays the same but the volume increases, the density of the air decreases. The rising water level indicates the expansion of the air/gas.
Discover, Design, and Debate (Page 18)
Activity Explanation: These are project ideas.
Dead Sea: Research why things float easily there (high salt content increases density).
Solubility: Test dissolving salt in vinegar and oil to see that it dissolves in vinegar (which has water) but not in oil.
Debate: Discuss if water is the best solvent (it dissolves more substances than any other liquid).