Case Based Questions: The Amazing World of Solutes, Solvents, and Solutions | Science Curiosity Class 8 - New NCERT PDF Download

Q1: Read the source below and answer the questions that follow:

At a picnic, friends make lemonade by stirring sugar into water. After adding too much sugar, they notice some grains settle at the bottom of the glass. They realise no more sugar can dissolve at that temperature, no matter how much they stir.

(a) What does it mean when sugar settles at the bottom and does not dissolve further? (1 mark)
(b) What type of solution is formed before and after the sugar settles? (1 mark)
(c) How can temperature changes affect how much sugar dissolves in water in this scenario? (2 marks)

Ans:
(a) It means the solution is saturated; no more sugar can dissolve at that temperature.
(b) Before settling, the solution is unsaturated; after settling, it is saturated.
(c) Increasing temperature usually increases solubility; thus, more sugar can dissolve in warmer water, while decreasing temperature may cause sugar to crystallise out.

Q2: Read the source below and answer the questions that follow:

A farmer uses shallow ponds to dry up seawater under the sun. After days of evaporation, he sees sparkling salt crystals forming at the bottom of the pond. This happens because the water evaporates, leaving the salt behind.

(a) Why do salt crystals appear when seawater evaporates? (1 mark)
(b) What causes the salt to separate from the water? (1 mark)
(c) How does the concept of solubility explain this salt-harvesting process? (2 marks)

Ans:
(a) Because water evaporates, leaving salt behind as crystals.
(b) Water evaporates, reducing solvent quantity, surpassing the salt's solubility, causing it to crystallise.
(c) Solubility limits the maximum amount of salt water that can be held; evaporation decreases solvent volume, making the solution saturated and salt precipitates.

Q3: Read the source below and answer the questions that follow:

Two soda bottles are opened—one at room temperature and one from the refrigerator. The room-temperature bottle quickly loses its fizz, while the chilled soda stays bubbly for longer. This shows how gas escapes differently at different temperatures.

(a) Why does soda lose fizz faster at room temperature than in the fridge? (1 mark)
(b) What property of gas solubility does this demonstrate? (1 mark)
(c) Explain how temperature affects the solubility of gases in liquids as seen in this case. (2 marks)

Ans:
(a) Gas escapes faster from soda at higher temperatures, reducing fizz.
(b) Gas solubility decreases as temperature increases.
(c) Higher temperatures give gas particles more energy to escape from the liquid; lower temperatures keep gases dissolved longer.

Q4: Read the source below and answer the questions that follow:

Riya adds sugar to two cups, one with hot tea and one with cold tea. The sugar in hot tea disappears almost instantly, but in cold tea, it takes a long time to dissolve. She realises temperature affects how fast solids dissolve in liquids.

(a) Why does sugar dissolve faster in hot tea than in cold tea? (1 mark)
(b) What does this illustrate about the solubility of solids in liquids with respect to temperature? (1 mark)
(c) How can this knowledge help in preparing beverages or medicines? (2 marks)

Ans:
(a) Hot tea’s solvent particles move faster, allowing quicker dissolution of sugar.
(b) Solubility of solids in liquids generally increases with temperature.
(c) Heating beverages or medicines helps dissolve solids faster, making preparation efficient and ensuring proper dosage.

Q5: Read the source below and answer the questions that follow:

During a picnic, some oil accidentally spills into a bowl of water. The oil floats on top, forming separate droplets, and does not mix with the water. This shows oil and water do not form a solution but stay as a mixture.

(a) Why does oil not dissolve in water and float instead? (1 mark)
(b) How does this relate to the idea of solutes and solvents? (1 mark)
(c) What does this tell us about the nature of solutions and mixtures? (2 marks)

Ans:
(a) Oil molecules do not mix with water molecules and are less dense, so they float.
(b) Water is a solvent that dissolves polar solutes; oil is non-polar and does not dissolve in water.
(c) This shows that not all mixtures are solutions; solute and solvent must be chemically compatible for a solution to form.

Q6: Read the source below and answer the questions that follow:

After a heavy rainfall, Anil notices white powdery patches on the leaves in his garden. On closer look, he realises the salt from the soil had dissolved in rainwater and then crystallised back when the water evaporated.

(a) How does salt dissolve in rainwater to form a solution? (1 mark)
(b) Why does salt crystallise again on the leaves after evaporation? (1 mark)
(c) What does this scenario illustrate about solubility and evaporation? (2 marks)

Ans:
(a) Salt dissolves in water by its particles dispersing uniformly in the solvent.
(b) When rainwater evaporates, salt concentration exceeds solubility, leading to crystallisation.
(c) Solubility limits how much solute stays dissolved; evaporation reduces the solvent, causing the solute to settle.

Q7: Read the source below and answer the questions that follow:

Sara prepares jelly by dissolving gelatin powder in hot water. When cooled, the jelly becomes semi-solid, but on reheating, it dissolves again. She understands that some changes in solutions are reversible depending on temperature.

(a) What role does temperature play in dissolving gelatin in water? (1 mark)
(b) Why does jelly solidify upon cooling? (1 mark)
(c) How does this reversible change demonstrate solution properties? (2 marks)

Ans:
(a) Hot water dissolves gelatin powder faster due to increased particle movement.
(b) On cooling, gelatin particles link, forming a semi-solid structure trapping water.
(c) It shows that solubility and physical state depend on temperature and can be reversible.

Q8: Read the source below and answer the questions that follow:

Rohit drops a fizzy tablet into water, and it immediately starts bubbling while dissolving. The bubbles are carbon dioxide gas escaping as the tablet reacts with water. In warm water, the tablet dissolves much faster than in cold water.

(a) Why do bubbles form when the tablet dissolves? (1 mark)
(b) What causes the tablet to dissolve faster in warm water compared to cold water? (1 mark)
(c) Explain how solubility and temperature together affect the rate of dissolution. (2 marks)

Ans:
(a) Bubbles are gas (carbon dioxide) released during the chemical reaction.
(b) Warm water increases particle energy, causing faster dissolution.
(c) Higher temperature increases solubility and particle movement, speeding up the dissolving process.

Q9: Read the source below and answer the questions that follow:

A mother tries to wash oil stains from clothes using plain water, but it doesn’t work. When she adds soap, the oil breaks into tiny droplets and mixes with water. Soap helps join oil and water, so the stain can be removed.

(a) Why is soap necessary to remove oil stains with water? (1 mark)
(b) What happens to oil particles with soap in water? (1 mark)
(c) How does soap act to make oil soluble in water? (2 marks)

Ans:
(a) Oil does not dissolve in water alone because it is nonpolar and water is polar.
(b) Soap molecules surround oil particles, breaking them into smaller particles that mix with water.
(c) Soap acts as an emulsifier with one end binding oil and the other water, allowing oil to dissolve in water.

Q10: Read the source below and answer the questions that follow:

On a snowy day, workers scatter salt over icy roads. The salt mixes with the thin water layer on the ice and lowers its freezing point, causing the ice to melt. This makes the roads safer for vehicles to travel on.

(a) How does salt help in melting the ice? (1 mark)
(b) What effect does salt have on the freezing point of water? (1 mark)
(c) Why is salt effective even though ice’s melting point is 0°C? (2 marks)

Ans:
(a) Salt dissolves in the thin layer of water on ice, lowering the freezing point.
(b) Salt lowers the freezing point of water, causing ice to melt below 0°C.
(c) Salt disrupts water’s freezing process, meaning ice melts even when the temperature is below the normal freezing point.