Case Based Questions: The Amazing World of Solutes, Solvents, and Solutions

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 has reached saturation: the water cannot dissolve any more sugar at that temperature, so the extra sugar remains undissolved and settles as solid.
(b) Before the sugar settles, the lemon water is an unsaturated solution because more sugar could dissolve. After settling, it is a saturated solution because it contains the maximum amount of dissolved sugar at that temperature.
(c) Temperature affects how much sugar can dissolve. Increasing the temperature usually increases the solubility of sugar, so warming the drink can dissolve the settled sugar again. Decreasing the temperature reduces solubility and may cause dissolved sugar to crystallise out. Heating therefore allows more sugar to dissolve; cooling can make sugar come out of solution.

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) As the sun evaporates water from the pond, the dissolved salt becomes more concentrated and eventually forms solid crystals that appear at the bottom.
(b) Evaporation reduces the amount of solvent (water). When the remaining water can no longer hold all the dissolved salt (the concentration exceeds solubility), the excess salt separates and crystallises.
(c) Solubility tells us the maximum amount of salt that can be dissolved in a given amount of water at a particular temperature. Evaporation lowers the volume of water so the solution becomes saturated and then supersaturated; the excess salt precipitates as crystals. In practice, slowing evaporation or changing temperature can change how much salt stays dissolved.

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) At higher temperatures the dissolved carbon dioxide has more energy and escapes from the liquid more quickly, so the room-temperature bottle loses its fizz faster than the chilled one.
(b) This demonstrates that the solubility of gases in liquids decreases as temperature increases.
(c) When temperature rises, gas particles gain energy and are more likely to leave the liquid as bubbles, so less gas stays dissolved. At lower temperatures gas molecules have less energy and remain dissolved for longer, which is why chilled soda stays bubbly longer than warm soda.

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) In hot tea the water molecules move faster, causing more frequent and energetic collisions with sugar crystals; this makes the sugar dissolve more quickly.
(b) It illustrates that, for many solids in liquids, solubility and the rate of dissolution increase with temperature - warmer liquids usually dissolve more solid and do so faster.
(c) This knowledge helps in practice: heating a drink or medicine helps solids dissolve faster and more completely, making preparation quicker and ensuring the correct dose is properly mixed. Stirring also speeds up the process.

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 does not dissolve in water because oil molecules are non-polar, while water molecules are polar, so they do not mix; oil is also less dense than water and therefore floats on top.
(b) This shows that for a substance to act as a solut,e it must be chemically compatible with the solvent. Water dissolves polar solutes easily, but non-polar substances like oil are not dissolved by water.
(c) It tells us that not all mixtures are solutions. A solution forms only when the solute dissolves uniformly in the solvent; when substances do not dissolve but remain as separate parts, the result is a heterogeneous mixture, as seen with oil and water.

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 when its particles separate and disperse evenly through the rainwater, forming a salt solution.
(b) As the water on the leaves evaporates, the remaining solution becomes more concentrated; when the concentration exceeds the salt's solubility at that temperature, salt comes out of solution and forms white crystals on the leaves.
(c) This shows how evaporation reduces the amount of solvent and can force a dissolved substance to crystallise once solubility is exceeded. It also demonstrates the reversible nature of dissolving: adding water again would cause the crystals to dissolve.

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 increases the movement of particles and helps gelatin powder dissolve quickly and thoroughly.
(b) On cooling, dissolved gelatin molecules link together to form a network that traps water, so the mixture becomes semi-solid or gelled.
(c) The change is reversible with temperature: heating breaks the network and the gel dissolves again, showing that solubility and the physical state of a solution can depend on temperature and can be reversed by heating or cooling.

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 form because the tablet produces carbon dioxide gas as it reacts with water; the gas escapes as visible bubbles.
(b) Warm water increases the energy and movement of particles, so the tablet's substances dissolve and react faster in warm water than in cold water.
(c) Higher temperature usually increases solubility and speeds up particle motion, so solids dissolve more quickly and reactions that release gas happen faster. At lower temperatures, the process is slower and gases are less likely to escape as quickly.

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) Soap is necessary because oil is non-polar and does not dissolve in polar water; soap helps bridge the difference so oil can be removed.
(b) Soap molecules surround the oil and break it into many tiny droplets that disperse through the water, forming an emulsion.
(c) Soap molecules have a part that binds to oil and a part that attracts water; this allows oil droplets to be held in suspension in water so they can be rinsed away, effectively making the oil behave as if it were soluble 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 liquid water on the ice and lowers the temperature at which water freezes, so the ice begins to melt.
(b) Salt lowers the freezing point of water, a phenomenon that means water must be colder than 0°C to freeze when salt is present.
(c) Salt is effective because even a thin film of liquid on ice will dissolve some salt and the resulting solution freezes at a lower temperature than pure water. This prevents the thin layer from refreezing at 0°C, so ice melts or stays melted at temperatures below the normal freezing point.