1
A group of students set up an experiment to separate a mixture of sand and camphor using sublimation. They heated the mixture in a china dish covered with an inverted funnel, with cotton plugged at the nozzle.(a) What is the purpose of plugging the funnel nozzle with cotton?(b) Explain why camphor separates from sand in this process.(c) If the mixture contained sand, salt, and naphthalene, how would the separation process change? 5marks
2
Assertion (A): Milk is considered a colloid, not a true suspension.Reason (R): The particles in milk are large enough to be seen with the naked eye and settle down when left undisturbed.Choose the correct option:(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
3
A student prepared a solution by dissolving 15 g of salt in 85 g of water.(a) Calculate the mass by mass percentage of the solution.(b) If the student had dissolved 15 g of salt in water to make 100 mL of solution, what would be the mass by volume percentage? 3marks
4
Sunlight entering a dark room through a small window appears as a beam of light. This phenomenon is observed because the light is scattered by the particles present in the air.(a) Name the scientific phenomenon responsible for this observation.(b) Explain why this phenomenon is observed in a dark room but not in a well-lit room.(c) Would this phenomenon be observed if the air was completely free of dust and smoke particles? Justify your answer. 5marks
5
Assertion (A): Distillation is a suitable method to separate a mixture of two miscible liquids with very close boiling points.Reason (R): In distillation, the separation is based on the difference in boiling points. If the difference is very small, it becomes difficult to collect pure vapours of the lower boiling point liquid before the other liquid starts to vaporise significantly.Choose the correct option:(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
6
In Activity 5.1, three mixtures were prepared: (A) salt and water, (B) chalk powder and water, and (C) milk and water. The laser light was passed through these mixtures.(a) Describe the observation when the laser beam passes through mixture A.(b) Describe the observation when the laser beam passes through mixture B.(c) Which mixture among A, B, and C exhibits the Tyndall effect, and why? 5marks
7
A fisherman wants to dry his nets after use. He hangs them in the sun. Explain, using scientific principles, why the nets dry faster on a sunny day than on a cloudy day. 3marks
8
A laboratory assistant accidentally mixed two miscible liquids, acetone (boiling point 56 °C) and water (boiling point 100 °C). He needs to separate them.(a) Identify the appropriate separation technique to use.(b) Draw a labelled diagram of the apparatus used for this separation.(c) Explain the principle behind this separation technique. 5marks
9
Assertion (A): Crystallization is a purification technique.Reason (R): In crystallization, the desired substance is obtained in the form of crystals, leaving impurities behind in the solution.Choose the correct option:(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
10
Consider the solubility curves of compounds A and B given in Activity 5.2.(a) If 100 g of water is saturated with compound A at 60 °C and then cooled to 40 °C, how much of compound A will separate out?(b) At 20 °C, which compound dissolves more in 100 g of water, A or B?(c) What can be inferred about the effect of temperature on the solubility of compound B? 5marks
11
A farmer is preparing a pesticide spray by mixing a liquid pesticide concentrate with water. He wants to ensure the correct concentration for effective pest control without damaging the crops.(a) If he mixes 5 mL of pesticide concentrate in enough water to make a final volume of 100 mL of spray, what is the volume by volume percentage concentration of the pesticide?(b) Why is it important to maintain the correct concentration of the pesticide? 3marks
12
Assertion (A): Paper chromatography is used to separate pigments in black ink.Reason (R): Different pigments in black ink have different solubilities in the solvent used and different affinities for the paper.Choose the correct option:(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
13
An oil spill occurs in the sea, creating a layer of oil on the surface of the water.(a) Explain why the oil forms a separate layer on top of the water.(b) Name the apparatus that can be used to separate the oil and water.(c) Draw a simple diagram of the apparatus mentioned in part (b) and briefly explain its working. 5marks
14
Why is a saturated solution prepared at a higher temperature and then cooled slowly in the crystallization process? 2marks
15
A student is given a mixture of common salt and chalk powder.(a) Suggest a method to separate common salt from chalk powder.(b) Explain the steps involved in your suggested method. 3marks
16
Assertion (A): Centrifugation is used to separate cream from milk.Reason (R): Cream is less dense than milk and therefore rises to the surface when milk is spun at high speed.Choose the correct option:(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
17
A solution is prepared by dissolving 20 g of sugar in 100 g of water.(a) Calculate the mass by mass percentage of the solution.(b) If 30 g of sugar were dissolved in the same amount of water, would the new solution be more or less concentrated? Justify your answer. 3marks
18
You are given a mixture of iron filings and sulfur powder.(a) How can you separate iron filings from sulfur? State the principle behind the method.(b) What would happen if you tried to separate iron filings from sulfur using sublimation? Justify your answer. 5marks
19
Assertion (A): All mixtures are heterogeneous.Reason (R): In heterogeneous mixtures, the components are not uniformly distributed throughout the substance.Choose the correct option:(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark
20
A laboratory technician is preparing a 5% glucose solution for an experiment.(a) How many grams of glucose would be needed to prepare 200 mL of this solution?(b) What does the "% m/v" in the concentration indicate? 3marks
Answers
1
(a) The cotton plug absorbs the camphor vapour that rises with the funnel, preventing it from escaping into the atmosphere. It allows the vapour to cool down and deposit as solid camphor on the inner walls of the funnel, while larger sand particles remain in the china dish.(b) Camphor undergoes sublimation, meaning it changes directly from a solid to a gaseous state upon heating, without melting into a liquid. Sand, on the other hand, does not sublime. When heated, camphor vaporises, leaving the sand behind. The vapour then cools inside the inverted funnel and deposits back as solid camphor.(c) If the mixture contained sand, salt, and naphthalene, naphthalene would also sublime. So, the process would be similar, with naphthalene forming deposits on the inner wall of the funnel along with camphor. Salt does not sublime. Therefore, a two-step separation might be needed, or a modification to collect different sublimates. Initially, sublimation could be used to remove naphthalene and camphor. Then, the remaining salt and sand could be separated by dissolving the salt in water, filtering out the sand, and then evaporating the water to obtain pure salt.
2
(iii) A is true, but R is false.Explanation: Milk is a colloid where the dispersed particles are small enough not to settle down readily, though they are larger than in a true solution. The particles in milk are not visible to the naked eye, and they do not settle down when left undisturbed.
3
(a) Mass of salt (solute) = 15 gMass of water (solvent) = 85 gTotal mass of solution = Mass of solute + Mass of solvent = 15 g + 85 g = 100 gMass by mass percentage = (Mass of solute / Mass of solution) × 100= (15 g / 100 g) × 100 = 15% m/m
(b) Mass of salt (solute) = 15 g
Volume of solution = 100 mL
Mass by volume percentage = (Mass of solute / Volume of solution) × 100
= (15 g / 100 mL) × 100 = 15% m/v
4
(a) The scientific phenomenon responsible for this observation is the Tyndall effect.(b) In a well-lit room, the ambient light is so intense that the scattering of light by dust and smoke particles is not noticeable. However, in a dark room, the only light present is the beam from the window. The dust and smoke particles in the air scatter this light in all directions, making the path of the light beam visible.(c) No, this phenomenon would not be observed if the air was completely free of dust and smoke particles. The Tyndall effect occurs because the dispersed particles (dust and smoke) scatter the light. If there are no such particles, the light beam would pass through without being scattered, and its path would not be visible.
5
(iv) A is false, but R is true.Explanation: Distillation is effective for separating miscible liquids with a significant difference (at least 25 °C) in their boiling points. If the boiling points are very close, fractional distillation is required for effective separation. The reason (R) correctly explains why separating liquids with close boiling points is difficult using simple distillation.
6
(a) When the laser beam passes through mixture A (salt and water), which is a true solution, the particles are too small to scatter the light. Therefore, the path of the laser beam will not be visible.(b) When the laser beam passes through mixture B (chalk powder and water), which is a suspension, the relatively large, undissolved chalk particles will scatter the light. The path of the laser beam will be visible as it passes through the mixture.(c) Mixture B (chalk powder and water) and mixture C (milk and water) would exhibit the Tyndall effect. Although mixture C appears homogeneous, the dispersed particles of milk are large enough to scatter light, making the beam visible. Mixture A (salt and water) is a true solution, and its particles are too small to scatter light.
7
The nets dry faster on a sunny day due to increased evaporation. Sunshine provides heat energy, which increases the kinetic energy of water molecules in the wet nets. This increased energy allows more molecules to overcome the intermolecular forces and escape into the atmosphere as water vapour. Additionally, sunny days often have lower humidity and stronger winds, both of which enhance the rate of evaporation.
8
(a) The appropriate separation technique is distillation.(b) [Diagram of a distillation setup: distillation flask containing the mixture, thermometer, condenser with water inlet and outlet, and a receiving flask to collect the distillate.](c) The principle behind distillation is the difference in the boiling points of the miscible liquids. The mixture is heated, and the liquid with the lower boiling point (acetone, 56 °C) vaporises first. The vapour rises, passes through the condenser where it is cooled by circulating water, and condenses back into liquid form. This pure liquid (distillate) is collected in the receiving flask. The liquid with the higher boiling point (water, 100 °C) remains in the distillation flask.
9
(i) Both A and R are true, and R is the correct explanation of A.Explanation: Crystallization is a process used to obtain pure solid crystals from a solution. The process relies on the difference in solubility of the pure substance and the impurities at different temperatures. As the solution cools, the solubility of the pure substance decreases, causing it to crystallize out, leaving the impurities dissolved in the remaining solution.
10
(a) From the graph in Activity 5.2, the solubility of compound A at 60 °C is approximately 287 g per 100 g of water, and at 40 °C it is approximately 241 g per 100 g of water.Mass of compound A that will separate out = Solubility at 60 °C - Solubility at 40 °C= 287 g - 241 g = 46 g.
(b) At 20 °C, the solubility of compound B is approximately 60 g per 100 g of water, while the solubility of compound A is approximately 10 g per 100 g of water. Therefore, compound B dissolves more in 100 g of water at 20 °C.
(c) The solubility of compound B increases significantly with an increase in temperature, from approximately 60 g at 20 °C to 287 g at 60 °C. This indicates a strong positive correlation between temperature and the solubility of compound B.
11
(a) Volume of pesticide (solute) = 5 mLTotal volume of solution = 100 mLVolume by volume percentage = (Volume of solute / Volume of solution) × 100= (5 mL / 100 mL) × 100 = 5% v/v
(b) Maintaining the correct concentration of the pesticide is crucial for several reasons:
- Effectiveness: Too little pesticide may not be effective in killing the pests, leading to crop damage.
- Crop Safety: Too much pesticide can be toxic to the crops, causing wilting, burning, or stunted growth.
- Environmental Impact: Excessive pesticide can harm beneficial insects, soil microorganisms, and contaminate water sources, leading to environmental pollution.
12
(i) Both A and R are true, and R is the correct explanation of A.Explanation: Paper chromatography separates components of a mixture based on their differential partitioning between a stationary phase (the paper) and a mobile phase (the solvent). Pigments have different solubilities in the solvent and different affinities for the paper, causing them to travel at different speeds and separate.
13
(a) Oil forms a separate layer on top of the water because oil is immiscible with water, meaning they do not mix. Furthermore, oil is less dense than water, causing it to float on the water's surface.(b) A separating funnel can be used to separate the oil and water.(c) [Diagram of a separating funnel: A pear-shaped glass vessel with a stopper at the top and a tap (stopcock) at the bottom. The mixture of oil and water is poured into the funnel. The less dense oil forms the upper layer, and the denser water forms the lower layer. The tap is opened to drain the lower layer (water) into a container. When the interface between oil and water reaches the tap, it is closed, leaving the oil in the funnel.]Working: The separating funnel is used to separate immiscible liquids based on their density. The mixture is allowed to stand until distinct layers form. The stopcock at the bottom is then opened carefully to drain the lower, denser liquid, and closed just as the interface reaches the tap. The upper, less dense liquid remains in the funnel.
14
In the crystallization process, a hot saturated solution is prepared because the solubility of most solid solutes increases with temperature. Preparing a hot saturated solution ensures that a maximum amount of solute is dissolved. When this hot solution is cooled slowly, the solubility decreases, and the excess solute crystallizes out as pure solid. Slow cooling allows for the formation of larger, well-formed crystals, as the particles have enough time to arrange themselves in a regular lattice structure. Rapid cooling can lead to the formation of smaller, less pure crystals or amorphous solids.
15
(a) The mixture can be separated by dissolving the common salt in water and then filtering out the insoluble chalk powder.(b) Steps:
1
Dissolution: Add water to the mixture of common salt and chalk powder and stir well. Common salt will dissolve in water, forming a salt solution, while chalk powder, being insoluble, will remain as a suspension.
2
Filtration: Filter the mixture using filter paper in a funnel. The insoluble chalk powder will be retained on the filter paper as residue. The filtrate will be the salt solution.
3
Evaporation: Heat the filtrate (salt solution) in a beaker or china dish. The water will evaporate, leaving behind the solid common salt crystals.
16
(i) Both A and R are true, and R is the correct explanation of A.Explanation: Centrifugation works by spinning the mixture at high speed. This generates a centrifugal force that pushes the denser components outwards more strongly. Cream, being less dense than milk, is pushed inwards towards the centre of rotation, while the denser milk is pushed outwards, effectively separating the cream. Thus, the statement in R is slightly inaccurate; it's the denser component being pushed outwards. However, the principle of separation based on density difference is correctly applied.
17
(a) Mass of sugar (solute) = 20 gMass of water (solvent) = 100 gTotal mass of solution = Mass of solute + Mass of solvent = 20 g + 100 g = 120 gMass by mass percentage = (Mass of solute / Mass of solution) × 100= (20 g / 120 g) × 100 = (1/6) × 100 ≈ 16.67% m/m
(b) If 30 g of sugar were dissolved in the same amount of water (100 g), the new solution would be more concentrated. This is because a larger amount of solute (sugar) is dissolved in the same amount of solvent (water), leading to a higher percentage of sugar in the solution.
18
(a) Method: Use a magnet to separate the iron filings from sulfur.Principle: Iron is a magnetic substance, while sulfur is not. When a magnet is brought near the mixture, the iron filings will be attracted to the magnet and can be separated, leaving the sulfur behind.
(b) If you tried to separate iron filings from sulfur using sublimation, it would not work. Sublimation is the process of a solid directly changing into a gas and back into a solid. Neither iron filings nor sulfur undergoes sublimation. Therefore, heating the mixture would not result in the separation of these components.
19
(iii) A is true, but R is false.Explanation: While many mixtures are heterogeneous, not all are. Homogeneous mixtures, also known as solutions, have a uniform composition. The reason (R) correctly defines heterogeneous mixtures, but the assertion (A) is a false statement as it claims all mixtures are heterogeneous.
20
(a) The concentration is given as 5% m/v, which means 5 grams of solute in 100 mL of solution.To prepare 200 mL of solution:Amount of glucose needed = (5 g glucose / 100 mL solution) × 200 mL solution= 10 g glucose
(b) The "% m/v" in the concentration indicates mass by volume percentage. It tells us the mass of the solute (in grams) present in 100 millilitres of the solution.
