1. 1
   A student adds 5 g of sodium chloride, 5 g of glucose and 5 cm³ of olive oil to three separate beakers of water and stirs each one. The sodium chloride and the glucose dissolve completely within a minute. The olive oil does not dissolve and forms a separate layer on the surface.

   (a)

   Which interaction is mainly responsible for keeping sodium ions (Na⁺) in solution?

   [1 mark]

   • AAttraction between Na⁺ and the partially positive hydrogen atoms of water molecules
   • BCovalent bonds formed between Na⁺ and water molecules
   • CHydrophobic interactions that exclude Na⁺ from the water
   • DAttraction between Na⁺ and the partially negative oxygen atoms of water molecules

   (b)

   Why does glucose dissolve in water?

   [1 mark]

   • AThe hydroxyl (–OH) groups of glucose form hydrogen bonds with water molecules
   • BGlucose is an ionic compound that separates into charged ions
   • CGlucose is non-polar, so water molecules surround it
   • DGlucose reacts with water to form smaller molecules

   (c)

   Explain why the olive oil does not dissolve in the water.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Paramecium is a single-celled organism that lives in fresh water. It has no cell wall. Water that enters the cell is collected by a contractile vacuole, which fills and then empties to the outside. A student timed how often the contractile vacuole emptied when Paramecium was placed in different solutions. In pond water it emptied every 6 seconds. In 0.05 mol dm⁻³ sucrose solution it emptied every 15 seconds, and in 0.10 mol dm⁻³ sucrose solution every 40 seconds. In 0.15 mol dm⁻³ sucrose solution the vacuole stopped emptying altogether.

   (a)

   Compared with the cytoplasm of Paramecium, pond water is:

   [1 mark]

   • Ahypertonic, so water leaves the cell
   • Bhypotonic, so there is a net movement of water into the cell
   • Cisotonic, so no water crosses the membrane
   • Dhypotonic, so there is a net movement of water out of the cell

   (b)

   Which statement best describes water movement across the membrane of Paramecium in the 0.15 mol dm⁻³ sucrose solution?

   [1 mark]

   • ANo water molecules cross the membrane
   • BWater moves out of the cell faster than it enters
   • CWater molecules move in and out at equal rates, so there is no net movement
   • DWater only moves into the contractile vacuole

   (c)

   Explain the trend in the rate at which the contractile vacuole empties as the sucrose concentration increases.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   To find the solute concentration of potato tissue, a student cut 25 potato cylinders of similar size, blotted and weighed them, and placed five into each of five sucrose solutions: 0.0, 0.2, 0.4, 0.6 and 0.8 mol dm⁻³. After 24 hours the cylinders were blotted and reweighed. The mean percentage changes in mass were: 0.0 mol dm⁻³, +18.2 %; 0.2 mol dm⁻³, +6.1 %; 0.4 mol dm⁻³, −3.9 %; 0.6 mol dm⁻³, −12.4 %; 0.8 mol dm⁻³, −19.8 %. The standard deviation of the five results at 0.2 mol dm⁻³ was 4.8 %; at every other concentration it was between 1.1 % and 1.6 %.

   (a)

   Determine the sucrose concentration that is isotonic with the potato tissue. Show your working.

   [3 marks]

   (b)

   Standard error can be calculated as SD/√n, where n is the number of repeats. Evaluate how reliable the result for 0.2 mol dm⁻³ is, and why the student used percentage change in mass rather than change in mass.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In a class practical, three types of cell were placed first in distilled water and then, separately, in a concentrated (1.0 mol dm⁻³) sodium chloride solution, and observed after 20 minutes. Human red blood cells swelled and burst in distilled water, and in the salt solution became shrunken with a spiky outline. Onion epidermis cells became firm in distilled water and did not burst; in the salt solution the cytoplasm and membrane pulled away from the cell wall. Amoeba, a freshwater single-celled organism without a cell wall, survived in distilled water with its contractile vacuole emptying rapidly.

   (a)

   Explain the observations for the red blood cells and the onion cells.

   [6 marks]

   (b)

   Discuss how organisms, and doctors treating patients, avoid the harmful effects of water movement on cells that lack a cell wall.

   [6 marks]

   Total for question 4: 12 marks

End of questions