1. 1
   Pond skaters (Gerris lacustris) are insects that stand and run on the surface of still ponds without breaking through it. Their legs are covered in fine, water-repellent hairs. A student adds a few drops of washing-up liquid (a detergent) to a tray of pond water holding several pond skaters. Within seconds the insects sink through the surface.

   (a)

   Which property of water allows the pond skaters to stand on the pond surface?

   [1 mark]

   • AHigh specific heat capacity of water
   • BBuoyancy of water acting on the submerged body
   • CAdhesion of water to the hairs on the insects' legs
   • DCohesion between water molecules at the surface, producing surface tension

   (b)

   Which statement correctly describes the attraction between neighbouring water molecules?

   [1 mark]

   • AA covalent bond forms between the oxygen atoms of two water molecules
   • BAn ionic bond forms between H⁺ and OH⁻ ions
   • CA hydrogen bond forms between the partially positive hydrogen of one molecule and the partially negative oxygen of another
   • DA hydrogen bond forms between the two hydrogen atoms within one water molecule

   (c)

   Explain why adding the detergent caused the pond skaters to sink.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A coast redwood (Sequoia sempervirens) can be more than 100 m tall. Water evaporates from its leaves and is replaced by water drawn up through narrow xylem vessels from the roots, with the water in the xylem under tension (pulled rather than pushed). In the leaves, water also spreads through the narrow spaces between cellulose fibres in the cell walls.

   (a)

   Which property of water is mainly responsible for the column of water in the xylem not breaking while it is under tension?

   [1 mark]

   • ACohesion
   • BBuoyancy
   • CHigh specific heat capacity
   • DLow viscosity

   (b)

   Which pair of properties best explains how water moves through the narrow spaces in the cellulose cell walls of leaf cells?

   [1 mark]

   • AWater is hydrophobic and cellulose is non-polar
   • BAdhesion of water to hydrophilic cellulose, together with cohesion between water molecules
   • CThe high thermal conductivity of water and of cellulose
   • DActive transport of water by pumps in the cell wall

   (c)

   Explain how evaporation of water from the leaves causes water to rise up the xylem.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The black-throated loon (Gavia arctica) flies between lakes and dives underwater to catch fish, sometimes to depths of more than 10 m. A student looked up physical properties of water and air, both at 20 °C. Density: water 998 kg m⁻³, air 1.2 kg m⁻³. Viscosity: water 1.0 mPa s, air 0.018 mPa s. Thermal conductivity: water 0.60 W m⁻¹ K⁻¹, air 0.025 W m⁻¹ K⁻¹. Specific heat capacity: water 4.18 kJ kg⁻¹ K⁻¹, air 1.01 kJ kg⁻¹ K⁻¹.

   (a)

   Calculate how many times greater the thermal conductivity of water is than that of air, and use your answer to explain why the loon's plumage is important when it dives into cold lake water.

   [3 marks]

   (b)

   Using the data for density and viscosity, discuss the contrasting challenges the loon faces when moving in air and in water.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student writes in a revision summary: "Water is the medium of life. Because water is such a good solvent, every molecule in a cell works best when it is dissolved in water."

   (a)

   Explain how the polarity of water molecules makes water a suitable medium for metabolism and for transport in organisms.

   [6 marks]

   (b)

   Evaluate the student's claim that every molecule in a cell works best when it is dissolved in water.

   [6 marks]

   Total for question 4: 12 marks

End of questions