WaterEdexcel A-Level Biology B: Subtopic test

1. 1
   A pond skater is a small insect that walks and rests on the surface of a pond without sinking, even though its body is denser than water.

   (a)

   Which property of water allows the pond skater to rest on the surface?

   [1 mark]

   • AHigh specific heat capacity
   • BSurface tension caused by hydrogen bonding between water molecules
   • CIts ability to act as a polar solvent
   • DIncompressibility

   (b)

   Why can water molecules form hydrogen bonds with each other?

   [1 mark]

   • AThe oxygen and hydrogen atoms share electrons equally
   • BThe hydrogen atoms in each molecule carry a full negative charge
   • COxygen is more electronegative than hydrogen, so the molecule has slightly negative and slightly positive regions that attract those of other molecules
   • DEach molecule contains ionic bonds between oxygen and hydrogen

   (c)

   Explain how surface tension arises at the surface of the pond.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In winter the surface of a deep pond freezes, but the water at the bottom stays liquid at about 4 °C and the fish living in the pond survive.

   (a)

   Which statement about the density of water is correct?

   [1 mark]

   • AWater is densest at 0 °C when it becomes ice
   • BIce is denser than liquid water
   • CLiquid water is least dense at 4 °C
   • DLiquid water is densest at about 4 °C, and ice is less dense than liquid water

   (b)

   Which statement best explains why ice is less dense than liquid water?

   [1 mark]

   • AThe hydrogen bonds in ice hold the molecules in an open lattice, so they are further apart than in liquid water
   • BThere are no hydrogen bonds in ice
   • CThe molecules in ice are lighter than those in liquid water
   • DThe covalent bonds in each water molecule are longer in ice

   (c)

   Explain how the properties of water help the fish in the pond to survive the winter.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Blood plasma is about 90% water. It carries dissolved glucose, mineral ions and urea around the body, and it carries heat from active muscles to the skin. Blood is pumped at high pressure through narrow vessels without any noticeable change in its volume.

   (a)

   Explain why water is suitable as a solvent for transporting glucose and mineral ions in the blood.

   [3 marks]

   (b)

   Explain how the high specific heat capacity and the incompressibility of water make it suitable as the main component of blood plasma.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A lake in a cold climate freezes at the surface in winter, but the organisms living in it survive. The lake contains fish, aquatic plants that absorb mineral ions from the water, and pond skaters that stand on the surface. In summer the temperature of the lake changes far more slowly than that of the air above it.

   (a)

   Explain how the dipole nature of water leads to hydrogen bonding, and how hydrogen bonding gives water a high specific heat capacity and a maximum density at 4 °C.

   [6 marks]

   (b)

   Evaluate the significance of the properties of water to the organisms living in the lake.

   [6 marks]

   Total for question 4: 12 marks

End of questions