1. 1
   A student stands three clean glass capillary tubes of internal diameter 0.5 mm, 1.0 mm and 2.0 mm upright in a dish of water. Water rises highest in the 0.5 mm tube and least in the 2.0 mm tube. She then places the cut end of a celery stalk in water containing a dye and later sees the dye in the xylem strands near the top of the stalk.

   (a)

   Which explanation accounts for water rising in the glass tubes?

   [1 mark]

   • AAdhesion of water to polar glass combined with cohesion between water molecules
   • BWater is less dense than glass
   • CEvaporation of water from the top of the tubes pulls the water up
   • DWater is hydrophobic and is repelled from the dish

   (b)

   Which bonds are directly responsible for cohesion between the water molecules in the tubes?

   [1 mark]

   • APolar covalent bonds between oxygen and hydrogen within each molecule
   • BHydrogen bonds between neighbouring water molecules
   • CIonic bonds between water and glass
   • DPeptide bonds

   (c)

   Outline the importance of capillary action in soil for plants.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The ratio of deuterium (heavy hydrogen) to ordinary hydrogen in water acts as a chemical fingerprint of where the water came from. In Earth's oceans the ratio is about 1.56 × 10⁻⁴. Water locked in minerals of carbonaceous asteroids, sampled from meteorites, has ratios of roughly 1.4–1.8 × 10⁻⁴. Water measured on comet 67P by the Rosetta space probe has a ratio of about 5.3 × 10⁻⁴.

   (a)

   According to the hypothesis studied in this course, what was the main source of the water on Earth?

   [1 mark]

   • AIt was produced by the first cells during respiration
   • BIt was made by photosynthesis in early oceans
   • CIt was delivered by asteroids colliding with the early Earth
   • DIt formed only after life appeared, from oxygen released by bacteria

   (b)

   Which pair of factors explains why Earth retained its water?

   [1 mark]

   • ASurface temperatures high enough to keep all water as vapour, and weak gravity
   • BVery weak gravity, so asteroids could deliver water more easily
   • CConstant volcanic activity boiling the oceans, and the absence of an atmosphere
   • DGravity strong enough to hold water vapour, and temperatures low enough for water to condense

   (c)

   Explain how the data support the hypothesis that asteroids were the source of Earth's water.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Astronomers are choosing which of four rocky exoplanets orbiting similar stars to study for signs of life. Planet P has 1.1 times the mass of Earth, orbits inside its star's habitable zone and has an estimated mean surface temperature of 12 °C. Planet Q has 0.1 times the mass of Earth, also orbits inside the habitable zone, but has an extremely thin atmosphere. Planet R has 0.9 times the mass of Earth and orbits close to its star, with a surface temperature of about 430 °C. Planet S has 5 times the mass of Earth and orbits far from its star, with a surface temperature of about −150 °C.

   (a)

   Explain what is meant by the "Goldilocks zone" and deduce which planet is the most promising target in the search for life.

   [3 marks]

   (b)

   Evaluate the use of liquid water as the main criterion when searching for extraterrestrial life.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Mars has about 11% of the mass of Earth and orbits about 1.5 times further from the Sun. Dried river channels and minerals that form only in water suggest that liquid water flowed on its surface billions of years ago. Today Mars has a very thin atmosphere and its water exists almost entirely as ice, with only traces of vapour. Earth, by contrast, has had liquid oceans for around 4 billion years.

   (a)

   Explain how hydrogen bonding between water molecules gives liquid water properties that make it a suitable medium for life.

   [6 marks]

   (b)

   Using the information about Mars, discuss why Earth has retained liquid water for billions of years and the significance of this for life on Earth.

   [6 marks]

   Total for question 4: 12 marks

End of questions