A1.1 Water. Practice questions with markscheme.

• D: specific heat capacity is the energy needed to raise the temperature of a given mass by 1 °C, so it decides how much the water warms, not how fast heat leaves the iguana; heat passes faster into water because water has a much higher thermal conductivity than air; water, not air, has the higher specific heat capacity (A); a high specific heat capacity means the water warms only slightly (B); the sea and the air are at the same temperature, and specific heat capacity does not set a temperature (C);

• B: the cytoplasm and the plasma are aqueous solutions, so these reactions and urea transport take place in water; oxygen moves along the tracheae through air, which is why water is the medium for most, not all, of the processes of life;

• B: the O–H bonds are polar covalent because electrons are shared unequally, being pulled towards the more electronegative oxygen; A describes ionic bonding, which does not occur in water; C confuses proton number with charge on a neutral atom; D reverses cause and effect, since hydrogen bonds are a consequence of the polarity, not its source;

• (a) [1]
  • 2.0 MPa;

  Accept answers in the range 1.9 to 2.1 MPa. Units required.

• (b) [2]
  • in both species the capacity lost increases as the tension increases / both curves are S-shaped;
  • P loses capacity at much lower tensions than Q, e.g. half is lost at about 2 MPa in P but at about 5.5 MPa in Q;
  • P has lost (almost) all of its capacity by about 3.5 to 4 MPa, whereas Q has lost only about 6 % at 4 MPa (and still conducts some water at 7 MPa);
  • the loss in P rises over a narrower range of tension than in Q / the curve for P is steeper, OWTTE;

  At least one similarity and one difference are needed for 2 marks.

• (c) [2]
  • water is pulled up (by the tension generated at the leaves) as a continuous column;
  • cohesion / hydrogen bonding between water molecules passes the pull from molecule to molecule down the column;
  • the gas breaks the continuity of the column / there is no hydrogen bonding across the gas;
  • so the pull is not transmitted to the water below the gas, which is no longer drawn up, OWTTE;
• (d) [1]
  • when the soil is dry the tension in the xylem becomes very high, and Q keeps (most of) its water columns intact at tensions that would block almost all of P's vessels;
  • so Q can go on supplying water to its leaves in drought, whereas P would stop conducting water / wilt, OWTTE;

• xylem vessel drawn as a long, narrow tube with its wall labelled;
• water molecules shown inside the vessel with hydrogen bonds between them, labelled as cohesion;
• attraction between water molecules and the vessel wall shown and labelled as adhesion;
• arrow showing upward movement of the water column / tension (pull) from above due to evaporation/transpiration in the leaves;
• annotation that cohesion keeps the column continuous / prevents it breaking under tension;
• annotation that adhesion helps the water move up the wall / prevents the column slipping back;

Beyond this subtopic. Award if given; the marks above can be earned without it.

• B3.2 vessel shown open at both ends (no end walls) / wall labelled as lignified;

Accept water molecules drawn as circles or as H–O–H. Do not award the cohesion mark if the 'hydrogen bonds' are drawn within one molecule. Max [4].

• (a) [1]
  • (90.2 + 91.5 + 90.4) ÷ 3 = 90.7 (g);

  Accept 90.7 or 90.70 g. Units not required.

• (b) [1]
  • ionic and polar substances dissolve (in large amounts) whereas the non-polar substance does not dissolve, OWTTE;
• (c) [2]
  • volume of water (100 cm³);
  • temperature (20 °C);
  • type/source of water (distilled);
  • stirring / rate of stirring;

  Mark the first two only. Do not accept 'the substance' or 'the mass dissolved'.

• (d) [3]
  • water molecules are polar, with a partially negative oxygen and partially positive hydrogens;
  • the partial charges attract the (positive and negative) ions of sodium chloride, so each ion becomes surrounded by water molecules and is separated from the crystal;
  • glucose has polar (hydroxyl) groups that form hydrogen bonds with water molecules, so it dissolves;
  • olive oil/triglyceride molecules are non-polar, so they form no attractions with water molecules;
  • water molecules are attracted more strongly to each other than to the oil, so the oil is excluded / forms a separate layer, OWTTE;

  Accept 'hydrophobic' for non-polar. Do not accept 'oil is less dense than water' as a reason.

• D: cohesion (hydrogen bonding between water molecules) produces surface tension strong enough to support small animals;

• A: the hydrophobic tails exclude water and form the barrier at the centre of the bilayer, so insolubility is essential to the function; glucose (A) and sodium ions (D) are hydrophilic and function dissolved in water, and haemoglobin (B) is a water-soluble protein whose transport role depends on being in solution;

• D: high thermal conductivity of water accelerates heat loss, so endotherms in water insulate (blubber) and respire more to compensate;

• C: water forms hydrogen bonds with (adheres to) the polar surfaces of cellulose and glass fibres, so it is drawn up the narrow spaces between the fibres by capillary action; wax is non-polar, so there is no adhesion and no rise; cohesion is a property of water itself and is the same in every strip (B);

• C: water dissolves the molecules from which cells are built and allows them to move, meet and react, so the chemistry that gave rise to the first cells, and the metabolism of cells ever since, takes place in an aqueous medium; A is wrong because water contains no carbon, so it cannot supply the carbon of organic molecules; B is wrong because the first cells were not photosynthetic; D is wrong because the early atmosphere contained almost no oxygen;

• B: the D/H ratio of Earth's oceans lies within the range for asteroid material but below that of most comets, supporting the hypothesis that Earth's water was delivered mainly by asteroids;

• B: cohesion keeps the water column continuous and adhesion attaches water to xylem/cell walls (capillary action); specific heat capacity affects how fast the water's temperature changes, not its movement;

• A: Q receives 0.01 × (1 ÷ 0.1)² = 1 times the energy per unit area that Earth receives, so its surface temperature could allow liquid water, whereas P receives only 1% and any water would freeze; a planet's gravity depends on its mass, not on its distance from the star (B);

• covalent bonds join the oxygen and hydrogen atoms within one molecule, whereas hydrogen bonds form between neighbouring molecules;
• covalent bonds involve (unequal) sharing of a pair of electrons, whereas hydrogen bonds are attractions between the partial charges of δ⁻ oxygen on one molecule and δ⁺ hydrogen on another;
• covalent bonds are strong/stable, whereas individual hydrogen bonds are weak and constantly break and re-form, OWTTE;

• xylem vessel drawn as a long, narrow tube with its wall labelled;
• water molecules shown inside the vessel with hydrogen bonds between them, labelled as cohesion;
• attraction between water molecules and the vessel wall shown and labelled as adhesion;
• arrow showing upward movement of the water column / tension (pull) from above due to evaporation/transpiration in the leaves;
• annotation that cohesion keeps the column continuous / prevents it breaking under tension;
• annotation that adhesion helps the water move up the wall / prevents the column slipping back;

Beyond this subtopic. Award if given; the marks above can be earned without it.

• B3.2 vessel shown open at both ends (no end walls) / wall labelled as lignified;

Accept water molecules drawn as circles or as H–O–H. Do not award the cohesion mark if the 'hydrogen bonds' are drawn within one molecule. Max [4].

• water is a good solvent for polar/ionic substances, so glucose, amino acids, ions and mineral salts dissolve and are carried dissolved in plasma/xylem sap;
• cohesion (hydrogen bonding) keeps the water column in the xylem continuous / unbroken under tension, so water can be pulled up to the leaves (transpiration stream);
• adhesion of water to (hydrophilic) cell walls helps water move through walls and up narrow xylem vessels (capillary action);
• high specific heat capacity means the blood/sap resists rapid temperature change, helping distribute heat and stabilize the organism's temperature;
• limitation: hydrophobic substances (e.g. lipids/O₂) dissolve poorly, so they need carriers (lipoproteins, haemoglobin), a constraint imposed by water's polarity;
• (on balance) water's polarity and hydrogen bonding suit it well as a transport medium, with the main limitation being the transport of non-polar substances, OWTTE;

A balanced answer should include at least one limitation to reach the top band.