A1.1 Water. Practice questions with markscheme.

• D: glucose and amino acids are polar/hydrophilic so they dissolve directly in the aqueous plasma; hydrophobic lipids need carriers (coated lipoprotein particles) to travel in water;

• B: hydrogen bonds between water molecules hold the column together under tension; A adhesion helps water cling to the walls but does not hold water molecules to each other; C heat capacity affects temperature change, not the ability to withstand tension; D low viscosity eases flow but does nothing to stop the column separating;

• water is polar, so a very wide range of substances dissolves in it: ionic compounds such as sodium chloride and polar molecules such as glucose and amino acids;
• because so many solutes dissolve, water is the medium in which almost all metabolic reactions occur and the transport medium of blood, xylem and phloem;
• however, non-polar substances dissolve poorly or not at all: lipids, steroid hormones and oxygen are (almost) insoluble in water;
• such substances must be carried in other ways, e.g. lipids as lipoprotein complexes and oxygen bound to haemoglobin, showing that water is not a solvent for everything;
• the insolubility of lipids in water is itself essential, because it allows membranes to form and to remain stable;
• overall the description is a useful exaggeration: water dissolves more substances than any other common solvent, but 'universal' is not literally correct, OWTTE;

A judgement is required for the final mark. Award a maximum of three marks if only strengths, or only weaknesses, are given.

• soil particles have polar/charged surfaces (e.g. clay minerals) to which water adheres / adhesion;
• adhesion is due to attraction (hydrogen bonding) between polar water molecules and the polar/charged surface;
• water molecules are also attracted to each other by hydrogen bonds / cohesion;
• adhesion together with cohesion produces capillary action, which holds/draws water through narrow spaces (even against gravity);
• as roots absorb water, water is drawn from neighbouring pores towards the root by capillary action / to replace the water removed, OWTTE;

Do not accept 'surface tension' as the reason for retention in soil.

• (a) [2]
  • the coastal town has a much smaller daily temperature range (than the inland town);
  • (consistent with the idea that) the sea warms and cools slowly / buffers/moderates the surrounding air temperature, OWTTE;
• (b) [4]
  • independent variable = the substance (water vs sand); dependent variable = temperature, measured/recorded at intervals (e.g. every minute);
  • controlled variables (any two): equal mass/volume of each substance, same starting temperature, same heat source and distance/time of heating, same container/thermometer position;
  • method: heat each for a set time (or cool each from the same start) and plot temperature against time, comparing the rate of change;
  • replication: repeat each condition several times and use means / a control such as an empty container or a substance of known heat capacity, OWTTE;

  Must identify IV and DV, at least two controlled variables, and replication for full marks.

• (c) [2]
  • heating water must also break/loosen (some of) the hydrogen bonds between molecules, not only speed the molecules up;
  • (so) a large amount of heat energy raises the temperature only slightly, OWTTE;

• cohesion: water molecules stick to each other (because of hydrogen bonding);
• cohesion allows water columns (in xylem) to be pulled up plants under tension (without breaking);
• cohesion produces surface tension, allowing small animals (e.g. pond skaters) to live on the water surface;
• adhesion: water sticks to (polar/hydrophilic) surfaces such as cellulose;
• adhesion draws water through/along cell walls / capillary action (e.g. keeping leaf cell walls moist);
• water rises in narrow spaces (against gravity) by capillary action, OWTTE;