Question 1
Paper 1A style
What causes hydrogen bonds to form between water molecules?
- Unequal sharing of electrons in O–H covalent bonds makes the molecules polar
- Water molecules lose and gain electrons to form H⁺ and OH⁻ ions
- Electrons are shared equally between the oxygen and hydrogen atoms
- Hydrogen atoms of neighbouring molecules form covalent bonds with each other
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Answer: A [1]
Question 2
Paper 1A style
Which property of water is mainly responsible for water being drawn through the cellulose cell walls of leaf mesophyll cells?
- High specific heat capacity
- Adhesion to hydrophilic cellulose
- Buoyancy
- Low viscosity
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Answer: B [1]
Question 3
Paper 1A style
Which substance is transported in blood plasma combined with proteins, because it does not dissolve well in water?
- Glucose
- Sodium ions
- Amino acids
- Cholesterol
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Answer: D [1]
Question 4
Paper 1B style
The table shows some physical properties of water and air at 20 °C. (a) Calculate how many times greater the thermal conductivity of water is than that of air. [1] (b) Using the data, explain why a ringed seal (Pusa hispida) swimming in cold Arctic water needs a thick layer of blubber. [2] (c) Suggest, using the data, why a black-throated loon (Gavia arctica) needs a streamlined body to catch fish underwater. [2]
| Property | Water | Air |
|---|---|---|
| Density / kg m⁻³ | 998 | 1.2 |
| Viscosity / mPa s | 1.00 | 0.018 |
| Thermal conductivity / W m⁻¹ K⁻¹ | 0.60 | 0.026 |
| Specific heat capacity / J g⁻¹ °C⁻¹ | 4.18 | 1.01 |
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- (a) 0.60 ÷ 0.026 = 23 (times) / accept 23.1 [1]
- (b) water conducts heat (about 23 times) faster than air / has a higher thermal conductivity [1]
- (b) so heat is lost from the seal's body much faster in water [1]
- (b) blubber is an insulator that reduces heat loss / helps maintain body temperature [1]
- (c) water is much more viscous than air / about 55 times more viscous [1]
- (c) so there is more drag / resistance to movement through water [1]
- (c) streamlining reduces drag so the loon can swim fast enough to catch fish / uses less energy [1]
- [max 5]: max 1 for (a), max 2 for (b), max 2 for (c)
Question 5
Paper 2A style
Explain how cohesion allows water to be transported up the xylem of a tall tree.
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- cohesion is attraction between water molecules due to hydrogen bonds [1]
- evaporation of water from leaves / transpiration pulls water up the xylem [1]
- water in the xylem is under tension / negative pressure [1]
- cohesion keeps the water column continuous / stops the column breaking under tension [1]
- adhesion of water to xylem walls helps support the column [1]
- [max 3]
Question 6
Paper 2B style
Explain how the polarity of water molecules makes water a suitable medium for metabolism and for transport in organisms.
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- electrons are shared unequally in the O–H covalent bonds / oxygen is more electronegative [1]
- oxygen is δ− and hydrogens are δ+, so water molecules are polar [1]
- hydrogen bonds form between water molecules / between water and other polar molecules [1]
- polar molecules and ions are hydrophilic / attracted to water molecules [1]
- water molecules surround ions / polar molecules so they dissolve OWTTE [1]
- named examples of hydrophilic solutes: glucose / amino acids / mineral ions [1]
- dissolved substances can move and collide, so reactions can happen [1]
- most enzymes catalyse reactions in aqueous solution / cytoplasm is mostly water [1]
- water is a reactant in hydrolysis / is used or released in metabolic reactions [1]
- blood plasma transports dissolved glucose / amino acids / urea / hormones [1]
- xylem transports dissolved mineral ions / phloem transports dissolved sucrose [1]
- first cells originated in water / water remains the medium for most life processes [1]
- [max 6]