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]
Question 7
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanOutline the hypothesis for the origin of water on Earth and why Earth has retained its water. Suggest why astronomers searching for life focus on planets in the Goldilocks zone.
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- water was delivered to Earth by asteroids / extraplanetary origin [1]
- Earth's gravity is strong enough to hold its atmosphere / water vapour [1]
- temperatures on Earth are low enough for water to condense into liquid [1]
- the Goldilocks zone is the distance from a star where liquid water can exist on a planet's surface [1]
- life as we know it requires liquid water as its medium [1]
- [max 3]