▶️ Answer/Explanation
(a)
- Water molecules are polar and form hydrogen bonds between molecules.
- Methane molecules are non-polar and do not form hydrogen bonds.
- Hydrogen bonds require more energy to break, so water has:
- Higher boiling point and melting point
- Higher heat capacity
- Higher heat of vaporization
- Methane lacks these strong intermolecular forces, so its thermal properties are much lower.
(b)
- Methane’s boiling point is −160°C (very low).
- Earth’s typical temperatures are well above −160°C.
- Therefore, methane is naturally a gas under Earth conditions.
(c)
- Methane has a low heat of vaporization (~760 J/g) vs water (~3357 J/g), so it removes less heat when evaporating.
- Methane does not form hydrogen bonds, so less energy is absorbed during phase changes.
- Methane’s boiling point (−160°C) means it is already a gas at body temperature, so it cannot evaporate from skin to remove heat.
- Water, with strong hydrogen bonds and high heat of vaporization, absorbs much more heat when it evaporates, making it a more effective coolant.
Markscheme:
(a)
a. Water forms hydrogen bonds but methane does not/hydrogen bonds form between water molecules, but are absent in methane;
b. Energy needed to break hydrogen bonds/intermolecular attractions;
c. Hydrogen bonds raise the freezing point/boiling point/heat capacity/heat of vaporization.
(b)
a. Boiling point of methane is \(-160^\circ \text{C}\)
OR
Methane is in gaseous state when temperatures are above/higher than \(-160^\circ \text{C}\);
b. Temperatures on Earth are always above \(-160^\circ \text{C}\).
(c)
a. Heat of vaporization is low/heat of vaporization is only \(760 \, \text{J g}^{-1}\)
OR
Methane has a lower heat of vaporization compared to water;
b. No hydrogen bonds need to be broken;
c. Not enough heat removed when methane evaporates;
d. Methane boils at \(-160^\circ \text{C}\) so would already be a gas (in/on the human body).
▶️ Answer/Explanation
(a)(i)
10 water molecules are shown.
(Each molecule consists of 1 oxygen atom and 2 hydrogen atoms.)
(ii)
Hydrogen bonds (represented by dashed lines) between water molecules.
(b) (i)
- a. Heat energy increases the molecular motion or vibration of water molecules.
- b. This causes hydrogen bonds between water molecules to break.
- c. Evaporation occurs when water molecules escape into the air (liquid → gas).
- d. This process removes heat from the skin, leading to cooling.
(ii)
- a. To cool the body and remove excess heat.
- b. To maintain a stable internal temperature (thermoregulation/homeostasis) or keep body temperature around 37 °C.
Markscheme:
(a) (i) 10;
(ii) hydrogen bonds/H bond;
(b) (i)
a. heat increases molecular motion/vibration;
b. hydrogen bonds break/bonds between water molecules break;
c. water evaporation is separation of water molecules/water changes from liquid to gas/vapour;
d. heat removed from skin surface/body;
(ii)
a. cooling/removal of heat/lowering body temperature;
b. to prevent overheating
OR
to help maintain body temperature/temperature homeostasis/for thermoregulation
OR
to keep temperature at \( 37\,^{\circ}\mathrm{C} \);
▶️ Answer/Explanation
(a)
- Anaerobic respiration occurs in the absence of oxygen.
- Both humans and yeast use glycolysis, producing a small amount of ATP (2 ATP per glucose).
- In yeast, the products are ethanol and carbon dioxide – this is called alcoholic fermentation.
- In humans (especially in muscles during intense exercise), the product is lactate (lactic acid) – this is lactic acid fermentation.
(b) (i)
- Water has a higher boiling point and melting point than methane.
- Water has a higher specific heat capacity – it takes more energy to change its temperature.
- Water has a higher latent heat of vaporization – it absorbs more heat when it evaporates.
- These differences are due to hydrogen bonding in water (from polarity), which is absent in methane, a non-polar molecule.
(ii)
- Methane is a potent greenhouse gas – it traps heat in the atmosphere.
- It is much more effective than carbon dioxide at trapping heat over short periods.
- Although it has a shorter atmospheric lifespan than CO₂, it degrades into CO₂, still contributing to long-term warming.
Markscheme:
a)
a. (in both) anaerobic respiration gives a small amount of ATP/2 ATP/energy from glucose;
b. anaerobic respiration occurs when there is no oxygen;
c. anaerobic respiration in yeast produces ethanol and carbon dioxide/alcoholic fermentation;
d. anaerobic respiration in humans (in muscle) produces lactate/lactic acid/lactic acid fermentation;
e. both undergo glycolysis;
b) i.
a. water has higher boiling/melting point;
b. water has a higher specific heat capacity;
c. water has a higher latent heat of vaporization;
d. differences due to water having many H-bonds/polarity between the molecules while methane has no H-bonds/polarity;
ii.
a. methane is a greenhouse gas OR methane causes an increase in temperature of the atmosphere;
b. methane is one of the most powerful greenhouse gases / more powerful than CO2;
c. methane has a relatively short lifespan compared to CO2/decomposes to CO2;
▶️ Answer/Explanation
(a)
(b)
- Water is a polar molecule – it has a slightly negative charge near the oxygen atom and a slightly positive charge near the hydrogen atoms.
- This allows it to form hydrogen bonds with other polar molecules or ions.
- Because of these attractions, water can dissolve many substances, making it a universal solvent.
Example:
- Sodium chloride (NaCl) dissolves in water:
- Na⁺ ions are attracted to the oxygen (negative pole) of water.
- Cl⁻ ions are attracted to the hydrogens (positive poles) of water.
- Water molecules surround the ions and pull them away into solution.
Markscheme:
a.
• Similar water molecule drawn with oxygen on one molecule facing hydrogen on the other water molecule
• One hydrogen bond drawn as a dotted/dashed line between the two water molecules and labelled
O and H do not have to be labelled but must be positioned correctly
Example:
Can get this mark even if atoms incorrect
b.
• Water molecule is polar
OR
• Water has weak positive and negative charges
• Water forms hydrogen bonds with polar substances
• Positive/hydrogen side/pole of water attracted to negative ions
OR
• Negative/oxygen side/pole attracted to positive ions
• Glucose/other example dissolves because it is polar
OR
• Sodium chloride/other example dissolves because ions are attracted to water