Water

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Water as the Medium for Life

The first cells are believed to have evolved in a watery environment, possibly near hydrothermal vents in the deep oceans.
Water in its liquid state allows dissolved molecules to move and collide, enabling chemical reactions.
Most life processes occur in water, and it makes up 70% to 95% of the mass of a cell.
Water is a major habitat, covering 71% of the Earth's surface.
Astronomers look for evidence of water on other planets and moons as an indicator of possible life.

Hydrogen Bonds and Polarity

Water is composed of one oxygen atom covalently bonded to two hydrogen atoms.
The sharing of electrons is uneven: oxygen attracts electrons more strongly, creating a weak negative charge (δ-) on oxygen and weak positive charges (δ+) on hydrogen.
This separation of charge is called a dipole, making water a polar molecule.
Hydrogen bonds form between the δ- oxygen of one water molecule and the δ+ hydrogen of another.
Hydrogen bonds are weak individually and constantly break and reform, but collectively they form a strong structure.
Hydrogen bonding is important in many biological molecules, including DNA base-pairing, protein secondary and tertiary structure, and cellulose and collagen tensile strength.

Cohesion and Adhesion

Cohesion is the attraction between water molecules due to hydrogen bonds, allowing columns of water to move under tension in the xylem (mass transport).
Cohesion creates surface tension at the water-air interface, forming a film that supports small insects like pond skaters.
Adhesion is the bonding of water to other polar or charged molecules, such as cellulose, via hydrogen bonds.
Adhesion enables water to move up the xylem during transpiration and to be drawn up narrow soil channels by capillary action.
Capillary action also draws water through plant cell walls, allowing flow through plant tissues.

Water as a Solvent

Hydrophilic means 'water-loving'; hydrophobic means 'water-hating'.
Polar molecules and those with positive or negative charges can form hydrogen bonds with water and are generally hydrophilic.
Non-polar molecules cannot form hydrogen bonds with water and are generally hydrophobic; they tend to group together due to hydrophobic interactions.
Because most biological molecules are hydrophilic and can dissolve, water is regarded as the universal solvent.
Different metabolites have different solubilities: sodium chloride and urea are highly soluble, while fats are insoluble.
Oxygen is sparingly soluble but soluble enough for aquatic animals to breathe; in blood, it combines with haemoglobin for sufficient transport.
Phospholipids have hydrophobic hydrocarbon tails that form the hydrophobic core of cell membranes.

Specific Heat Capacity and Thermal Properties

Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1°C.
Water has a high specific heat capacity (4200 J/kg/°C) compared to air (1000 J/kg/°C), due to its many hydrogen bonds.
This means water temperature does not fluctuate greatly, providing stable aquatic habitats and helping organisms maintain a constant internal temperature.
Stable water temperatures are vital for maintaining optimal conditions for enzyme activity.
Thermal conductivity of water is almost 30 times higher than that of air, so air is a good insulator for organisms in cold climates.
Ice has lower thermal conductivity than liquid water, so it insulates the water below, helping aquatic organisms survive.

Density, Buoyancy, and Viscosity

Ice is less dense than liquid water, so it floats, forming a habitat on and below the ice.
Buoyancy is the ability of an object to float in water; the black-throated loon has solid bones to overcome buoyancy, while the ringed seal has blubber that improves buoyancy and insulation.
Viscosity is the resistance of a fluid to flow; water has higher viscosity than air.
The black-throated loon can fly through air with little friction, and its body shape and webbed feet reduce drag in water.
The ringed seal uses flippers to propel itself through water.

Enzymes and Water

Most enzymes require water to hold their shape and maintain stability.
Water enables enzymes to catalyse reactions in aqueous solutions.
Hydrogen bonds often facilitate the binding of the enzyme active site to its substrate, forming an enzyme-substrate complex.

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1.Which of the following 3-carbon compounds will not dissolve in water?
Easy
- APropanoic acid
- BPropanol
- CGlycerol
- DPropane
2.Which of the following is a correct definition of specific heat capacity?
Easy
- AThe heat required to change one mole of liquid into one mole of gas
- BThe heat required to raise the temperature of 1 kg of liquid by 1°C
- CThe heat required to change one mole of solid into one mole of liquid
- DThe ability of a solid to transfer heat to a liquid
3.Which of the following are NOT examples of hydrogen bonding?
Medium
- ABase-pairing between two strands of DNA
- BThe forces that hold water molecules together
- CThe bond that joins one nucleotide to its neighbour in a strand of DNA
- DInteractions between water and the polar R groups of certain amino acids
4.Water (H₂O) is a polar molecule, whereas methane (CH₄) is nonpolar. Which of the following properties of methane is explained by methane's lack of polarity?
Medium
- ALow molecular weight
- BLow boiling point
- CFlammability
- DGreenhouse gas effect
5.Which of the following observations is NOT explained by water's high latent heat of vaporisation and specific heat capacity?
Medium
- AIce is less dense than liquid water, so it floats on water
- BWater exists in all three physical states (solid, liquid and gas) on Earth
- CA small volume of water can dissipate a lot of heat from an organism
- DA lot of heat energy is required to raise the temperature of water
6.Which row of the table lists the four common metabolites in decreasing order of solubility in water?
Medium
- Aoxygen → sodium chloride → cholesterol → hydrophobic amino acid
- Bsodium chloride → oxygen → hydrophobic amino acid → cholesterol
- Chydrophobic amino acid → oxygen → sodium chloride → cholesterol
- Dsodium chloride → hydrophobic amino acid → oxygen → cholesterol
7.Which of the following properties of water are a result of intermolecular forces? 1. High surface tension. 2. Good solvent. 3. Cohesiveness. 4. High specific heat capacity.
Medium
- AI and II
- BI, II and III
- CI, II and IV
- DAll
8.Which of the following properties of water stops enzymes from being denatured during transpiration? I. Water retains a lot of heat. II. Water forms hydrogen bonds with other polar and nonpolar molecules. III. A lot of heat is required to evaporate water. IV. Water is cohesive.
Medium
- AI only
- BI and II
- CII, III and IV
- DIII only

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9.Sodium hydrogencarbonate (NaHCO₃) dissociates into sodium ions (Na⁺) and bicarbonate ions (HCO₃⁻) when dissolved in water. The bicarbonate ions create an alkaline environment suitable for certain enzymes, and in the blood they help buffer lactic acid during exercise. How does water help the production of bicarbonate ions?
Medium
- AThe oxygen atoms form ionic bonds with bicarbonate ions
- BThe oxygen atoms in water attract bicarbonate ions
- CThe hydrogen atoms form hydrogen bonds with bicarbonate ions
- DThe hydrogen atoms in water attract bicarbonate ions
10.Which of the following does NOT rely on the specific heat capacity of water?
Medium
- AHeat loss through evaporation of sweat
- BEnzyme function
- CAquatic ecosystems withstanding extreme temperature change
- DHumans living in sub-zero temperatures
11.Which of the following correctly describes the term hydrophilic?
Easy
- AWater-loving; polar or charged molecules that can form hydrogen bonds with water
- BWater-hating; non-polar molecules that cannot form hydrogen bonds with water
- CThe attraction between water molecules and other water molecules
- DThe attraction between water molecules and other polar or charged surfaces
12.Which of the following statements about hydrogen bonds in water is correct?
Medium
- AHydrogen bonds are strong, permanent bonds that do not break
- BHydrogen bonds form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another
- CHydrogen bonds form between the slightly negative hydrogen of one water molecule and the slightly positive oxygen of another
- DHydrogen bonds are covalent bonds within a water molecule
13.Which of the following are properties of water that result from hydrogen bonding? (Select all that apply.)
Medium
- AHigh surface tension
- BHigh specific heat capacity
- CCohesion between water molecules
- DAbility to dissolve non-polar molecules
- ELow viscosity
14.Which of the following statements about water as a solvent are correct? (Select all that apply.)
Medium
- AWater is a universal solvent due to its polarity
- BNon-polar molecules dissolve easily in water
- CSodium chloride and urea are highly soluble in water
- DHydrophobic molecules join together due to hydrophobic interactions
- EOxygen is completely insoluble in water
15.Water is a polar molecule because the sharing of electrons between oxygen and hydrogen atoms is uneven, creating a dipole.
Easy
True or false?
16.Hydrogen bonds are strong, permanent bonds that do not break and reform.
Easy
True or false?
17.Cohesion refers to water molecules sticking to other materials, while adhesion refers to water molecules sticking to each other.
Easy
True or false?
18.Match each term with its correct description.
Medium
- Cohesion
- Adhesion
- Hydrophilic
- Hydrophobic
- Water molecules sticking to each other
- Water molecules sticking to other materials
- Water-loving; polar or charged molecules that dissolve in water
- Water-hating; non-polar molecules that do not dissolve in water
19.Place the following steps in the correct order to describe the evolution of the first cells in water.
Medium
- Water and solutes became trapped within a membrane
- Chemical reactions began occurring within the membrane-bound structure
- The first cells evolved in a watery environment
- This led to the evolution of cells
20.Which of the following statements about hydrogen bonding in water are correct? (Select all that apply.)
Medium
- AHydrogen bonds form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of an adjacent water molecule.
- BHydrogen bonds are strong, permanent bonds that hold water molecules together permanently.
- CWhen few hydrogen bonds are present they are constantly breaking and reforming.
- DHydrogen bonds form because water is a non-polar molecule.
- EHydrogen bonds are responsible for water's high specific heat capacity.
21.Which of the following are examples of hydrogen bonding in biological molecules? (Select all that apply.)
Hard
- ABase-pairing between two strands of DNA
- BThe bond joining one nucleotide to its neighbour in a strand of DNA
- CThe hydrogen bonds between strands of cellulose that give it tensile strength
- DInteractions between mRNA and tRNA during protein synthesis
- EThe covalent bond between oxygen and hydrogen within a single water molecule
22.Which of the following properties of water are a result of hydrogen bonding between water molecules? (Select all that apply.)
Medium
- AHigh surface tension
- BHigh specific heat capacity
- CCohesion between water molecules
- DAbility to dissolve non-polar substances such as fats
- EAbility to act as a solvent for polar and charged molecules
23.Which of the following statements about the thermal properties of water are correct? (Select all that apply.)
Hard
- AWater has a specific heat capacity of 4200 J/kg/°C.
- BThe high specific heat capacity of water is due to the many hydrogen bonds present.
- CWater's thermal conductivity is almost 30 times higher than that of air.
- DThe specific heat capacity of water is lower than that of air.
- EA lot of thermal energy is required to break the hydrogen bonds in water, so its temperature does not fluctuate greatly.
24.Match each property of water with its biological example or consequence.
Medium
- High specific heat capacity
- High latent heat of vaporisation
- Cohesion and adhesion
- Low density of ice
- Stable aquatic habitats and stable enzyme activity
- Cooling effect when sweat evaporates
- Water moving up the xylem of tall trees
- Ice floats, forming a habitat for seals
25.Match each biological molecule or process with the role of hydrogen bonding involved.
Medium
- DNA
- Cellulose and collagen
- Protein synthesis
- Enzyme action
- Base-pairing between two strands
- Tensile strength
- Interactions between mRNA and tRNA
- Holding the enzyme's shape and binding the substrate
26.Place the following steps in the correct order to describe how water moves up a tall tree.
Medium
- Water evaporates from the leaves, pulling the column of water upwards.
- Hydrogen bonds between water molecules create cohesion.
- Water is drawn into the roots and moves into the xylem.
- Water molecules adhere to the cellulose in the xylem walls.
27.Place the following statements in order from most soluble to least soluble in water.
Hard
- Oxygen
- Sodium chloride
- Cholesterol
- Hydrophobic amino acid
28.The high specific heat capacity of water is due to the many hydrogen bonds present between water molecules.
Easy
True or false?
29.Which of the following is the best definition of specific heat capacity?
Medium
- AThe energy required to raise the temperature of 1 kg of a substance by 1°C.
- BThe energy required to change one mole of liquid into one mole of gas.
- CThe energy required to change one mole of solid into one mole of liquid.
- DThe ability of a substance to conduct heat.
30.Which of the following is not an example of hydrogen bonding?
Medium
- ABase-pairing between two strands of DNA
- BThe forces that hold water molecules together
- CThe bond that joins one nucleotide to its neighbour in a strand of DNA
- DInteractions between water and the polar R groups of certain amino acids
31.Which of the following properties of water stops enzymes from being denatured during transpiration?
Medium
- AI only
- BI and II
- CII, III and IV
- DIII only
32.With reference to a thermal property of water, explain how the experiment can demonstrate the response of the human body to prevent overheating.
Medium
- AThe covered cup cools more slowly because water has a high specific heat capacity, similar to how sweating cools the body.
- BThe uncovered cup cools more slowly because water has a high latent heat of vaporisation, similar to how sweating cools the body.
- CThe covered cup cools more quickly because water has a low specific heat capacity, similar to how sweating cools the body.
- DThe uncovered cup cools more quickly because water has a high latent heat of vaporisation, similar to how sweating cools the body.
33.A student measures the energy needed to raise the temperature of 1 kg of an unknown liquid by 1 °C and obtains a value of 2100 J/kg/°C. Compared with water, how much energy does this liquid require, and what does this suggest about the hydrogen bonding in the liquid?
Medium
- AHalf as much energy; the liquid has fewer hydrogen bonds than water
- BTwice as much energy; the liquid has more hydrogen bonds than water
- CThe same energy; the liquid has the same number of hydrogen bonds as water
- DOne quarter as much energy; the liquid has no hydrogen bonds at all
34.A sample of water is cooled from 20 °C to 4 °C. Which statement best describes what happens to its density and why?
Medium
- ADensity decreases because water molecules move slower and pack less closely
- BDensity increases because the molecules lose kinetic energy and move closer together
- CDensity stays the same because water is incompressible
- DDensity decreases because hydrogen bonds break and the structure collapses
35.A student places a drop of water on a clean glass slide and another drop on a slide coated with a thin layer of oil. The water spreads out on the glass but forms a rounded bead on the oil. Which statement best explains this difference?
Hard
- AWater molecules are cohesive and adhere to the polar glass but not to the non-polar oil
- BWater molecules are adhesive and cohere to the non-polar oil but not to the polar glass
- CThe oil has a higher specific heat capacity than glass, so the water evaporates more slowly
- DThe glass is hydrophobic and the oil is hydrophilic, so water behaves differently on each
36.A scientist discovers a new liquid on an exoplanet. Tests show it has a high specific heat capacity, high surface tension, and dissolves ionic compounds. Which statements about this liquid are most likely correct? (Select all that apply.)
Medium
- AIt is likely to be polar.
- BIt probably forms hydrogen bonds between its molecules.
- CIt is likely to be non-polar.
- DIt probably has weak intermolecular forces.
- EIt could support stable aquatic habitats.
37.A student investigates the cooling of two identical cups of hot water. Cup A is wrapped in a wet cloth; Cup B is left bare. The graph shows Cup A cools more slowly than Cup B. Which conclusions are supported by this result? (Select all that apply.)
Medium
- AEvaporation of water from the cloth removes heat from Cup A.
- BWater has a high latent heat of vaporisation, so it absorbs a lot of energy when evaporating.
- CThe wet cloth increases the specific heat capacity of the water inside Cup A.
- DThe wet cloth acts as an insulator, reducing heat loss by conduction.
- ECup B loses heat faster because it has a lower specific heat capacity.
38.A pond skater is observed walking on the surface of a pond. Detergent is then added to the pond, and the pond skater sinks. Which statements explain this observation? (Select all that apply.)
Hard
- ADetergent molecules disrupt hydrogen bonds between water molecules at the surface.
- BSurface tension is reduced by the detergent.
- CDetergent increases the density of water, so the pond skater can no longer float.
- DDetergent molecules are non-polar and break covalent bonds within water molecules.
- EThe pond skater relies on the high surface tension of water to support its weight.
39.The thermal conductivity of water is almost 30 times higher than that of air, which makes air a good insulator for organisms in cold climates.
Easy
True or false?
40.The density of ice is lower than the density of liquid water, so ice floats and can form an insulating layer above water.
Medium
True or false?
41.Complete the definition of specific heat capacity.
Easy
Specific heat capacity is a measure of the energy required to raise the temperature of 1 kg of a substance by ____ °C.
42.Name the property of water that allows its molecules to form a meniscus as the water makes contact with the inner surface of a glass container.
Easy
The property is ____.
43.Define the term hydrophilic.
Easy
44.What is the specific heat capacity of water in J/kg/°C?
Medium
45.What is the specific heat capacity of air in J/kg/°C?
Medium
46.By approximately how many times is the thermal conductivity of water higher than that of air?
Medium
47.Complete the sentence about the polarity of water.
Easy
In a water molecule, the oxygen atom attracts electrons more strongly than hydrogen, creating a weak negative region (δ−) on the oxygen and a weak positive region (δ+) on the hydrogen atoms. This separation of charge is called a ____.
48.Calculate the energy required to raise the temperature of 2 kg of water by 10 °C. Use the specific heat capacity of water as 4200 J/kg/°C. Give your answer in joules.
Medium
49.A 0.5 kg sample of an unknown liquid requires 10 500 J of energy to raise its temperature by 10 °C. Calculate the specific heat capacity of the liquid in J/kg/°C.
Medium
50.Explain how the properties of cohesion and adhesion are useful to living organisms.
Medium
51.Compare and contrast the terms adhesion and cohesion in the context of water molecules in biological tissues.
Medium
52.Explain the importance of hydrogen bonding in water.
Hard
53.Explain why some biological molecules are hydrophilic, some are hydrophobic and others are amphipathic, using named examples to support your answer.
Hard
54.Other than the starting temperature of the water (70°C), state two conditions that must be kept the same for each cup throughout the experiment.
Medium
55.What is the specific heat capacity of water in J/kg/°C?
Medium
- A5040 J/kg/°C
- B4620 J/kg/°C
- C3780 J/kg/°C
- D4200 J/kg/°C
56.What is the specific heat capacity of air in J/kg/°C?
Medium
- A1200 J/kg/°C
- B1100 J/kg/°C
- C900 J/kg/°C
- D1000 J/kg/°C
57.By approximately how many times is the thermal conductivity of water higher than that of air?
Medium
- A30 times
- B27 times
- C33 times
- D36 times
58.Calculate the energy required to raise the temperature of 2 kg of water by 10 °C. Use the specific heat capacity of water as 4200 J/kg/°C. Give your answer in joules.
Medium
- A75600 J
- B84000 J
- C100800 J
- D92400 J
59.A 0.5 kg sample of an unknown liquid requires 10 500 J of energy to raise its temperature by 10 °C. Calculate the specific heat capacity of the liquid in J/kg/°C.
Medium
- A1890 J/kg/°C
- B2310 J/kg/°C
- C2100 J/kg/°C
- D2520 J/kg/°C

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