Water
Laura Armstrong
Teacher
Recall Questions
This topic requires knowledge of water from GCSE Chemistry. You can test your knowledge on this below.
Why is water described as a polar molecule?
Water has an uneven charge distribution; oxygen is slightly negative (δ-) and hydrogen is slightly positive (δ+), making it a polar molecule.
What type of bonding occurs between water molecules?
Hydrogen bonding.
Why does ice float on water, and why is this important for aquatic life?
Ice is less dense than liquid water because hydrogen bonds form a lattice structure, creating air spaces. This insulates water below, allowing aquatic life to survive in cold temperatures.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains water or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Properties of Water and Their Biological Importance
Water is essential for life due to its unique properties, which result from hydrogen bonding between molecules.
1. Water as a Solvent
- Water dissolves polar molecules (e.g., glucose, amino acids, and salts) and allows biochemical reactions to occur faster in solution (such as in the cell cytoplasm).
- Transport medium: Blood plasma and xylem transport dissolved substances (e.g., oxygen, glucose, minerals).
2. High Specific Heat Capacity
- Definition: The amount of heat needed to raise 1g of water by 1°C.
- Water has a high specific heat capacity because hydrogen bonds absorb energy before breaking.
- Biological Importance:
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Buffers against temperature changes, maintaining stable aquatic and cellular environments.
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3. Large Latent Heat of Vaporisation
- Definition: The energy required to convert water from a liquid to a gas.
- Biological Importance:
- Allows cooling by evaporation (e.g., sweating and transpiration).
4. Cohesion and Surface Tension
- Water molecules stick together due to hydrogen bonding (cohesion).
- Biological Importance:
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Enables capillary action in xylem for water transport in the xylem of plants.
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Supports small organisms like pond skaters that rely on surface tension.
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5. Water as a Reactant
- Hydrolysis reactions: The hydrolysis of larger molecules (e.g., digestion of proteins, carbohydrates, lipids).
- Condensation reactions: Involve the removal of water to form larger molecules (e.g., protein synthesis, DNA formation).
- Photosynthesis: Water is a reactant in photosynthesis. Used to make glucose in plants.
6. Water’s Density and Ice Formation
- Ice is less dense than liquid water because hydrogen bonds form a lattice structure.
- Biological Importance:
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Ice insulates water beneath, preventing large bodies of water from freezing entirely.
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Provides aquatic habitats for organisms in polar regions.
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7. Water is transparent
- Water being transparent is essential for aquatic plants because it allows light to penetrate through the water. This is important because aquatic plants rely on sunlight to carry out photosynthesis.
Key Terms
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Hydrogen bonding: Weak interactions between the δ+ hydrogen of one molecule and the δ- oxygen of another.
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Polar molecule: A molecule with an uneven charge distribution.
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Specific heat capacity: The energy required to raise the temperature of 1g of a substance by 1°C.
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Latent heat of vaporisation: The energy required for a liquid to change to a gas.
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Cohesion: Attraction between water molecules due to hydrogen bonding.
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Adhesion: Attraction between water molecules and other surfaces.
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Solvent: A liquid in which substances dissolve.
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Hydrolysis: A reaction that breaks bonds using water.
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Condensation: A reaction that forms bonds while releasing water.
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Exam Tip:
When explaining the importance of water’s properties, always link the property to a biological function.
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Explain how the properties of water make it important for organisms. (5 marks)
- Water is a universal solvent, allowing chemical reactions to occur faster in the cell cytoplasm.
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It has a high specific heat capacity, buffering temperature changes in organisms and aquatic habitats.
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Water has a large latent heat of vaporisation, enabling cooling through the evaporation of sweat or transpiration in plants.
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Cohesion due to hydrogen bonding helps water move up in the xylem of plants in the transpiration stream.
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Water is a reactant in metabolic reactions, including hydrolysis and photosynthesis.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!