The big idea: Almost every chemical reaction in a living thing happens in water.
Water is an excellent solvent: it can dissolve many substances, so the chemicals of life are carried around and react while dissolved in water.
Water can do this because it is a polar molecule — and that one fact is the root of its solvent power.
Water is polar (oxygen δ−, hydrogens δ+). The δ+ and δ− ends grip the charged parts of polar and ionic solutes — that is why water dissolves them.
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- Solvent
- A liquid that dissolves another substance to form a solution. In cells, the solvent is water.
- Solute
- A substance that dissolves in a solvent.
- Solution
- The mixture formed when a solute is dissolved in a solvent (e.g. salt dissolved in water).
Because water is polar (oxygen is δ−, the hydrogens are δ+), the charged ends of water molecules are attracted to other charged or polar particles.
When a substance like salt or glucose meets water, the water molecules surround its particles — the δ+ hydrogens face the negative parts and the δ− oxygen faces the positive parts. This pulls the particles apart and holds them in solution. We say the substance has dissolved.
How salt dissolves: water molecules surround each ion. The δ− oxygens point toward the positive Na⁺; the δ+ hydrogens point toward the negative Cl⁻. This pulls the ions apart and holds them in solution.
Interactive diagram
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| Type of substance | Examples | Dissolves in water? |
|---|---|---|
| Polar molecules | glucose, amino acids | Yes — water grips their charged parts |
| Ionic substances | sodium chloride, mineral ions (Na⁺, Cl⁻, Cu²⁺, NO₃⁻) | Yes — water surrounds each ion |
| Non-polar molecules | fats, oils, lipids | No — they have no charged parts for water to grip |
- Hydrophilic
- “Water-loving” — a polar or charged substance that dissolves in water.
- Hydrophobic
- “Water-fearing” — a non-polar substance (such as a fat) that does not dissolve in water.
- Mineral ion
- A charged particle of an element (e.g. Cu²⁺, NO₃⁻) that organisms need; it travels dissolved in water.
Two jobs that depend on water being a solvent: 1. Metabolism (reactions). The chemical reactions of life happen between solutes dissolved in water, so the reacting particles can move and meet.
2. Transport. Dissolved substances are carried around — in blood plasma in animals, and in xylem (water + minerals) and phloem (dissolved sugars) in plants.
Fats and oils are non-polar, so they do not dissolve in water and need a different way to be transported.
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How this is tested: Paper 1A (multiple choice) can ask you to identify that water dissolves polar or ionic solutes, or which substance will not dissolve.
Paper 2 likes a short Explain question that follows an element through a food chain — a mineral the plant takes up dissolved in water, which then passes to an animal that eats the plant. You have to link solvent → uptake → food chain.
IB-style question — a mineral shortage passed along a food chain
A field has very little copper in its soil. Cattle that graze only on grass grown in this field develop a copper deficiency. Copper is a mineral that animals can only obtain through their diet. Explain how the copper shortage in the soil leads to a copper deficiency in the cattle. [2]
How to score both marks
- Link the soil to the plant. Plants take up copper as mineral ions dissolved in water from the soil. With little copper in the soil, the grass takes up little copper, so the grass contains very little copper.
- Link the plant to the animal. The cattle obtain copper only by eating the grass (it passes along the food chain). Because the grass is low in copper, the cattle take in too little — so they become copper-deficient.
- Answer the command term (Explain). State the cause-and-effect chain clearly: low copper in soil → low copper taken up by grass → low copper passed to the grazing cattle → copper deficiency.
Final answer
Plants absorb copper as ions dissolved in soil water; little soil copper means the grass contains little copper; the cattle get copper only by eating that grass, so they take in too little and become deficient.
Why the solvent point matters here: The plant can only take copper up because it is dissolved in soil water — the element moves as mineral ions in solution. An Explain answer should make the cause → effect chain explicit, not just say 'there was no copper'.