Solvation at Higher Level: The same statement as SL. At HL it underpins solute potential: dissolved particles lower the water potential (4.6.8 and 4.6.10).
Practise this as you read
- Explain how ions and polar molecules dissolve in water.
- State which end of a water molecule faces a positive ion.
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The big idea: Solvation is water molecules surrounding each particle of a dissolved substance. It happens because water is polar.
The points to remember
- In a water molecule the oxygen is slightly negative and the hydrogens are slightly positive.
- Positive ions attract the oxygen ends of water molecules.
- Negative ions attract the hydrogen ends of water molecules.
- Polar molecules such as glucose form hydrogen bonds with water.
- Each dissolved particle ends up in a shell of water: it is solvated.
Remember it as: Opposite charges face each other.
Real example: sugar in phloem sap. Plants move sucrose from leaves to roots and fruits dissolved in phloem sap. Sucrose is polar, so it forms hydrogen bonds with water and stays in solution.
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What solvation allows: Because so many substances dissolve in water, water is the medium of life.
| Dissolves in water? | Examples | Why |
|---|---|---|
| Yes: ions | Sodium, potassium, chloride | Attracted to the charged ends of water molecules |
| Yes: polar molecules | Glucose, amino acids, sucrose | Form hydrogen bonds with water |
| No: non-polar molecules | Fats and oils | No charges, so water is not attracted to them |
Why it matters: Dissolved substances can be transported in blood plasma and phloem sap, and can react with each other in the cytoplasm.
How it is asked: Short answers on why ions and polar molecules dissolve in water, and 1A items on which end of a water molecule faces an ion.
IB-style questionExplain[3 marks]
Explain how sodium chloride and glucose dissolve in water.
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See the answer broken into clear points, with guidance on how each point earns marks.