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: salt in your blood. Sodium and chloride ions are carried dissolved in blood plasma. Each ion travels inside its own shell of water molecules.
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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.
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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.