The big idea: Lipids are the molecules of life that dissolve in non-polar solvents but hardly at all in water.
The points to remember
- A lipid dissolves in non-polar solvents (such as hexane) but only sparingly in water.
- So lipids are hydrophobic: water-repelling.
- Fats (solid at room temperature) and oils (liquid) are lipids.
- Waxes and steroids (such as cholesterol and testosterone) are lipids too.
Fats and oils
- Butter is a fat: solid.
- Olive oil is an oil: liquid.
Waxes
- Beeswax in honeycomb.
- The waxy cuticle on leaves.
Steroids
- Cholesterol in membranes.
- Testosterone, a hormone.
Real example: a cabbage leaf: Rain forms round drops on a cabbage leaf and rolls off. The leaf is covered in a waxy cuticle, a lipid that water cannot wet.
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The big idea: Lipids are non-polar, so they cannot form hydrogen bonds with water. Water keeps them out.
The points to remember
- Lipids are mostly long chains of carbon and hydrogen, which are non-polar.
- Non-polar molecules have no charges, so they cannot form hydrogen bonds with water.
- Water molecules hydrogen-bond to each other instead and push the lipid out.
- So lipid droplets join together and float on water as a separate layer.
Remember it as: Non-polar, no hydrogen bonds with water: hydrophobic.
Polar (hydrophilic)
- Glucose, salts, amino acids
- Form hydrogen bonds or attract water
- Dissolve in water
Non-polar (hydrophobic)
- Fats, oils, waxes, steroids
- No hydrogen bonds with water
- Dissolve in non-polar solvents
Sparingly, not never: Lipids are only sparingly soluble in water: a very little dissolves. Say "insoluble or only sparingly soluble in water, soluble in non-polar solvents".
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How it is asked: No paper has set this statement on its own so far. It comes up inside longer lipid questions, so practise a short answer in the Paper 2 style.
IB-style questionOutline[2 marks]
Outline why lipids such as triglycerides are hydrophobic.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.