Water as a solvent at Higher Level: The same statement as SL. At HL you will meet these ideas again inside membranes, enzymes and transport in the blood and the plant, so learn the mechanism here: polar water surrounds ions and hydrogen-bonds to OH groups; non-polar molecules give it nothing to hold.
Practise this as you read
- Explain every case with the bond: hydrogen bonds to a polar molecule, attraction to a charged ion, or neither.
- Pair a property with its function: soluble, so transported; insoluble, so stored or kept as a barrier.
- Quote a real case: urea in plasma, maple sap, a duck's preen oil.
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The big idea: Water is the solvent of life. Being polar, it dissolves an enormous range of solutes, and every reaction of life takes place in the solution that results.
The points to remember
- A solvent dissolves a solute to make a solution. In every living thing the solvent is water.
- Water is polar: a δ− oxygen and δ+ hydrogens. Its charged ends are attracted to charged or polar particles.
- Water molecules surround each particle, pull it away from its neighbours and hold it: the substance has dissolved.
- Hydrophilic substances (polar or charged) dissolve: glucose, amino acids, salts and mineral ions.
- A polar molecule such as glucose dissolves because its OH groups form hydrogen bonds with water.
Remember it as: Charged water dissolves charged things.
The root of it all: water is polar, with a δ− oxygen and δ+ hydrogens, so its ends grip the charged parts of other substances.
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A pinch of salt in water, and what the water molecules do to it:
Glucose has no full charges, only polar OH groups, yet it dissolves just as well. Draw why, and why oil cannot:
Real example: the Dead Sea: The Dead Sea holds about 340 g of dissolved salts in every litre, nearly ten times sea water. Every gram of it is ions wrapped in water molecules. So much is dissolved that a swimmer floats without trying.
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In early spring the sap of a sugar maple is about 2 % sucrose dissolved in water, moving through the trunk. Tap it and boil 40 litres down, and you have 1 litre of maple syrup.
The tree can move that sugar only because it is in solution. metabolism and transport both depend on water dissolving things first.
The points to remember
- Metabolism: most enzymes work in aqueous solution. The enzyme and the substance it acts on must be dissolved to move and meet.
- Transport in animals: blood plasma carries dissolved glucose, amino acids, ions, urea and hormones.
- Transport in plants: xylem sap carries water and dissolved mineral ions up; phloem sap carries dissolved sugar.
- Plants take up minerals only in solution: ions dissolved in the soil water enter the roots.
- Dissolve first. Nothing is carried, and nothing reacts, until it is in solution.
Remember it as: Dissolve first, then move, then react.
In animals: blood plasma
- About 92 % water
- Carries dissolved glucose and amino acids to cells
- Carries ions, hormones and the waste urea
In plants: sap
- Xylem sap: water and dissolved mineral ions, roots to leaves
- Phloem sap: dissolved sugar, leaves to the rest of the plant
- Minerals enter the root only in solution
Real example: urea in the blood: Urea, the waste from breaking down amino acids, is highly soluble (108 g in 100 g of water). Because it dissolves so readily, the plasma can carry it from the liver to the kidneys, where the enzymes and transporters that handle it all work in solution.
The same rule links the soil to the animals that graze on it:
Minerals travel in solution, all the way up a food chain: A plant takes up copper, or nitrate, only as ions dissolved in the soil water. An animal then gets the mineral by eating the plant.
So if the soil water holds little copper, the grass takes up little, and the cattle that graze it become copper-deficient.
The big idea: Being insoluble can be a job in itself. hydrophobic molecules keep water out, stay put as stores, and build barriers, while hydrophilic ones dissolve and travel.
The points to remember
- Hydrophobic means non-polar: no charged parts for water to grip, so it does not dissolve. Fats, oils and waxes.
- An oil molecule is a chain of carbon and hydrogen. It cannot hydrogen-bond with water, so oil molecules stay together, apart from the water.
- Fat stores stay as droplets: insoluble, so they do not dissolve away and do not upset the cell's water balance.
- A waxy cuticle on a leaf is hydrophobic: water cannot pass through it, so the leaf does not dry out.
- Phospholipids have hydrophobic tails that turn away from water: that is what makes a cell membrane a barrier.
Remember it as: No charges, no grip, no dissolving.
Measured at 20 C: how much of each substance 100 g of water can dissolve.
Real example: a duck's preen oil: A duck spreads hydrophobic oil from a gland near its tail over its feathers. Water cannot wet the oiled feathers, so the duck stays dry, warm and afloat. Wash the oil off and the feathers soak through.
When you explain why a lipid does not dissolve, give the reason, not the result:
Why oil does not dissolve: Do not write 'oil is lighter' or 'oil floats' as the reason. Floating is the result. The reason is that oil is non-polar: it cannot form hydrogen bonds with water, so water leaves it alone and the oil molecules stay together.
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How this is tested: Paper 1A: one mark for the substance that will not dissolve, or for matching 'soluble' to its use (transport).
Paper 2: a four-mark explain on why a sugar dissolves and a lipid does not, or a two-mark property-and-function pair: soluble, so transported in blood or sap.
Honey, which is mostly glucose and fructose, stirs into a cup of tea and vanishes. A piece of beeswax from the same hive floats on the tea unchanged. Explain why the sugars dissolve while the wax does not.
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