Chemistry · Rung 14 of 24

Water, the solvent

Water is a bent molecule, and the oxygen pulls harder on the electrons than the hydrogens do, so one side ends up slightly negative and the other slightly positive even though the whole molecule is neutral. Almost everything follows from that tiny asymmetry. The rule is “like dissolves like”: water, being polar, dissolves polar and ionic things — salt, sugar, alcohol — and rejects non-polar ones — oil, wax, petrol. And it is not a matter of stirring harder: water molecules hold on to each other better than they would to oil, and push it out. Soap solves that by having two halves, a non-polar tail that buries itself in grease and a polar head that stays in the water: it does not dissolve grease, it wraps it and carries it off. The same thing explains why water boils at a hundred degrees when its size says it should boil far below, why ice floats, and why almost everything that moves inside a living thing travels dissolved.

Where this sits

Comes after
Bonds

Exercises from this rung

Generated by the app, from easiest to hardest

  1. What does it mean that water is polar?

    • That one side is slightly positive and the other slightly negative
    • That it conducts electricity on its own
    • That it freezes at a very high temperature
    • That the whole molecule carries an electric charge

    Why That the molecule is bent and the oxygen pulls harder on the electrons than the hydrogens do, so one side ends up slightly negative and the other slightly positive, even though the whole molecule is neutral. Almost everything strange about water comes from that tiny asymmetry: that it dissolves salt, that it boils at a hundred degrees when it should boil far below, and that ice floats.

    easier

  2. Why does soap remove grease and plain water does not?

    • Because it has one polar end and one non-polar end
    • Because it makes the water hotter
    • Because it dissolves grease the way alcohol does
    • Because it reacts with the grease

    Why A soap molecule is a thing with two halves: a non-polar tail that buries itself in grease and a polar head that stays in the water. They surround the grease droplet with tails in and heads out, and now the water can carry it away. It does not dissolve grease: it wraps it and drags it off.

    in between

  3. How do minerals from the soil reach a tree's leaves?

    • Pushed by the wind
    • As a gas
    • The leaf makes them itself
    • Dissolved in water

    Why Dissolved in water, which is what makes water the transport system of almost all life. A solid mineral cannot travel up a tube a tenth of a millimetre wide; dissolved, it can. The same happens in blood and in soil: almost nothing that moves inside a living thing travels any other way.

    harder