Density doesn't decide what mixes with water
- The idea
- Predict
- See it
- Why
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The idea
“Whether something dissolves or mixes in water depends on whether it's heavier or lighter than water.”
You've watched oil float on water your whole life, so density seems like the reason the two don't mix.
The short version
The mistake: Whether something dissolves or mixes in water depends on whether it's heavier or lighter than water.
What's actually true: Water attracts polar and charged things, so polarity decides what dissolves, mixes or wets. Density only sets which layer floats.
- Water is polar. Its oxygen is partly negative and its hydrogens partly positive. Polar molecules like sugar hydrogen bond with water and dissolve.
- Oils and fats are nonpolar and hydrophobic. Water molecules hydrogen bond to each other and crowd the oil into a layer that floats.
- Wetting works the same way. Water spreads on glass, which attracts it. That's adhesion. On nonpolar wax, cohesion wins and water beads up.
On the exam: Dissolving, mixing or wetting? Ask what attracts what. Density only tells you which layer is on top.