CHEM1001 Lecture Notes - Lecture 17: Sugar Alcohol, Vegetable Oil, Glycerol

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1 Aug 2016
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Lecture 17
When an object is placed in water, the object's relative density determines whether it floats or
sinks.
If the object has a lower density than water, it will float to the top of the water.
Cork has a density of 240 kg/m3, so it will float.
Air has a density of approximately 1.2 kg/m3, so it rises immediately to the top of a water
column.
The metals sodium and potassium will both float on water, while lead will sink.
The sugar alcohol glycerol will sink into the water and form a separate layer until it is
thoroughly mixed.
Vegetable oil will float on water, and no matter how vigorously mixed, will always return as a
layer on the water surface.
Even if the pressure is consistent, water's density will change based on the temperature.
Recall that the three basic forms of matter are solid, liquid and gas.
As a rule of thumb, almost all materials are more dense in their solid or crystalline form than in
their liquid form; place the solid form of almost any material on the surface of its liquid form,
and it will sink.
Water, on the other hand, does something very special: ice floats on liquid water.
Look carefully at the relationship between water's temperature and its density.
Beginning at 100 °C, the density of water steadily increases, as far as 4 °C. At that point, the
density trend reverses.
This table lists the densities of water at different temperatures and constant pressure.
The implications of this simple fact are enormous: when a lake freezes, ice crusts at the surface
and insulates the liquid below from freezing, while at the same time allowing the colder water
to sink to the bottom.
If ice did not float, it would sink to the bottom, allowing more ice to form and sink, until the
lake froze solid! Scuba divers and swimmers often encounter these water temperature
gradients, and they might even encounter a water layer at the very bottom of a lake with a
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