Float Race

The float race, or what is viscosity?

Why does each float move at a different speed when all three cylinders are filled with oil? Because each cylinder holds oil of a different viscosity.

Viscosity is often confused with density, but they are two different things:

• Density tells you how much a substance weighs.
• Viscosity tells you how readily a liquid flows. The higher the viscosity, the more the liquid resists the float’s movement.

What would the race look like with honey, water and chloroform in the cylinders? Both chloroform and honey are considerably denser than water (a litre of either weighs noticeably more than a litre of water). Their viscosities, however, differ greatly. Honey is several thousand times more viscous than water, while chloroform is almost half as viscous. In honey the float would sink incredibly slowly. In chloroform it would sink fastest of all, even though chloroform is denser than water — it would win our race!

And why do we not use honey, water and chloroform? With honey you would wait an eternity for the result, and sugar crystals would soon form. Water could freeze in winter and burst the cylinders. Chloroform may be the sprinter, but it evaporates so easily that it would soon vanish from the cylinder.

Viscosity changes with temperature. Heat the oil and its viscosity drops sharply, so the float speeds up — while the density (weight) barely changes. That is why the floats sink noticeably faster in summer than in winter. Come back in a different season and see the difference!