Floating

The density of a material governs whether it will sink or float in different liquids or gases.


So a wooden block floats in water while an iron one sinks. A hydrogen-filled balloon will rise in the air but one filled with carbon dioxide sinks to the ground.
This is best explained by Archimedes principle.



A ship will float because the weight of water displaced is equal to the weight of the ship. The ship will sink deeper into the water until this is true.


The density of the liquid is also very important. A ship will float lower in low density fresh water than it will in higher density salt water. The weight of liquid displaced each time is the same - the weight of the ship itself.

HYDROMETER

This property is also used to measure the density of a liquid using an instrument called a HYDROMETER. This is a glass tube with a scale on the side weighted at the bottom with lead shot. The hydrometer floats in the liquid and if the liquid is dense the hydrometer does not need to sink very low to displace its own weight of liquid. If the liquid has a lower density it will sink much deeper.
Hydrometers are used to check the density of car battery acid, beer, milk and wine. Two types are drawn below.