Hydrometer A hydrometer is an instrument used to measure the density of liquids. It is usually made of glass and consists of a cylindrical stem and a bulb weighted with lead shot to make it float upright.The hydrometer is gently lowered into liquid to be tested and allowed to come to rest floating freely.The point at which the surface of the liquid touches the stem of the hydrometer is read from a scale written on the stem.
The hydrometer is based on Archimedes’ principle.The less dense the liquid, the lower the hydrometer will sink. This is because more of the liquid has to be displaced before it equals the weight of the hydrometer.
LAWS AND PRINCIPLES
Boyle’s Law Pressure is inversely proportional to volume for a fixed mass of gas at constant temperature.
p r 1 V
for a fixed mass of gas at constant temperature.
This is equivalent to saying: Pressure by volume is a constant for a fixed mass of gas at constant temperature. pV k for a fixed mass of gas at constant temperature.
Hydrometer in use, measuring the density of a liquid
p1 initial pressure, V1 initial volume, p2 final pressure, V2 final volume p1V1 p2V2 BOYLE’S LAW
SAMPLE PROBLEM 4H
A gas syringe has 8 cm3 of air inside it, while held just above a tank of water.What volume of air will be in the syringe when it is lowered 2 m into the tank of water? You may assume the temperature of the gas in the syringe does not change.Atmospheric pressure is 1 013 hPa; density of water is 1 000 kg m3.
SAMPLE ANSWER 4H
Using Boyle’s law, you may leave volume in cm3 rather than change it to the standard unit of m3, but you must remember to give your answer in cm3 if you do this.
