**Kedar Kulkarni**

After learning to find the density of solids materials – those which sink and those which float on water – students were curious to find out the density of liquids. They knew that oil floating on water meant the oil is lighter than water and hence the density of the oil must be less than the density of water.

They were eagerly waiting for the next experiment that the teacher was going to show them. The question that bothered them was ‘how does one weigh liquid?’

Raju said: “Teacher weighed the oil by pouring it into a plastic bag. Let us measure the volume of oil using the measuring jar.”

Anthony was not convinced and said: “Some oil will stick to the bag, and the volume will not be accurate.” The teacher agreed.

Students voiced various suggestions as the teacher listened to all of them.

Mira stood up and said: “Teacher, what if we measure the empty measuring jar first, and then pour oil in it to 500 ml and weigh it again. We will get the combined weight of jar and oil. If we subtract the weight of the empty measuring jar we will get the mass of the oil. Now that we know the mass and the volume, we can easily find the density of the oil.”

Teacher was very happy and said: “Excellent Mira, that is the easiest way to find the density of oil and that is what we are going to do.” They then found the density of the oil. It was 0.92 g/cc.

The density of this oil is less than 1. It is less than that of water which is why it floats on water. This is also why petrol floats on water.

There is one more way to measure the relative density of liquids. The instrument used for the same is called a hydrometer. It uses the Archimedes principle. It will sink deeper in a liquid which has a lower density than one with a higher density. It is made of a long glass tube stem with a bulb at the other end and has lead for stability. A long jar of appropriate diameter is also used. The liquid to be tested is poured in the jar, and the hydrometer is lowered in this liquid. Depending on the density of the liquid the hydrometer will sink. The point where the liquid surface touches the long stem has a calibrated scale that indicates the numerical value of the density.

(Writer is a mechanical engineer and runs a hands-on science activity centre at Margao)