Q1.
Have you noticed that some packets of ghee or oil are labelled with a volume of 1 litre but a weight of only say 910 grams (Fig. 9.11)? What does this tell us about the density of the oil, and is it less or more than that of water?
Answer
It tells us the oil's density directly — and it is less than that of water.
Volume of oil = 1 litre = 1000 mL
Mass of oil = 910 g
Density = Mass / Volume
Density = 910 g ÷ 1000 mL
Density of oil = 0.91 g/mL
Density of water ≈ 1 g/mL, so relative density of oil = 0.91 ÷ 1 = 0.91 (no unit)
Mass of oil = 910 g
Density = Mass / Volume
Density = 910 g ÷ 1000 mL
Density of oil = 0.91 g/mL
Density of water ≈ 1 g/mL, so relative density of oil = 0.91 ÷ 1 = 0.91 (no unit)
0.91 g/mL is less than 1 g/mL, so the same 1 litre of water would weigh about 1000 g while the oil weighs only 910 g. Since the oil is lighter for the same volume, oil floats on water — exactly what you see when a drop of oil is added to a glass of water.
Why it happens: The label is not saying the packet is short of oil. It is a full litre; oil simply packs less mass into each millilitre than water does, because its particles are bulkier and cannot be as closely packed. Mass and volume are two different quantities, and it is their ratio — the density — that decides floating and sinking.
Check it yourself: Look at a 1 litre milk packet. It will say about 1030 g, a little more than a litre of water, because milk carries dissolved sugar, salts and proteins. Milk therefore sinks in water; oil floats on it.