Practical: Liquid levels in connected vessels
Water and vegetable oil do not mix and oil is lighter than water. Pour water into one side of a U-tube and then oil into the other. The oil pushes the water down on its side and up on the other. At balance the pressures are equal at the boundary level: ρ_water × g × h_water = ρ_oil × g × h_oil. Both heights are measured from the horizontal level where the oil–water boundary stands (from the boundary to the top of the oil, and from that same level to the top of the water). From this ρ_oil = ρ_water × h_water ÷ h_oil. Because oil is lighter, its column is taller and the calculated ρ_oil comes out below 1000 kg/m³ (about 900–920 kg/m³ for vegetable oil).
Practical: make a U from clear plastic hose and fix it upright to a stand or books. Pour in water first and then carefully add oil through a funnel. With a ruler, measuring from the boundary level, record h_water and h_oil in 3 trials with different amounts of oil and calculate the oil’s density. Safety: oil is slippery – wipe spills at once with paper towels and tell the teacher; use plastic hose instead of glass tubing; drink or taste nothing; wash hands with soap afterwards.