1. The problem statement, all variables and given/known data A column of water 40 cm high supports a 30 cm column of an unknown liquid.What is the density of the liquid? 2. Relevant equations 3. The attempt at a solution As per the instruction of the teacher I equated their pressures. h1ρ1g=h2ρ2g. And hence arrived at the answer. But I don't understand why we are doing this.Why should the pressures of both the columns be equal? I am trying to think of what would happen if they weren't equal but I can't get it. The unknown liquid column will be acted on by atmospheric pressure at the top. What will the pressure at the interface of the 2 liquids be?