The density of any substance is the mass (in kg) of the substance posessed by unit volume( 1 cubic meter) of the substance. The Density of Water is approximately 1000Kg/m3. Hence converted in litre, density of water becomes 1 Kg/Litre. hence mass of 1 L of water is 1 kg.
https://www.quora.com/Why-is-it-that-1-litre-of-water-weighs-1-kg-while-this-doesnt-apply-for-other-liquids
https://www.quora.com/Why-is-it-that-1-litre-of-water-weighs-1-kg-while-this-doesnt-apply-for-other-liquids
