Is 1 kg the same as 1 liter?

Not in all cases. The litre is equal 1/1000 m3. This volume of water = 1 kilogram of water at 4oC (the maximum density of water). A litre of other liquids will not necessarily have a mass of one kilogram, as examples a litre of pure alcohol will have a mass of about 0,8 kg and liter of Mercury (Hg) will have amass of 13,6 kg.