Milk and Cream

You are given two identical bottles, one with milk and the other with cream, both filled to the top. Quick now, which is heavier? And is light cream lighter than heavy cream?

Answer

The milk is “heavier,” meaning more dense. Cream floats to the top in a milk cream mixture, so the cream is less dense. Quite often people confuse mass density with liquid flow sluggishness.

The two properties are unrelated. Many of these people will think that the cream is more dense. Anyone who has milked a cow or a goat has seen the cream at the top. A pint of light cream weighs about 1.5 grams more than a pint of heavy cream on average. Why? Because heavy cream has more fat per volume, and fat is less dense than water.

Separation of materials by density has been an important method for millennia. Gold and platinum have been separated from other elements by dumping the ores into a hot bath of lead. The specific density of lead is 11.36, of gold 19.32, and of platinum 21.45, so gold and platinum atoms sink and practically all other elements and compounds float. Of course, the workers must not breathe in the lead atoms in the vapors.