QUESTION #157

# What is the density of water?

This is a very powerful question. The answer to it tells one a lot about us as a species that interacts with the world. The simple answer is that the density of water is one. That's it! Well, OK, actually I've given you the specific gravity of water. The actual density of water is one gram per milliliter. You can see that the specific heat of a substance is the same as its density without the units.

So, now that you know this, what does it tell you about us as a species? Well, we are the ones who said that the density of water is one gram per milliliter. How did we come to this decision? We had to first decide what a meter is so that we could know what a decimeter is so we could make a liter (a cubic decimeter) and then we could make a millimeter which we could fill with water and call it a gram! So, if you ever find yourself in a desert island you too could make a milliliter by getting a gram of water!

Of course it cannot stay this simple! You have a gram of water in one milliliter only if it is at 4 degrees centigrade. As you know materials expand and contract with temperature. Water's density is one gram per milliliter only at 4 degrees centigrade and at one atmosphere of pressure since pressure also affects the density of liquids.

So, in the end, if you ask what is the density of anything, you have to define its temperature and its location so that we can know the atmospheric pressure.
Answered by: Tom Young, B.A. Science teacher, Whitehouse High School

Although the density of water varies somewhat with temperature and pressure, and is higher for salt water than fresh water, you can use about 62 lb per cubic foot for its weight density in English units.

Water was used as the basis for establishing the metric unit of mass, however, so it is easier to remember that a cubic centimeter of it has a mass of 1 gm. Knowing that there are 1000 cubic centi- meters in a liter, you can also use 1 kilogram (1000 grams) per liter for water's mass density.
Answered by: Mr. Paul Walorski, B.A. Part-time Instructor