Density & Seawater

Density can be a really hard idea to understand. To start, try thinking back to a time when you were in a swimming pool, and how hard or easy it was to float. Then think about floating in the ocean. It’s easier to float in the ocean than in a pool because ocean water has a higher density. As water gets more and more dense, it’s easier and easier for you to float in it.

So why is seawater so dense? And what changes the density of seawater?

1) Temperature. This is the big one. Cold water is denser than hot water. Remember drawing the molecules in solids, liquids, and gases? In liquids, the molecules aren’t as tightly packed as in solids, but they’re tighter than in a gas. High temperatures make molecules move faster, so they can’t stay close together. A hot liquid looks a little more like a gas, and cold liquid looks a little more like solid:2) Salinity. Salinity is a measure of how much salt is in the water. Salt makes water denser, but a change in temperature will have a bigger effect on density than a change in salinity will. The salt molecules keep the water molecules closer together by holding on to them through chemical bonds.

3) Pressure. Picture what each of those boxes I drew above would look like if you sat on them. All of the molecules would be pushed together! That’s what happens to the molecules in the seawater that’s down at the bottom of the ocean. The weight of all that water on top of them packs them tightly.

Here are plots of temperature, salinity, and density from our last station:
A few tips to help you understand these graphs:
- The units for pressure are “db” which stands for decibars. The cool thing is that in the ocean, a decibar of pressure is equal to a meter of water. So when you see 100 db, you know it’s 100 m down.
- The units for salinity are “psu.” That stands for practical salinity units, which doesn’t really mean anything at all. So don’t worry about it! Just remember that higher numbers mean saltier water.
- The units for density are kg/m³. That tells you how much one cubic meter of water would weigh. So when you see a density of 1030 kg/m³, that means that one cubic meter of water (about 260 gallons) weighs 1030 kg (about 2,270 pounds). For comparison, tap water at room temperature has a density of about 1000 kg/m³.

So if you were holding a gallon of tap water, it would weigh 8.3 pounds. But if you were holding a gallon of seawater, it would weigh 8.6 pounds.