The content and activity in this topic will work towards building an understanding of how salinity and temperature differences affect the density and buoyancy of water layers in the ocean.

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Density is a measure of how much mass there is in a given volume or amount of space. The density of any substance is calculated by dividing the mass of the matter by the volume of the matter.


In Fig. 2.2, volume is represented by boxes and individual particles of matter are represented by colored shapes.

Box A has five spheres.Box B is the same size, and has the same volume as box A, but box B has 10 spheres.Box C has the same mass as box A, with five spheres, but box C has a larger volume than boxes A and B.Box D has the same volume and number of green spheres as part A, but also includes other types of matter than the rest of the boxes—red circles and blue cubes.

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Image by Byron Inouye

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If the amount of matter is increased without changing the volume, then the density increases (Fig. 2.2 A to 2.2 B). If volume increases without an increase in mass, then the density decreases (Fig. 2.2 A to 2.2 C). Adding additional matter to the same volume also increases density, even if the matter added is a different type of matter (Fig. 2.2 A to 2.2 D).


Salinity Affects Density

When salt is dissolved in fresh water, the density of the water increases because the mass of the water increases. This is represented by the addition of red spheres and blue cubes to the box from Fig. 2.2 A to Fig. 2.2 D. Salinity describes how much salt is dissolved in a sample of water. The more salt there is dissolved in the water, the greater its salinity. When comparing two samples of water with the same volume, the water sample with higher salinity will have greater mass, and it will therefore be more dense.


Temperature Affects Density

The density of water can also be affected by temperature. When the same amount of water is heated or cooled, its density changes. When the water is heated, it expands, increasing in volume. This is represented by the increase in the size of the box from Fig. 2.2 A to 2.2 C. The warmer the water, the more space it takes up, and the lower its density. When comparing two samples of water with the same salinity, or mass, the water sample with the higher temperature will have a greater volume, and it will therefore be less dense.


Relative Density

In Fig. 2.3, the beaker of liquid models a body of water like the ocean or a lake. The bag of liquid simulates a layer of water. The relative density of the liquid in the bag compared to the liquid in the beaker can be determined by observing whether the bag sinks or floats.

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In Fig. 2.3 A, the bag rose to the top of the beaker and is now floating on the surface. The yellow liquid and the bag are less dense than the liquid in the beaker.In Fig. 2.3 B, the bag is floating in mid-water (subsurface floating). The orange liquid and the bag are equal in density to the liquid in the beaker.In Fig. 2.3 C, the bag sank to the bottom of the beaker. The green liquid and the bag are more dense than the liquid in the beaker.