Why Is Sea Water Blue in Color & Tap Water Clear in Color?

Water color depends on several factors.

Looking across the ocean on a sunny day, you will see what appears to be a blue expanse of water. However, if you fill a glass with tap water, it appears clear. There are many factors that affect water color, including the volume of liquid and how many particles are present.

Volume

One major difference between looking at the ocean and looking at a glass of water is that there is more water in the ocean. If you scoop up and filter a glass of ocean water, it will appear clear. The larger the body of water, the more light it can absorb and reflect for us to see. A glass does not have the necessary volume of water to produce a color that our eyes can see.

Wavelengths

The huge volume of ocean water absorbs wavelengths of light. Water molecules reflect back only a fraction of the wavelengths they take in. One wavelength the molecules scatter and reflect back is the long blue variety. Our eyes see this reflection as the ocean being blue. A glass of water does the same thing, but it is not deep or large enough for our eyes to detect a color difference.

Particles

Tap water has very little particulate matter in it, while ocean water has plant pieces, dirt and other particles. These particles also absorb and reflect certain wavelengths and affect how blue the ocean appears.

References

• Library of Congress: Everyday Mysteries: Why is the Ocean Blue?
• Causes of Color: What Color is Water?