The short answer

Buoyancy has to do with the density of an object compared to the density of the fluid it’s in. You are more buoyant in salt water because the water has a higher density due to the dissolved salt, making it easier for you to float.

The long answer

Have you ever noticed that it is easier to float in an ocean than in a lake?

I was recently swimming in a particularly salty slice of ocean, and I felt a noticeable difference in buoyancy. So, what is it about salt water that makes our bodies float more easily?

First let's dive into (pun intended) how buoyancy works:

What is buoyancy and how does it work?

Buoyancy is the ability for an object to float in a fluid, most commonly water. Whether something floats depends on the density of the object compared to the density of the fluid.

When you place an object in water, gravity pulls it down, but at the same time, the water pushes back up against the object. This upward push is what we call buoyancy.

There are three types of buoyancy:

1. Positively buoyant: The object's density is less than the density of the water it displaces. The object will float.

2. Neutrally buoyant: The object's density is exactly equal to the density of the water it displaces. The object will suspend in the water and neither sink nor float.

3. Negatively buoyant: The object's density is greater than the density of the water it displaces. The object will sink.

So both the density of the object (you) and the density of the fluid (water) matter in terms of how positively buoyant you are while swimming. Keep this in mind as we tackle the key question:

Why are you more buoyant in salt water?

The answer comes down to the increased density of salt water.

Salt water = salt + water. Adding salt to water increases the density of the fluid. A cubic foot of salt water weighs (on average) about 64.1 lbs, while a cubic foot of fresh water only weighs around 62.4 lbs.

Adding salt to water increases the density of the fluid. For example, a cubic foot of salt water weighs (on average) about 64.1 lbs, while a cubic foot of fresh water only weighs around 62.4 lbs. The added density of salt in the water increases the force of buoyancy that pushes you against gravity, making you feel more buoyant and "floaty.".

Not all salt water is equal though. Famously, the Dead Sea has a much higher density because it is a lot saltier than other bodies of salt water: a cubic foot of water there weighs about 77.3 lbs! This extreme density is why people float so effortlessly in the Dead Sea.

Temperature also affects the density of water. Cold water is denser than warm water, so you'll float more easily in frigid waters (but hopefully you also float more easily because you're wearing a wet suit!).

In conclusion, buoyancy depends on the density of both the object and the fluid. Since salt water is denser than fresh water, you are more buoyant in it.

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Sources

Gibb, N. (2018, June 10). Buoyancy in Salt Water vs Fresh Water. TripSavvy. https://www.tripsavvy.com/buoyancy-salt-water-vs-fresh-water-2962936

Petrone, C. J. (n.d.). Buoyancy. BRIDGE Ocean Education. https://masweb.vims.edu/bridge/datatip.cfm?Bridge_Location=archive1207.html