Why are you more buoyant in salt water?
The short answer
Buoyancy has to do with the density of you compared to the density of the water you're displacing. You're more buoyant in salt water because the water has a higher density due to the dissolved salt.
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 the feeling of increased buoyancy was noticeable. What is it about salty water that lifts our bodies 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 has to do with 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 push is buoyancy. There are three types of buoyancy:
Positively buoyant: The object's density is less than the density of the water it displaces. The object will float.
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.
Negatively buoyant: The object's density is greater than the density of the water it displaces. The object will sink.
Just now realizing the scale I created here. Enjoy the massive rubber ducky!
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 the 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.
So the added density of the salt water increases the force of buoyancy pushing you against gravity, letting you feel more floaty.
Here's a GIF I found on Wikipedia of some dude just chilling, enjoying a float in the Dead Sea.
Not all salt water is equal though. Famously, the Dead Sea has a much higher density because it's a lot saltier than other salt water: a cubic foot of water there weighs about 77.3 lbs!
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!).
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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
If you've ever wondered why your food tastes so good when it's cooked in oil, it's because of a little thing called heat transfer.