Objects will float better in salt water than in fresh water. Why? It is explained in the following paragraph.
The (in)ability of an object to float in any fluid* is called buoyancy. Several answers on ScienceLine already cover this topic, but I will summarize the salient points here.
When an object is placed into a fluid (for example, a swimmer in water), some of the fluid is displaced to make room for the object. The fluid pushes with a net upward force on the object, called the buoyant force. If that force is large enough, then the object floats; if not, the object sinks.
The force exerted by the fluid on the object is equal to the weight of fluid displaced, and the weight of the fluid is given by the equation
weight = density * volume.
Since salt water is denser than freshwater, for a given volume of water the salt water will have a greater mass, and therefore greater upward buoyant force. Since the upward force from the salt water is greater, objects will float better in salt water than in fresh water.
*Note that buoyancy is for any fluid, which encompasses both liquids and gases, not only liquids like water.
For additional details on the physics behind the buoyant force, check here and here.
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