Why?

Objects are more buoyant in salt water than in fresh water because salt water has a higher density than fresh water. Because salt water is denser than fresh water the object displaces a greater weight of salt water and weighs less. The denser the liquid, the easier it is to float in.

Definitions

Buoyancy~

Buoyancy is the upward force that keeps things afloat. When placed in water, an object will float if its buoyancy is greater than its weight. And it will sink if its weight is greater than its buoyancy. When it floats it’s called positive buoyancy, when it sinks it’s called negative buoyancy, and when it neither floats nor sinks it’s called neutral buoyancy.

Density~

Look at these two boxes! Each box has the same volume, and each ball weighs the same. So which one is heavier? The box that has more balls has more mass per unit of volume. This is called density. Density is the weight of something in relationship to its volume.