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The density of a fluid plays a fundamental role in how objects behave when submerged. Fluid density is essentially the mass of the fluid per unit volume, often measured in kilograms per cubic meter (kg/m³). It indicates how compact or concentrated the mass is within the fluid.
If a fluid is denser, it means more mass is contained within a smaller space. For instance, saltwater is generally denser than freshwater because it contains dissolved salts, increasing its mass per unit volume. When a fluid has higher density, it exerts more upward pressure or buoyancy on objects within it.
This concept is important for understanding why the same object might float higher in one fluid compared to another. In denser fluids, like saltwater, an object will displace less volume to achieve the same buoyant force, allowing it to float higher.

Buoyant force is the upward force exerted by a fluid on an object submerged in it. This force is what keeps objects afloat and is responsible for the phenomenon of buoyancy. The reason behind this force is the pressure difference exerted by the fluid on the object.
According to Archimedes' principle, the buoyant force acting on an object is equal to the weight of the fluid that the object displaces. For example, when a ship is in water, it pushes aside a certain amount of water. The weight of this displaced water equals the buoyant force pushing up.
Factors affecting buoyant force include:

• Volume of the displaced fluid: Larger objects displace more fluid, increasing the buoyant force.
• Density of the fluid: Denser fluids exert a stronger buoyant force for the same volume displaced.

In practice, if a ship moves from freshwater to saltwater, the change in buoyant force will make it ride higher in the saltwater due to the increased fluid density.

One of the key differences between freshwater and saltwater is their density. Freshwater, coming from lakes and rivers, has fewer dissolved particles and salts compared to the ocean. Because of this difference, freshwater has a lower density than saltwater.
This density difference has practical implications for floating objects like ships. In freshwater, a ship must displace more water to achieve the same buoyant force it easily obtains in saltwater. This results in the ship sitting lower in the water in a freshwater lake compared to the ocean.
The contrast can be summarized as follows:

• Saltwater: Higher density, increasing buoyant force, allowing ships to float higher.
• Freshwater: Lower density, requiring more displacement, meaning ships float lower.

So, when comparing the two, ships will ride higher in the ocean due to saltwater's greater density, resulting in a greater buoyant force with less water displacement.