During summer, though the surrounding is hot, the water in the lake will not reach very high temperature mainly due to its high specific heat capacity. In general, cool water sinks as it is denser and warm water rises as it is less dense, the bottom of the lake will always be lower temperature compared to near the surface. The bottom of the lake may not necessary be 4 degree Celsius.
Density is the mass per unit volume.
Common units for density are g/cm3 or kg/m3. SI unit is kg/m3.
Density of a pure substance is fixed, regardless of it’s size!
Floating and Sinking
A denser (heavier) substance will sink in liquid which is less denser (lighter).
A less dense substance will float in a liquid which is denser.
If substance has the same density as the liquid, it will be suspended in the liquid.