This
is not how thermal properties of materials work. It
is a common misconception (and an urban myth) though.
When you say , you are saying two contradicting things.
Rapidity of "letting go of heat"
is thermal conductivity. In addition to that, association of any thermal property with
density is erroneous, as it has nothing to do with it:
The density of diamond (for example) is little more than a third of the
density of
copper (3.51 vs 8.96 kg/m3), yet the
thermal conductivity of natural
diamond is about 2200 W/(m·K), which is approximately five times greater than
copper. Isotopicallly pure
diamond has the highest
thermal conductivity of any known solid at room temperature at 3320 W/(m·K).
Specific heat of natural
diamond is 0.520 joule/gram K, which is closer to
copper at 0.385 joule/gram K than
aluminium at 0.870 joule/gram K, even though it's
density is much closer to
aluminium than to
copper.
The speed of heat transfer is defined by thermal conductivity. This is, for example, why, when you walk over ceramic, glass, stone or metal, it feels colder than walking over cloth, wood, polymers, etc. Materials with higher thermal conductivity conduct heat from your body faster, which makes them feel colder at the exact same temperature than materials with a lower value.
With the basics of
thermal conductivity out of the way, let us address and reintroduce the, often misunderstood, "elephant in the room", which is, of course,
specific heat, otherwise known as
thermal (or
heat)
capacity.
Specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. The temperature change is required to take effect without phase change, as heat released or gained during a phase change does not change the temperature.
The misconception is that because aluminium has a higher value than copper (0.870 vs 0.385 joule/gram K) it is better at releasing heat. This, of course is incorrect, as specific heat has no relation to the speed of heat transfer either way (gain or loss). Pure distilled, deionised, demineralised water has the highest specific heat value of the common substances at 4.186 joule/gram K . This is why water is the best liquid for any heat transfer applications where the operating conditions permit.
What this means is that a gram of water can hold the most heat before being saturated and thus elevates its temperature.
All of the above means that substances with higher specific heat value change temperature slower (with mass being equal), however, this is only because they can hold more heat per unit mass.
Fun fact: inhomogeneous systems that do not meet the strict definition of thermodynamic equilibrium, such as stars and galaxies have negative specific heat values. This property can result in negative temperature.
