The **Dulong–Petit law**, a chemical law proposed in 1819 by French physicists Pierre Louis Dulong and Alexis Thérèse Petit, states the classical expression for the molar specific heat capacity of a crystal. Experimentally the two scientists had found that the heat capacity per weight (the mass-specific heat capacity) for a number of substances became close to a constant value, *after* it had been multiplied by number-ratio representing the presumed relative atomic weight of the substance. These atomic weights had shortly before been suggested by Dalton.

In modern terms, Dulong and Petit found that the heat capacity of a mole of many solid substances is about 3*R*, where *R* is the modern constant called the universal gas constant. Dulong and Petit were unaware of the relationship with *R*, since this constant had not yet been defined from the later kinetic theory of gases. The value of 3*R* is about 25 joules per kelvin, and Dulong and Petit essentially found that this was the heat capacity of crystals, per mole of atoms they contained.

The modern theory of the heat capacity of solids states that it is due to lattice vibrations in the solid, and was first derived in crude form from this assumption by Albert Einstein, in 1907. The Einstein solid model thus gave for the first time a reason why the Dulong–Petit law should be stated in terms of the classical heat capacities for gases.

Read more about Dulong–Petit Law: Equivalent Forms of Statement of The Law, Application Limits, Derivation

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**Dulong–Petit Law**- Derivation

... Thus we have Using energy we have This gives specific heat which is independent of the temperature. ...

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