Which one is heavier 10 gram of sodium or potassium?

DENSITY Gradually increases in moving down from Li to Cs. Because the increase in mass dominates the increase in volume.
But Potassium is less dense when compared to Sodium. The reason is, the unusual increase in atomic size i. e., atomic volume. This happens because of the addition of empty 3d orbitals. This increases the atomic volume of the atom. Hence we see that anomaly.

Answerδh > 0explanationc₆h₆  (s)  → c₆h₆ (l)if you are changing it from a solid to a liquid, you are pretty much melting the solid. this requires heat energy. therefore, the reaction is endothermic (takes in energy); the system enthalpy increases as the reaction proceeds: δh > 0.