Specific heat capacity

$C_p$ and $C_V$.

When the matter is heated, there are two conditions. First, when the matter is free to expand, the matter expands its volume instead of increasing its pressure. Examples are seawater and the Earth’s atmosphere. In this situation, the heat capacity is $C_p$, under constant pressure. Second, when the matter is heated and the volume is limited like material in a metal can, the relevant heat capacity is $C_V$, not $C_p$.

FYI: water $C_p$ ≈ 4× air $C_p$.