A liquid in a thermally insulated vessel receives heat at a constant rate of $30\,\text{W}$ from an electrical heater. The temperature of the liquid becomes constant at $50\,^\circ\text{C}$ while the surrounding temperature is $20\,^\circ\text{C}$. After the heater is switched off, the liquid cools from $40\,^\circ\text{C}$ to $39.9\,^\circ\text{C}$ in $10\,\text{s}$. The heat capacity of the liquid is