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# What is observed during isothermal expansion of an ideal gas?

(a) Enthalpy remains constant
(b) Enthalpy decreases
(c) Internal energy increases
(d) Internal energy decreases

Toolbox:
• During Isothermal expansion of an ideal gas $\Delta E=0, \Delta T=0$
During isothermal expansion of an ideal gas, $\Delta E=0, \Delta T=0$
From the definition of enthalpy,
$H=E+PV$
or $\Delta H = \Delta E + \Delta (PV)$
or $\Delta H = \Delta E + \Delta (nRT)$ {Since, PV=nRT for an ideal gas}
or $\Delta H = \Delta E + nR \Delta T$
or $\Delta H = 0$