The expression for entropy change, ΔS = nC_p.ln$$\left( {\frac{{{{\text{T}}_2}}}{{{{\text{T}}_1}}}} \right),$$  is valid for the __________ of a substance.

Actually the Entropy of an ideal gas is given by:
$$\Delta S = \int_{{T_i}}^{{T_f}} {} \frac{{{C_p}dT}}{T} - nR\ln \left( {\frac{{{P_f}}}{{{P_i}}}} \right).$$
$$ \Rightarrow $$  it depends upon both temperature and pressure.
Under constant pressure process we can write $$\Delta S = {C_p}.\ln \left( {\frac{{{T_f}}}{{{T_i}}}} \right).$$    The final temperature can be greater or lesser than the initial temperature.