Concept:
At constant temperature, Boyle’s law is applicable:
\[
P_1V_1 = P_2V_2
\]
Step 1: Write given values.
At NTP:
\[
P_1 = 1~atm
\]
\[
V_1 = 2.5~dm^3
\]
New pressure:
\[
P_2 = 1.25~atm
\]
\[
V_2 = ?
\]
Step 2: Apply Boyle’s law.
\[
P_1V_1 = P_2V_2
\]
\[
(1)(2.5) = (1.25)V_2
\]
\[
V_2 = \frac{2.5}{1.25}
\]
\[
V_2 = 2.0~dm^3
\]
Step 3: Find change in volume.
\[
\Delta V = V_1 - V_2
\]
\[
\Delta V = 2.5 - 2.0
\]
\[
\Delta V = 0.5~dm^3
\]
Step 4: Conclusion.
Hence, the change in volume is:
\[
\boxed{0.5~dm^3}
\]