Final answer:
Ideal gas behavior occurs at high temperatures and low pressure. A real gas deviates most from an ideal gas at low temperatures and high pressures, while gases are most ideal at high temperatures and low pressures.
Step-by-step explanation:
Ideal gas behavior occurs at high temperatures and low pressure. When a gas is put under high pressure, its molecules are forced closer together and the assumption that the volume of the particles themselves is negligible becomes less valid. When a gas is cooled, the decrease in kinetic energy of the particles causes them to slow down, making attractive forces between them more prominent. Therefore, a real gas deviates most from an ideal gas at low temperatures and high pressures, while gases are most ideal at high temperatures and low pressures.