Introduction:
Liquefaction is the process of converting a gas into a liquid state. It is achieved by altering the conditions of temperature and pressure. It is important to note that not all gases can be easily liquefied, and the conditions required for liquefaction vary depending on the gas in question.

Explanation:
Increase in Pressure:
When the pressure exerted on a gas is increased, the gas molecules are forced closer together. This reduces the intermolecular space between the molecules, causing them to interact more strongly with each other. As a result, the attractive forces between the molecules become significant, and the gas molecules start to move slower. Consequently, the gas is converted to a liquid state.

Decrease in Temperature:
When the temperature of a gas is decreased, the kinetic energy of the gas molecules decreases as well. As a result, the motion of the molecules slows down, and the attractive forces between the molecules become more significant. At a certain temperature called the critical temperature, the intermolecular attractive forces become strong enough to overcome the kinetic energy of the molecules, causing the gas to condense into a liquid state.

Combined Effect of Pressure and Temperature:
Liquefaction of gases often requires a combination of increased pressure and decreased temperature. By manipulating both pressure and temperature, the gas molecules can be brought closer together and their kinetic energy can be reduced. This allows the intermolecular forces to dominate and cause the gas to liquefy.

Real-life Applications:
The liquefaction of gases has numerous practical applications. Some examples include:

1. Production of Liquefied Petroleum Gas (LPG): LPG, which consists of propane and butane gases, is commonly used as fuel for heating, cooking, and transportation. By subjecting these gases to high pressure and low temperature, they can be liquefied and stored in compact containers for easy transportation and use.

2. Cryogenics: In cryogenic applications, gases such as nitrogen, oxygen, and helium are liquefied at extremely low temperatures. This allows for the efficient storage and transportation of these gases, which are used in various scientific, medical, and industrial applications.

3. Natural Gas Processing: Natural gas, which primarily consists of methane, is often liquefied for transportation purposes. By cooling and compressing the gas, it can be converted into liquefied natural gas (LNG), which occupies significantly less volume and can be transported more easily.

Conclusion:
In conclusion, gases can be liquefied by increasing the pressure exerted on them. This causes the gas molecules to come closer together, leading to the formation of a liquid state. While a decrease in temperature can also contribute to the liquefaction process, a combination of pressure and temperature manipulation is often required to achieve successful liquefaction.