Question

Indicate whether the following statements are true or false. Explain.

1. The specific enthalpy of an ideal gas is a function of temperature and pressure.
2. Air cannot be regarded as a pure substance.
3. Liquid alternatively referred to as compressed liquid because its pressure is less than atmospheric pressure.
4. The pressure and temperature are not dependently variable for a system contains a two-phase liquid–vapor mixture at equilibrium.

Answer 1

1). The specific enthalpy of an ideal gas is a function of temperature and pressure.

-False

The specific enthalpy of an idea gas is a function of temperature only.

As we know that idea gas follows the equation PV =nRT, that means product of pressure and volume(work) is a function of temperature only since R and n are constants.

We also know that internal energy is a function of temperature i.e u(t)

Now enthalpy h = u + Pv

This proves that enthalpy of an ideal gas is function of temperature only as u and Pv are also function of temperature only.

2). Air cannot be regarded as a pure substance.

- True, air cannot be regarded as a pure substance.

Pure substances are the one which are made up of only one kind of material, For e.g iron is a pure substance as it contains only iron molecules. As we no that air comprises of different gases like Oxygen, Nitrogen, Carbon dioxide etc. This gases are different altogether, thus air is not a pure substance.

3). Liquid alternatively referred to as compressed liquid because its pressure is less than atmospheric pressure.

False,

A liquid remains compressed when its pressure is above its saturation pressure(P_s). Once the pressure is reduced than atmospheric pressure, the liquid tends to boil earlier. (Considering temperature to be constant). Saturation pressure is the one at which the liqud starts boiling. There is a particular saturation pressure for each value of temperature.

For e.g at 99.97 degree celsius, saturation pressure is 1 atm (For water). If we increase pressure than 1atm, the liquid can be considered to be compressed. However if we decrease the pressure of liquid than 1 atm, It will no longer remain compressed and start to boil.

4). The pressure and temperature are not dependently variable for a system contains a two-phase liquid–vapor mixture at equilibrium.

False,

For a two phase mixture, pressure and temperature are dependent on each other. For e.g, considering example of wet steam(Water + Steam), At 1 atm pressure, the temperature of liquid-vapor mixture is 99.97 degree Celsius.

But for the same mixture present at 2 atm, the temperature at which the liquid vapor matter(wet steam) will come into existence is 120.628 degree celsius. Thus pressure and temperature are interdependent.