Pure Substances

Question 1

Define a pure substance and elaborate on what is a homogeneous and non-homogeneous mixture

Answer 1

Pure substances is a fixed chemical composition throughout more than one phase and can also be a homogeneous mixture

Homogeneous with more than one phase may not be pure substances air

Non-homogeneous mixture are not pure substances (oil, water)

Question 2

Explain the different phases of a pure substance and elaborate its characteristics

Answer 2

Solid
> molecules are arranged in a three-dimensional pattern
> fixed vol

Liquid
> molecular spacing is not much different
> not in fixed vol

Gas
> molecules are far apart
> order dont exist

Question 3

For a closed system comprising of a pure substance, explain the sequence of substance

Answer 3

Closed system of a pure substance heated at constant pressure

> state 1 (compressed liquid - liquid)
> state 2 (saturated liquid - liquid)
> state 3 (saturated liquid - vapour)
> state 4 (saturated vapour - vapour)
> state 5 (superheated vapour - vapour)

Question 4

Define saturation temperature and saturation pressure for a pure substances

Answer 4

Saturation temperature refers to temp where substance changes phase at a given pressure

Saturation pressure refers to where substance changes phase at a given temp

Question 5

What are the characteristic of the critical point

Answer 5

Critical point at which the saturated liquid and saturated vapour states are identical

Question 6

Define enthalpy and specific enthalpy and explain why it is considered a combined property

Answer 6

Enthalpy is a combination of the properties of internal energy plus pressure times volume. Extensive property.

Specific enthalpy of humid air is a calculated property combining the sensible and latent heat of 1 kg of dry air plus water vapour

H = U + PV occurs freq in open systems

Question 7

Define quality x of a pure substance

Answer 7

Quality x is used to determine the state in two-phase region