STPM Chemistry Form 6 Notes – Terminology
and Concepts: Liquid and Solid States (Part 5 –
Final)
Summary: Definition of the states of matter
State
Shape of substance Volume of substance
Solid
Definite
Definite
Liquid
Indefinite
Definite
Gas
Indefinite
Indefinite
Phase – refers to a single homogeneous physical state of a heterogeneous system. There are
three phases with the same composition solid, liquid and gas.
Triple point – the point of a condition of temperature and pressure at which the solid,
liquid and vapour phases exist simultaneously at equilibrium.
Critical point – is the highest temperature and highest pressure at which there is a
difference between liquid and vapour states. At either a temperature or a pressure over the
critical point, only a single fluid state exists, and there is a smooth transition from a dense,
liquid-like fluid to a tenuous, gas-like fluid/or pressure that is required to liquefy a gas at its
critical temperature.
Supercooling – metastable condition where a liquid can exist below its freezing point.
Phase Diagrams
In laboratory, experiments are being carried out on two environmental factors which is
temperature and pressure (referred to as independent variables).
A) The Phase Diagram of Water
- ice (solid), water (liquid) and water vapour / steam (gas)

Vapour Pressure Curve
- critical point = critical temperature (374˚C) and critical pressure (200 atmospheres)
- temperature above 374˚C and critical pressure 200 atmospheres, the vapour and
liquid are indistinguishable (no longer two separate phases) because the densities of the
gas and liquid are equal (meniscus separating a liquid from its vapour disappears).

Melting Temperature Curve
- melting temperature point decrease with pressure- supercooling is the cooling of a
liquid to below its freezing point without a change taking place from the liquid to the
solid state. A phenomenon (metastable condition) shows the vapour pressure of water
below its freezing point.

Triple Point
- Water triple point is at temperature 0.01˚C and pressure 0.006 atm (610 N m-2). All
the three phases (ice, water and water vapour) coexist at equilibrium.

Normal Melting Temperature Point
- the temperature at which both the solid and the liquid states of the substance exist
inequilibrium at a pressure of 1 atm (101 kNm-2)

Normal Freezing Temperature Point
- the temperature at which both the liquid and the solid states of the substance exist
inequilibrium at a pressure of 1 atm (101 kNm-2)
Unsual Behaviour of Water

i) Why ice can float?
- the volume of water increase when the change of phase from liquid to solid.
Reasons: Ice (solid) has an open structure (hydrogen bond).

ii) Why the melting temperature curve slopes to the left (melting point decreases with
pressure)?
- (In most of substances (except water), an increase in pressure will push the molecules
even closer / Increase in pressure favours the physical state which is higher density)
Reasons: Increasing the pressure favours the formation of liquid water due to the
latent heat of fusion is absorbed from the surroundings during melting.
B) The Phase Diagram of Carbon Dioxide
- solid carbon dioxide (dry ice), liquid carbon dioxide and gas carbon dioxide

Vapour Pressure Curve
critical point = critical temperature (374˚C) and critical pressure (217 atmospheres)
- temperature above 374˚C and critical pressure 217 atmospheres, the vapour and liquid
are indistinguishable because the densities of the gas and liquid are equal. At this point,
carbon dioxide gas can be liquefied.

Melting Temperature Curve
- melting temperature point increase with pressure
- melting temperature curve slopes to the right
- density of dry ice (solid carbon dioxide) is higher than the density of liquid carbon
dioxide. It is because the carbon dioxide molecules are held closer together (smaller
volume).
- Increasing the pressure favours the formation of solid carbon dioxide due to the latent
heat of fusion is liberated (given out) to the surroundings.

Triple Point
- Carbon dioxide triple point is at temperature -57˚C and pressure 5.1 atm. All the three
phases (solid, liquid and gas) coexist at equilibrium.

Normal Sublime Temperature Point
- the temperature at which both the solid and the gas states of the substance exist
inequilibrium at a pressure of 1 atm (101 kNm-2)
- at atmospheric pressure, solid carbon dioxide (dry ice) sublimes to form carbon
dioxide gas at -78˚C.
Important Points of Water Phase Diagram and Carbon Dioxide Phase Diagram
Water
Phase Diagram ice, water and water
vapour / steam
Carbon dioxide
solid carbon dioxide
(dry ice), liquid carbon
dioxide and gas carbon
dioxide
Vapour
Critical Point = 374˚C
Critical Point = 374˚C
Pressure Curve and 200 atmospheres
and 217 atmospheres
Triple Point
0.01˚C and 0.006 atm
-57˚C and 5.1 atm
Melting
Increasing the pressure Increasing the pressure
Temperature
favours the formation of favours the formation of
Curve
liquid water
solid carbon dioxide