STPM Chemistry Form 6 Notes – Terminology and Concepts: Liquid and Solid States (Part 5 – Final)

by BerryBerryTeacher

in Berry Reference (Notes)

Berry Berry Easy would like to present the final part on long-running topic of “Liquid and Solid States” for STPM Form 6 Chemistry. Despite being the final part in the series, this is certainly the most important part in the whole chapter due to the berry important phase diagram, triple point (uniqueness of three phases coexisting), the uniqueness of water (be thankful that water had the properties it had or else we would be living today). The aforementioned subtopics are rather popular in topical tests and major examinations. So make sure you understand them fully. Do also make use of the super summary table at the of the post. (Remember to draw out the phase diagrams)

STPM Chemistry Form 6 – Terminology and Concepts: Liquid and Solid States (Part 5)

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.

Supercoolingmetastable 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 in equilibrium 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 in equilibrium 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 in equilibrium 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.

Berry Important Points of Water Phase Diagram and Carbon Dioxide Phase Diagram

Water Carbon dioxide
Phase Diagram ice, water and water vapour / steam solid carbon dioxide (dry ice), liquid carbon dioxide and gas carbon dioxide
Vapour Pressure Curve Critical Point = 374˚C and 200 atmospheres Critical Point = 374˚C and 217 atmospheres
Triple Point 0.01˚C and 0.006 atm -57˚C and 5.1 atm
Melting Temperature Curve Increasing the pressure favours the formation of liquid water Increasing the pressure favours the formation of solid carbon dioxide

Now we have reached the conclusion of this topic. Be sure to understand this topic in full if you want to score in chemistry. Do also learn the Berry Important Points table of the water and carbon dioxide phase diagram by heart.

Previous post:

Next post: