Physical Properties (HSC SSCE Chemistry): Revision Notes

Physical Properties

Physical properties are fundamental characteristics that help us understand, identify, and separate different substances. Understanding these properties is essential for studying chemistry and working with materials in the laboratory.

What are physical properties?

Physical properties are the characteristics of a substance that can be observed or measured without changing it into a different substance. When you measure a physical property, the substance remains the same material before and after the measurement.

Examples of physical properties include:

• Melting point and boiling point
• Colour
• Density
• Solubility in specific solvents
• Hardness
• Electrical and thermal conductivity
• Viscosity
• Surface tension

In contrast, chemical properties are associated with the chemical changes (or chemical reactions) that occur when a substance is mixed with other substances, heated, or exposed to light. Chemical properties involve the transformation of a substance into a different substance.

Physical properties serve two important purposes:

• Separating mixtures: Different physical properties allow us to separate components of mixtures
• Identifying substances: The unique combination of physical properties helps identify pure substances

Homogeneous and heterogeneous substances

Substances can be classified based on the uniformity of their composition.

Homogeneous substances have uniform composition throughout. Every part of the substance has identical properties. Examples include:

• Pure water
• Sugar
• Aluminium foil
• Petrol
• Apple juice

Heterogeneous substances have non-uniform composition. Different parts of the material have different properties, and you can distinguish different components. Examples include:

• Fruit cake
• Concrete
• Wood
• Beef
• Orange juice (with pulp)

Colour

Colour can provide an initial clue for identifying substances. Some substances have distinctive colours:

• Liquid bromine: deep brown
• Solid copper: reddish brown
• Copper sulfate solution: pale blue
• Solid sulfur: yellow
• Solid iron(III) oxide: red
• Potassium permanganate crystals: deep purple

Colour is generally not useful for separating mixtures but can serve as a preliminary identification tool.

Magnetism

Magnetism is a highly useful physical property for separation purposes.

Some substances are strongly magnetic, including:

• Iron
• Steel
• Certain alloys (such as alnico, which contains iron, aluminium, nickel, and cobalt)

Magnetic substances can be easily separated from non-magnetic materials by using a magnet. This property has important practical applications:

• Separating a mixture of iron filings from sulfur powder
• Sorting steel cans at waste recycling facilities
• Separating ferrous (magnetic) metals from other metals at scrap metal yards

Particle size

Particle size refers to the dimensions of individual pieces of a solid substance. This property is not particularly useful for characterising substances because it can be easily changed by grinding a coarse solid into a finer powder.

However, particle size differences can be exploited to separate mixtures through sieving.

A sieve is a device containing uniform holes or a woven mesh that allows particles smaller than a certain size to pass through while retaining larger particles. Sieves can be made of:

• Shaped metal sheets with punched holes (like a kitchen colander)
• Woven mesh or gauze (metal or plastic) held in a frame

Applications of sieving include:

• Separating lumps from flour in cooking
• Separating fine sand from coarse gravel at quarries
• Laboratory and industrial separation of materials by size

Melting and boiling points

Melting point and boiling point are two of the most important physical properties for identifying substances and testing purity.

Melting point

The melting point of a solid is the lowest temperature at which the solid changes to a liquid at standard pressure. Although solids can melt at higher temperatures, the melting point specifically refers to the lowest temperature at which this transition occurs.

Standard pressure is $100.0$ kPa (kilopascals), which is very close to standard atmospheric pressure ($101.3$ kPa).

Freezing point

The freezing point of a liquid is the highest temperature at which the liquid can be converted to a solid. The freezing point of a liquid equals the melting point of the corresponding solid. Freezing is simply the reverse process of melting.

Boiling point

The boiling point of a liquid is the lowest temperature at which the liquid boils (changes from liquid to gas with visible bubbling) at a stated pressure (usually standard pressure).

The term normal boiling point refers specifically to the boiling point at a pressure of $100.0$ kPa, although the word "normal" is often omitted when the meaning is clear from context.

Key characteristics of boiling points:

• Pure substances boil at fixed temperatures at standard pressure
• Mixtures boil over a range of temperatures
• Non-volatile impurities (like salt in water) raise the boiling point
• More volatile impurities (like alcohol in water) lower the boiling point

Volatile and non-volatile substances

Volatile substances are easily converted to vapour, meaning evaporation occurs quite rapidly. Substances with low boiling points (less than about $100°\text{C}$) are considered volatile.

Examples of volatile substances:

• Ordinary alcohol (ethanol)
• Ethyl acetate (nail polish remover)

Non-volatile substances are not easily converted to vapour, meaning evaporation is quite slow. Substances with high boiling points (much greater than $100°\text{C}$) are non-volatile. All solids are non-volatile.

Examples of non-volatile substances:

• Cooking oil
• Ethylene glycol (motor car antifreeze)

Vapour versus gas

The terms vapour and gas both refer to the same physical state, but with a subtle difference:

A vapour is a gas that is easily liquefied or condensed. It is a gas that exists close to its boiling point.

We typically speak of "water vapour" in the atmosphere because water readily condenses out of air. In contrast, we refer to "oxygen gas" in the atmosphere because oxygen is much more difficult to liquefy.

Physical properties data

The following tables show melting points, boiling points (at $100$ kPa), and densities (at $25°\text{C}$) for common substances:

Substance	Melting point (°C)	Boiling point (°C)	Density (g mL⁻¹)
Acetic acid	$16.7$	$118$	$1.04$
Aluminium	$660$	$2450$	$2.7$
Argon	$-189$	$-186$	—
Carbon (graphite)	$3730$	$4830$	$2.3$
Chloroform	$-64$	$62$	$1.48$
Copper	$1083$	$2600$	$9.0$
Ethanol	$-114$	$78$	$0.79$
Ethyl acetate	$-84$	$77$	$0.90$
Ethylene glycol	$-16$	$198$	$1.11$
Hexane	$-95$	$69$	$0.66$

Substance	Melting point (°C)	Boiling point (°C)	Density (g mL⁻¹)
Hydrogen	$-259$	$-253$	—
Lead	$327$	$1740$	$11.4$
Mercury	$-39$	$357$	$13.6$
Nitrogen	$-210$	$-196$	—
Oxygen	$-219$	$-183$	—
Phosphorus	$44$	$280$	$1.8$
Sodium	$98$	$892$	$0.97$
Sulfur	$114$	$444$	$2.0$
Water	$0.0$	$100.0$	$1.00$
Zinc	$420$	$610$	$7.1$

Additional substances:

Substance	Melting point (°C)	Boiling point (°C)
Bromine	$-7$	$58$
Carbon disulfide	$-111$	$46$
Carbon tetrabromide	$91$	$190$
Gallium	$30$	$2400$
Hydrogen peroxide	$-2$	$158$
Lead bromide	$373$	$914$
Magnesium	$650$	$1110$
Neon	$-249$	$-246$
Phosphorus trichloride	$-91$	$74$
Sulfur dioxide	$-73$	$-10$
Sulfuric acid	$10$	$330$

Density

Density is a particularly useful physical property for identifying substances.

Density is defined as mass per unit volume:

$\text{density} = \frac{\text{mass}}{\text{volume}}$

Common units for density include:

• Grams per millilitre ($\text{g mL}^{-1}$ or $\text{g/mL}$)
• Kilograms per cubic metre ($\text{kg m}^{-3}$)

Determining density

Density of solids versus liquids

Particles in solids are generally packed more closely together than in liquids. Therefore, solids typically have greater densities than the same substances in liquid form.