Heating profile of a solid

The temperature at which vibrations of a particles of a solid become so great such that the mutual attractive forces are overcome and the particles break free from the fixed lattice positions and slide over one another in a liquid state is called melting point (m. pt). At this point, the solid state and the liquid phases are in equilibrium. Even though heat is continuously supplied to the solid, the temperature is constant at the time of melting.

Generally, the heat absorbed by a substance in changing from one phase to another is called the latent heat (hidden heat). The amount of heat absorbed, which causes the change of phase to occur at the melting point, is called the heat of fusion. The magnitude of this heat energy depends on the nature of the solid and the strength of the attractive forces between particles.

When heated, some solids however vaporize directly without passing through the liquid phase. On cooling, the vapour also turns to the solid phase. The phenomenon is known as sublimation. Solids having usually high vapour pressure exhibit this property. Examples of such solids include iodine crystal, ammonium chloride crystal, naphthalene, camphor and dry ice (solid carbon (IV) oxide).

Similarly, boiling point (b. pt) is the at which the liquid and vapour phases are in equilibrium. The boiling point of a liquid depends on the prevailing external atmospheric pressure. The lower the external pressure, the lower the boiling point of a liquid and vice-versa. The amount of heat absorbed that causes change of phase (liquid – vapour) to occur at the boiling point is the heat of vaporization.

Properties of Matter

Properties tell us what a substance is or what a substance can do. The properties of matter are classified as either physical or chemical.

Physical properties tell us everything about what a substance is, when no change is occurring to its constituents. Such properties are observed with our senses or instruments or pieces of apparatus. Examples are: state of matter, colour, odour, taste, texture, hardness, solubility in water, melting point, boiling point, density, viscosity and spectroscopic patterns (IR, UV, NMR).

Chemical properties are those properties that tell us about what a substance can do as regards to whether or not the substance reacts with other substances. Examples are flammability, rusting of iron, precipitation, decomposition of water by an electric current.

Changes of Matter

The changes that matter undergoes are associated with properties. The change are therefore categorized into physical and chemical.

A physical change is one that alters the physical properties of matter (particularly the form and state). In many cases, the changes of state is easily reversible. Examples are changes of state such as boiling, melting, freezing, condensation, sublimation, magnetization of metals, breaking glass and cutting wood.

A  chemical change is one in which new kind of matter is formed. It us always accompanied by energy changes (evolution or absorption of energy in form of heat or light or both). The process is not easily reversible. Examples are: combustion, rusting of iron, precipitation and souring of milk.

Classification of Matter

Matter is anything that has mass and occupies space i.e everything seen or unseen.

On a broad basis, matter can be subdivided into two general categories: pure or distinct substances and impure substances (mixtures).

Pure substances are distinct substances having the following general properties:

i. All parts are the same throughout (homogeneous).

ii. Composition is definite.

iii. They cannot be easily separated by physical means.

iv. Separation by physical methods is not easy.

v. They have a unique set of physical properties (boiling point, melting point, density, physical state, spectroscopic properties, etc.).

Physical properties are, in fact, used as the criteria for determining the purity level of substances. Pure solid substances have sharp melting point or very narrow melting range (Example: a reasonable pure benzoic acid sample has a melting point of 122°C or a melting range 120°C to 122°C). However, impurities (contaminants) cause a depression in melting points and widening of the melting range of solid substances (Example: an impure sample of benzoic acid has a melting range 110°C to 118°C).

For liquid, the criterion for determining the purity level of samples is the boiling point or relative density. A reasonable pure liquid has a constant boiling point or a low boiling range (Example: 99°C or 98°C to 100°C for distilled water). Impurities cause an elevation of the boiling point and the widening of the boiling range of a hitherto pure liquid (Example: 99°C to 105°C for an impure water sample).

Pure substances are either elements or compounds.

State of Matter and Phase Changes

Chemist usually describe substances as matter. Each form of Matter is made up of aggregates of small particles called molecules. Molecules consist of one or more tiny particles called atoms. A molecules may be monoatomic (e.g. He, Ne, Ar) or diatomic (e.g. H2, O2, N2, Cl2) or polyatomic (e.g. O3, P4, S8). Hence:

Atoms >>> Molecules >>> Matter

Matter exists in three phases commonly referred to as the states of matter. These are gas, liquid and solid.

The particles (molecules) of a gas are very far apart and are held together by very weak attractive forces, those of a liquid are much more closely held together by stronger attractive forces, those of solid are most closely held together by very strong attractive forces.

The phase transfer processes are freezing, melting, boiling, condensation and sublimation.

Solid     >>>     Liquid     (Melting)

Liquid    >>>     Solid      (Freezing)

Liquid    >>>     Vapour    (Boiling)

Vapour   >>>     Liquid     (Condensation)

Solid     >>>     Vapour    (Sublimation).

We are going to learn more about matter in the next article