8th Class Science Materials : Metals and Non-metals NCERT Summary - Metals Its Physical Properties

Metals and its Physical Properties

Metals and its Physical Properties

(a) Physical Change: A physical change is a change of state only and is not accompanied by an alteration in the chemical composition, weight or chemical properties of any substance, e.g., melting of ice, magnetising a needle, etc.

(b) Chemical Change: A chemical change gives rise to a new substance having composition and properties altogether different from those of the original substance, e.g., radioactive decay, rusting of iron, photosynthesis, etc. Energy is absorbed or released during a chemical change.

PHYSICAL AND CHEMICAL PROPERTIES

(a) Physical Properties: The properties which do not depend on reaction with any other substance, e.g., colour, melting point, boiling point, density, etc.

(b) Chemical Properties: The chemical properties of a substance are those that describe its reactions with other substances.

EMPIRICAL FORMULA

• The empirical formula gives the simple whole number ratio between the number of atoms of the different elements forming the compound, e.g., \[{{H}_{2}}O\] tells that there are twice as many H atoms as O atoms.

MOLECULAR FORMULA

• It indicates the number of atoms in a molecule of a molecular substance. Elements as well as compounds can be represented by a molecular formula, e.g.,\[{{H}_{2}},\]\[{{O}_{2}},\]\[{{N}_{2}},\] etc.

COLLOIDS

• Colloidal particles may be crystalline or non-crystalline. When these are dispersed in a liquid, solid or gas they result in the formation of a colloidal system, e.g., top soil of earth, protoplasm, etc.

(a) Sol: When a solid is dispersed in a liquid, the particles of a Sol will not settle out.

(b) Gel: In a gel, the liquid contains a colloidal solid dispersed throughout the system, but set in a structure which does not flow, e.g., gelatin, jellies, etc.

(c) Aerosol: The dispersion of either a solid or a liquid in a gas. When the dispersed colloidal particle is solid, the result is smoke and when it is liquid, the result is fog.

(d) Emulsion: When one liquid is dispersed into another in which it is not soluble, e.g., milk, paint, etc.

Properties of Colloids

• When the beam of light is passed through a true solution, the path of the beam is not visible but when it is passed through a colloidal solution, its path is visible. This effect is known as Tyndall Effect.

METALLURGY

• It comprises the various processes involved in the production of metal from the ore which generally contains a large percentage of rocky materials, called gangue.

RUSTING

• It is caused in iron due to presence of moisture, oxygen, \[C{{O}_{2}}\]in the air. Rusting is prevented by surface coating with film of oil, paint or metal coating such as chromium coating, nickel plating, tin plating and copper plating.

ALLOY

• It is a mixture of two or more metals, and small amount of non-metals also.

ORES

• These are minerals from which metals are produced.

MINERALS

• These are the natural materials extracted from the Earth that are formed through geological processes. Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms.