ICSE Chemistry Class 8 Notes Chapter 3 Elements, Compounds and Mixtures

→ “Element is a pure substance made up of one kind of atoms only.” It cannot be broken down into simpler substances by any means, e.g. If we go on breaking a gold piece, however small it be, yet it will be gold.

→ “Compound is a pure substance made up of two or more different elements in fixed ratio.” Compounds have formula. e.g. Common salt or table salt NaCl.

→ Atom : “The smallest particle of element.” It is neutral i.e. has no charge, may or may not exist independently.

→ All elements have symbol i.e. H for hydrogen, O for oxygen and there are about 118 elements found so far. Elements are in the form of metals, non-metals, metalloids and noble gases.

→ Molecule : “Atoms of the same element or different elements combine to form a molecule.”

→ Molecule is smallest particle of substance that can normally exist independently and can retain the physical and chemical properties of the substance, e.g. Cl2, CO2, H2O, O2, H2, Na, K etc.

→ Atomicity : “The number of atoms present in a molecule.” O2 is a molecule of oxygen having 2 atoms of oxygen. Na is a molecule of sodium substance having one atom of sodium.

→“Mixture is made up of two or more substances, elements or compounds or both mechanically mixed together in any proportion.” A mixture retains the properties of constituent elements or compound of which it is made up. Mixture has no formula.

→ Compound has new properties different from its constituent from which it is made. i.e. water is a compound (H2O) made up of two element hydrogen (H) and oxygen (O) in fixed ratio combined chemically 2 parts of hydrogen and one part of oxygen by volume and 2 x (1) : 16.
i. e. 2 : 16 or 1 : 8 by mass.
Hydrogen (gas) is combustible i.e. catches fire and oxygen

→ helps in burning is supporter of combustion but water is a liquid and puts out the fire.

→ “Metalloids are the element whose properties resemble both metals and non-metals.” Boron (B), Arsenic (As), Germanium (Ge), Antimony (Sb), Tellurium (Te), Polonium (Po).

→ Alloys are homogeneous mixtures of (metal, metal) or (metal, non-metals). Sublimable substance are ammonium chloride NH4Cl, Naphthalene balls, iodine, dry ice, camphor, benzoic acid and anthracene.

→ Noble gases or inert gases : occur in traces in the atmosphere are monoatomic. i.e. He, Ne, Ar, Kr, Xe, Rn.

→ Types of mixture and methods to separate :
ICSE Class 8 Chemistry Notes Chapter 3 Elements, Compounds and Mixtures - 1

ICSE Class 8 Chemistry Notes

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