Revision: Chemical Change and Chemical Bond General Science SSC (English Medium) 8th Standard Maharashtra State Board

The cell which converts electrical energy into chemical energy is called an electrolytic cell and the process which converts electrical energy into chemical energy is called electrolysis.

A chemical reaction that occurs in the presence of light is called a photochemical reaction.

A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change during the reaction.

Any chemical change in matter that involves its transformation into one or more new substances is called a chemical reaction.

An electrochemical reaction is a chemical reaction involving the transfer of electrons, where oxidation and reduction occur simultaneously.

Chemical reactions which are characterised by the formation of insoluble solid substances are called precipitates.

A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt as one of the products.

A chemical change is a permanent change in which the chemical composition of a substance is changed and one or more new substances with different chemical compositions and different properties are formed.

The substances formed as a result of a chemical reaction by formation of new bonds are called products.

A chemical change is a process in which the composition of matter changes, resulting in the formation of one or more new substances.

The substances taking part in chemical reaction are called reactants.

Reduction is defined as the phenomenon in which an atom gains an electron to form a negatively charged ion called an anion.
During the formation of ionic bond one atom undergoes oxidation while another atom undergoes reduction.

A chemical bond may be defined as the force of attraction between any two atoms in a molecule to maintain stability.

or

A chemical bond may be defined as the force of attraction between any two atoms, in a molecule, to maintain stability.

Oxidation is the loss of electrons during a reaction by a molecule, atom or ion. In terms of electron transfer, oxidation is defined as the phenomenon in which an atom loses an electron to form a positively charged cation.
During the formation of ionic bond one atom undergoes oxidation while another atom undergoes reduction.

An ion is a charged particle which is formed due to the gain or the loss of one or more electrons by an atom.

A metallic element, whose one atom readily loses electron(s) to form a positively charged ion, is an electropositive element.

A non-metallic element, whose atom readily accepts electron(s) to form a negatively charged ion, is an electronegative element.

The cation and the anion being oppositely charged attract each other and form a chemical bond. Since this chemical bond formation is due to the electrostatic force of attraction between a cation and an anion, it is called an electrovalent (or an ionic) bond.

A metallic atom, which loses electron(s), becomes a positively charged ion and is known as a cation.

The chemical compounds formed as a result of the transfer of electrons from one atom of an element to one atom of another element are called ionic (or electrovalent) compounds.

The number of electrons that an atom of an element loses or gains to form a electrovalent bond is called its electrovalency.

A non-metallic atom, which gains electron(s), becomes a negatively charged ion and is known as an anion.

The bond formed between two atoms by sharing a pair of electrons, provided entirely by one of the combining atoms but shared by both, is called a coordinate bond. 

The chemical bond formed between two combining atoms by mutual sharing of one or more pairs of electrons is called a covalent bond.

The molecule formed due to the sharing of electrons (covalent bond) is called a covalent molecule.

The chemical bond that is formed between two combining atoms by mutual sharing of one or more pairs of electrons is called a covalent (or a molecular) bond, and the compound formed due to this bond is called a covalent compound.

When an iron nail is kept in copper sulphate solution, a reddish brown coating is seen on the iron nail after sometime. Also, the colour of the copper sulphate solution changes from blue to green. This is observed because iron displaces copper from copper sulphate solution, as iron is more reactive than copper.

\[\ce{Fe_{(s)} + CuSO4_{(aq)} -> FeSO4_{(aq)} + Cu_{(s)}}\]

An ionic bond is formed by the complete transfer of one or more electrons from an electropositive atom to an electronegative atom, resulting in oppositely charged ions that attract each other.

Key conditions for ionic bond formation:

• One atom must have low ionisation enthalpy (easily loses electron) — typically a metal

• The other must have high electron affinity (easily gains electron) — typically a non-metal

• Large difference in electronegativity between the two atoms

Example: Na + Cl → Na⁺ + Cl⁻ → NaCl

• Sodium (2,8,1) loses 1 electron → Na⁺ (2,8)

• Chlorine (2,8,7) gains 1 electron → Cl⁻ (2,8,8)

Ionic solids are crystalline structures containing cations and anions held together by strong electrostatic ionic bonds.

• Carbon forms covalent bonds by sharing electrons to achieve a noble gas configuration.
• Covalent bonds can be single, double, or triple, as seen in molecules like H₂, O₂, and N₂.
• Covalent compounds have low melting and boiling points and are poor conductors of electricity.
• Carbon has allotropes such as diamond, graphite, and fullerene (C₆₀), each with different physical properties.