DEFINITIONS : CHEMISTRY

• Chemical Reaction: When a substance changes chemically during a reaction, the reaction is called a chemical reaction.


• Reactant: The substance that undergoes chemical change is called a reactant.


• Product: The new substance that is formed during a reaction is called a product.


• Skeletal Chemical Equation: A chemical equation which shows both reactants and products but which is not balanced as regards the mass of the substances involved is called a skeletal chemical equation.


• Combination Reaction: A reaction in which a single product is formed from two or more reactants is called a Combination reaction.


• Exothermic Reaction: A reaction in which heat is evolved along with the formation of the products is called an Exothermic reaction.


• Endothermic Reaction: A reaction in which heat is absorbed during the formation of products is called an Endothermic reaction.


• Decomposition Reaction: A reaction in which a single reactant breaks down to give more than one simpler product is called a Decomposition reaction.


• Thermal Decomposition: A reaction in which decomposition of reactant is carried out by heating it is called thermal decomposition.


• Displacement Reaction: A reaction in which a more reactive element displaces a less reactive element from its solution is called a displacement reaction.


• Double Displacement Reaction: A reaction which involves an exchange of ions between the reactants is called a double displacement reaction.


• Precipitation Reaction: A reaction in which an insoluble product(precipitate) is formed is called a precipitation reaction.


• Oxidation: If a substance gains oxygen or loses hydrogen it is said to undergo oxidation.


• Reduction: If a substance gains hydrogen or loses oxygen it is said to undergo reduction.


• Redox Reaction: A reaction in which one substance is oxidised and the other is reduced is called Redox reaction.[NOTE: A reaction cannot be either an oxidation reaction or a reduction reaction independently.]


• Corrosion: When a metal is attacked by substances like moisture, acids, etc. around it and converted into its salt it is said to corrode and the process is called corrosion.


• Rancidity: The change in the smell and taste of fats and oils due to their oxidation is called rancidity.


• Olfactory Indicators: Substances whose odour changes in acidic or basic mediums are called olfactory indicators when they are used to identify whether a substance is an acid or a base.


• Neutralisation Reaction: The reaction between an acid and a base giving a salt and water is called neutralisation reaction.


• Alkali: Bases which are soluble in water are called alkalies.


• Antacid: A base which is used to neutralise excess hydrochloric acid produced in stomach to reduce pain and irritation is called an antacid.


• Water of Crystallisation: The fixed number of water molecules present in one formula unit of a salt(compound) is called water of crystallisation.


• Ductility: The ability of metals to be drawn into thin wires is called ductility.


• Malleability: The ability of metals to be beaten(hammered) inti thin sheets is called malleability.


• Allotrope: Each of structurally different forms of an element is called an allotrope of that element.


• Ionic Compounds: Compounds formed by the transfer of electrons from a metal to a nonmetal are called ionic compounds.


• Covalent Compounds: Compounds formed by sharing of electron pairs between atoms are called covalent compounds.


• Minerals: The elements or compounds which occur naturally in the earth's crust are called minerals.[NOTE: Minerals in chemistry are compounds in general but Minerals in biology are elements. For example Na and Fe are minerals for a biologist]


• Ore: A mineral from which a particular metal can be extracted profitably is called an ore.


• Gangue: Impurities like sand present in an ore mined from the earth are collectively called gangue.


• Calcination: The process of heating carbonate ores strongly in limited supply of air to change them into oxides is called calcination.


• Roasting: The process of heating sulphide ores strongly in presence of excessive air to change them into oxides is called roasting.


• Galvanisation: The process of deposition(coating) of a thin layer of zinc on steel and iron to prevent their rusting is called galvanisation.


• Alloy: A homogeneous mixture of two or more metals or a metal and a nonmetal is called an alloy.


• Amalgam: An alloy containing mercury as one of the constituents is called an amalgam.


• Catenation: The property of carbon atoms by which one carbon atom can form covalent bonds with other carbon atoms giving rise to large molecules is called catenation.


• Isomers: Compounds with identical molecular formula but different structures(atomic arrangements) are called isomers.


• Functional Group: An atom or a group of atoms which confers specific properties to an organic compound regardless of the length and nature of the carbon chain is called a functional group.


• Homologous Series: A series of organic compounds having same functional group and in which there is a difference of -CH₂ between two successive compounds is called homologous series.


• Addition Reaction: A reaction in which an atom or group of atoms enters a molecule without replacing another atom from it is called an addition reaction.


• Substitution Reaction: A reaction in which an atom or group of atoms replaces another atom(or group of atoms) to enter the molecule is called a substitution reaction.


• Catalyst: A catalyst is a substance that causes a reaction to occur or proceed at a different rate without the reaction itself being affected.


• Isotope: Isotopes are atoms of an element with similar chemical properties but different atomic masses due to different number of neutrons in their nuclei.


• Atomic Radius: The distance between the centre of the nucleus and the outermost shell of an isolated atom is called its atomic radius.


• Oxidising Agent: A substance which supplies oxygen to or removes hydrogen from other substance is called an oxidising agent.


• Reducing Agent: A substance which supplies hydrogen to or removes oxygen from other substance is called an reducing agent.