cHEMISTRY DEFINITIONS

101 chemistry definitions

There about 101 chemistry definitions that are needed in every Olevel chemistry exam. I want every child writing any chemistry exam to look through these chemistry definitions because it will go a long way in helping you excel in your exams

101 chemistry definitions

  • Physical change : A physical change is one which is easily reversible and in which no new substance is formed.

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  • Chemical change :A chemical change is one which is NOT easily reversible and in which new substance is formed.

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  • Element: An element is a substance which cannot be split into simpler units by any ordinary chemical process.

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  • Compound: A compound is a substance which contains two or more elements chemically combined together.

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  • Mixture: A mixture contains two or more constituents which can easily be separated by physical means.

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  • Atom : An atom is the smallest part of an element which can take part in a chemical reaction.
  • Alloy: An alloy is a substance prepared by by adding one or more elements to a base or parent metal to obtain desirable products.

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  • Molecule : A molecule is the smallest particle of a substance that can exist alone and still retain the chemical properties of that substance.

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  • Ion: An ion is an atom or group of atoms that has an electrical charge.

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  • Mole: A mole is the amount containing as many elementary entities(particles) as the number of atoms in 12 grams of carbon-12.

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  • Empirical formula: This is the simplest formula of a compound that shows the relative number of atoms in a molecule of substance.

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  1. Relative atomic mass: Relative atomic mass is the number of times the average mass of one atom of an element  is heavier than one twelfth the mass of one atom of carbon-12.
  • Molecular formula: This is the formula that shows actual number of atoms in a molecule
  • Structural Formula: This is the formula of a compound that shows the bonds and atoms in a compound.

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  • Law of conservation of mass(matter):The law states that matter is neither created nor destroyed during chemical reactions but changes from one form to another.

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  • Law of Definite proportions: Law of definite proportions states that all pure samples of a particular chemical compound contain similar elements combined in same proportion by mass.

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  • Law of multiple proportions: Law of multiple proportions states that if two elements A and B combine to form more than one chemical compound then the various masses of one element A which combine separately with a fixed mass of B ae in simple multiple ratio.

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  • Valency: This is the combining power of an element.

OR

Valency is the number of electrons lost or gained in order to form ions.

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  • Isotopy: This is the existence of atoms of an element with the same atom in number but different mass numbers.
  • Isotopes : These are the atoms of an element with same number of protons (atomic number) but different mass numbers.

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  • Isotopes : These are atoms of an element with same atomic number but different mass numbers.

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  •  Pauli Exclusion Principle: No two electrons in an atom will have same values for all the four quantum number.

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  • Hund’s rule : Pairing will not start in degenerate orbitals until all the orbitals have been singly occupied.

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  • AufBau’s Principle : In building up of an atom, electrons occupy orbital of lower energy before occupying orbitals of higher energy.

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  • Hybridization: This is the merging of two orbitals to form an identical orbital of equal energy.

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  • Ionic bonding/Electrovalent bonding: This is the type of bonding that involves transfer of electrons from the metallic atom to the non metallic atom forming oppositely charged ions in the process

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  • Covalent bonding: This is the type of bonding that involves sharing of electrons between non metals in order to attain stable configurations.
  • Dative bonding/Co-ordinate covalent bonding: This is the type of bonding in which only one of the participating atoms contributes the electrons.

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  • Kinetic theory of gases: The kinetic theory of gases states that gases are constantly moving and so possess kinetic energy.

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  • Boyle’s law: Boyle’s law states that the volume of a fixed mass of gas is inversely proportional to its pressure provided that temperature remains constant.

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  • Charles’s law: The Charles law states that the volume of a fixed mass of gas is directly proportional to its temperature in Kelvin (absolute temperature) provided pressure remains constant.

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  • Dalton’s law of partial pressures: Dalton’s law of partial pressures states that if there is a mixture of gases which do not react chemically together, then the total pressure exerted by the gases is equal to the sum of the individual partial pressures.

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  • Gay-Lussac’s law of combining volumes: The law states that when gases react, they do so in volume which are in simple ratio to one another and to the volume of the product if gaseous provided temperature and pressure remains constant.

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  • Avogadro’s Hypothesis: The Avogadro’s hypothesis states that equal volume of all gases at the same temperature and pressure contain same number of molecules.

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  • Avogadro’s number: Avogadro’s number is the number of carbon atoms in 12g of carbon-12 and is numerically equal to 6.02 x 1023.

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  • Relative vapour density :This is the weight or mass of a pure volume of a gas or vapour when compared to the mass of equal volume of dry air.

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  • Grahams’s law of diffusion: Graham’s law of diffusion states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass (or vapour density) at constant temperature and pressure.

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  • Acid: An acid is a substance which will dissolve in water to produce hydrogen ion as the only positive ion.

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  • Basicity of Acid: This is number of replaceable hydrogen ion (H+) in one mole of an acid.

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  • Base : A base is a substance which neutralizes an acid to form salt and water.

                               OR

A base is a substance that dissolves in water to produce hydroxide ions as the only negative ion.

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  • Alkali: An alkali is a base that is very soluble in water.

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  • Neutralization reaction: A neutralization reaction is a reaction between an acid and an alkali to form salt and water only.

                             Or         

This is the combination of hydrogen ion(H+) and hydroxide ion(OH) to form one mole of water.

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  • pH: This is the degree of acidity or alkalinity of a solution.

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  • Acid-base indicator: An indicator is a weak organic acid or base that changes colour due to change in pH of the medium.

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  • Buffer solutions: A buffer solution is a solution that resists change in pH on dilution or addition of acid or base.

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  • Salt: A salt is a compound formed when all or part of the ionizable hydrogen ion from an acid has been replaced by metallic or ammonium ion.

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  • Efflorescence : This is the ability of a salt to lose part or all of its water of crystallization when exposed to the atmosphere.

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  • Hygroscopy : This is the ability of a salt to absorb moisture on exposure to the atmosphere and becomes sticky or moist.

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  • Deliquescence : This is the ability of a salt to absorb large amount of water and form a solution when exposed to the atmosphere.

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  • Allotropy: This is the existence of an element in different forms in the same physical state.
  • Allotropes : These are the different forms of an element in the same physical state.

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  • Destructive distillation of coal: This is heating coal to a very high temperature in the absence of air.

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  • Periodic law: The modern periodic law states that the property of an element is a periodic function of its atomic number.
  • Periodic property :The periodic property is any physical property of an element which changes due to change in atomkic number.

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  • Periodicity: Periodicity is the variations of properties of elements in a regular pattern down the group and across the period.

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  • Atomic radius (atomic size): Atomic radius is the one half the distance of closest approach between the nuclei of atoms in elemental substance.

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  • Ionization energy: This is the amount of energy required to remove the outermost electron from an atom of an element  in order to form an ion.

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  • Electron affinity: This is the energy released when a gaseous atom gains an electron to form a gaseous negative ion.

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  • Electronegativity : This is the power of an atom in a molecule to attract electrons to itself.

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  • Electropositivity : This is the power of an atom to give out or lose electrons.

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  • Catalyst : A catalyst is a substance which alters the rate of a chemical reaction but remains chemically unchanged at the end of the reaction.

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  • Oxidation : Oxidation is loss of electron.

Oxidizing agent: An oxidizing agent is a substance that accepts electrons.

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  • Reduction : Reduction is g\in of electrons.

Reducing agent: A reducing agent is a substance that donates electrons.

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  • Redox reaction: A redox reaction is a reaction that involves oxidation and reduction.

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  • Electrolysis: Electrolysis is the gradual chemical decomposition of a compound in molten or aqueous form by the passage of electricity.
  • Electrolyte: This is a compound that conducts electricity and becomes decomposed in the process.
  • Electrodes: These are conductors in form of wires, rods or plates through which electrons enter or leave the electrolyte

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  • Electrochemical cell :Electrochemical cell is a cell that converts chemical energy to electrical energy.

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  • Faraday’s first law of Electrolysis: Faraday’s first law of electrolysis states that the mass of an element discharged during electrolysis is directly proportional to the quantity of electricity passing through it.

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  • Faraday’s second law of Electrolysis : Faraday’s second law of electrolysis states that when the same quantity of electricity is passed through different electrolytes, then the number of moles of elements discharged are inversely proportional to the charges on the ions.

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  • Standard electrode potential :Standard electrode potential of a metal ion/metal system is the potential difference set up between he metal and one molar solution of its ions at 250C.

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  • Standard heat of reaction: This is the heat absorbed or evolved when a chemical reaction occurs between molar quantities of the substances as represented in the equation under standard conditions.

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  • Exothermic reaction : This is a reaction in which heat is liberated to the surrounding.
  • Endothermic reaction: This is a reaction in which heat is absorbed from the surrounding.

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  • Standard heat of formation: This is the heat evolved or absorbed when one mole of that substance is formed from its elements under standard conditions.
  • Heat of formation: This is the heat evolved or absorbed when one mole of that substance is formed from its elements.

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  • Standard heat of neutralization: This is the heat evolved when one mole of hydrogen ion from an acid reacts with one mole of hydroxide ion from an alkali to form one mole of water under standard conditions.
  • Heat of neutralization:This is the heat evolved when one mole of hydrogen ion from an acid reacts with one mole of hydroxide ion from an alkali to form one mole of water

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  • Standard heat of combustion: This is the heat evolved when one mole of a substance is burned completely in oxygen under standard conditions.
  • Heat of combustion: This is the heat evolved when one mole of a substance is burned completely in oxygen.

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  • Rate of reaction :This is the amount of products formed or reactants converted per unit time.

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  • Equilibrium : A reversible reaction is said to be in dynamic equilibrium when the rate of forward reaction is equal to the rate of backward reaction producing no net change in the concentrations of reactants and products.

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  • Le Chatelier’s principle: Le Chatelier’s principle states that if an external constraint such as change in temperature, pressure or concentration is imposed on a system in equilibrium, the equilibrium will shift so as annul or neutralize the constraint.

  • Solution : A solution is a uniform or homogeneous mixture of two or more solutes.
  • Suspension: A suspension is a heterogeneous mixture of undissolved particles in a given medium.
  • Unsaturated solution: A solution that contains less the maximum amount of solute that it can can dissolve at a temperature.
  • Saturated solution : A saturated solution is a solution which contains the maximum amount of solute that it can dissolve at a particular temperature in the presence of undissolved solute particles.
  • Supersaturated solution : A supersaturated solution is a solution which contains more of the solute that it can usually hold at that temperature.
  • Concentrated solution: A concentrated solution is a solution that contains a high amount of solute dissolved in a solvent at a particular temperature.
  • Solubility: This is the maximum of solute in moles or grams that will saturate I dm3 of the solvent at that temperature.

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  • Radioactivity: Radioactivity is the spontaneous emission of radiation by an element.

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  • Nuclear fission: Nuclear fission is the process in which the nucleus of a heavy element is split into two nuclei of nearly equal mass with a release of energy and radiation.

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  • Nuclear fussion :Nuclear fusion is a process in which two or more light nuclei fuse or combine to form a heavier nucleus with a release of energy and radiation

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  •  Allotropy: Allotropy is the existence of an element in different forms in the same physical state.

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  • Octane number: This is the rating used to check the quality of petrol.

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  • Homologous series: This is a family of organic compounds which follows a regular structural pattern in which successive member differs from the the other by –CH2 group.

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  • Functional group : A functional group is an atom, a radical or bond common to a homologous series and which determines the chemical properties of the series.

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  • Isomerism : Isomerism is the existence of two or more compounds with same molecular formula but different molecular structures.

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  • Hydrocarbons : Hydrocarbons are compounds that contain carbon and hydrogen only.

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  • Cracking :This is splitting larger molecules into smaller molecules by subjecting them to high temperatures and pressures.
  • Reforming: This is a conversion process  

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  • Substitution reaction:A substitution reaction is a reaction which involves direct displacement of atom or group by another atom or group.

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  • Addition reaction: An addition reaction is a reaction that involves direct addition of an attacking reagent across the double or triple bond of an unsaturated compound to yield a saturated product.

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  • Fermentation : Fermentation is the decomposition by micro-organism of large organic molecules like starch into smaller molecules such as ethanol.

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  • Esterification :This is the reaction of carboxylic acids and alkanols to produce esters(alkanoates) and water.

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  • Saponification:This is the alkaline hydrolysis of fats and oil to yield soap and propane 123 triol (glycerol)

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  • Polymerization : This is the process whereby two or more monomers link together to produce a compound of high molecular mass called the polymer.

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  • Addition polymerization: This is the process whereby two or more of the same monomers link together to form a polymer without the elimination of small molecule.

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  • Condensation polymerization : This is the process whereby two or more monomers link together to form a polymer with the elimination of small molecules.

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  • Thermoplasts :These are polymers that can be softened repeatedly by heat and remolded.

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  • Thermosets : These are polymers that cannot be softened or melted by heat and remoulded.

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  • Orbital: This is a region where there is a high probability of finding an electron.

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  • Activation energy /energy barrier: This is the minimum amount of energy required to be attained or surpassed before a reaction will occur.

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  • Water of crystallization: water of crystallization is the definite amount of water some salts chemically combine with when they form salts from aqueous solutions.

This is the compendium of definitions of terms in Chemistry but it is prepared for reference and quick memorization to help you learn better.

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