Essential Chemistry Concepts and Practice Questions

Posted on Apr 2, 2026 in Chemistry

Essential Chemistry Practice Questions

  1. Define Allotropy: The property of some chemical elements to exist in two or more different forms in the same physical state is called allotropy. These forms are called allotropes.
  2. Name the allotropes of phosphorus: The common allotropes are white phosphorus, red phosphorus, and black phosphorus.
  3. What is a catalyst? A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.
  4. Application of heterogeneous catalysis: A major application is the Haber process for the synthesis of ammonia (NH₃), where solid iron acts as a catalyst for gaseous nitrogen and hydrogen.
  5. What is transmutation? The conversion of one chemical element or an isotope into another through a nuclear reaction.
  6. Newman projection for propane: In a Newman projection of propane, one views down the C₁-C₂ bond. The front carbon is a dot with three bonds (two H, one CH₃ for the back carbon), and the back carbon is a circle.
  7. What is a homologous series? A series of organic compounds with the same functional group and similar chemical properties in which successive members differ by a -CH₂- unit.
  8. What are alkenes? Write their general formula: Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). General formula: CₙH₂ₙ.
  9. Two uses of ethene:
    • To manufacture polythene (plastic).
    • For artificial ripening of fruits.
  10. Physical properties of benzene:
    • Colorless liquid with a characteristic aromatic odor.
    • Immiscible in water but soluble in organic solvents.
  11. Two limitations of Valence Bond Theory (VBT):
    • It fails to explain the tetravalency of carbon.
    • It does not explain the paramagnetic nature of oxygen (O₂).
  12. Characteristics of d-block elements:
    • They are all metals.
    • They show variable oxidation states and form colored ions.
  13. Define Ionization Enthalpy: The minimum amount of energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state.
  14. Physical properties of dihydrogen:
    • Colorless, odorless, and tasteless gas.
    • It is the lightest known substance and is highly inflammable.
  15. Define Oxidation and Reduction:
    • Oxidation: Loss of electrons or addition of oxygen/electronegative element.
    • Reduction: Gain of electrons or addition of hydrogen/electropositive element.

Adsorption vs. Absorption

  • Adsorption: A surface phenomenon where molecules accumulate only at the surface (e.g., charcoal adsorbing gas).
  • Absorption: A bulk phenomenon where molecules are uniformly distributed throughout the body of the material (e.g., sponge absorbing water).

Hydrogenation

It is a chemical reaction between molecular hydrogen (H₂) and another compound or element, usually in the presence of a catalyst like nickel (Ni) or platinum (Pt).

  • Example: Converting unsaturated vegetable oils into solid saturated fats (Vanaspati Ghee).

Solvent Extraction

A process used to separate a substance from a mixture by dissolving it in a suitable solvent. It relies on the differential solubility of the solute in two immiscible liquids (usually water and an organic solvent).

TLC vs. Paper Chromatography

  • TLC (Thin Layer Chromatography): Uses a thin layer of adsorbent (like silica gel) on a glass/plastic plate. It is faster and gives better separation.
  • Paper Chromatography: Uses a specialized cellulose paper as the stationary phase. It is slower and generally used for simpler separations.

Condensed Orbital Notation

  • Lithium (Z=3): [He] 2s¹
  • Silicon (Z=14): [Ne] 3s² 3p²
  • Calcium (Z=20): [Ar] 4s²
  • Chlorine (Z=17): [Ne] 3s² 3p⁵

Shapes of p-orbitals

The p-orbitals are dumb-bell shaped. There are three types oriented along the axes: pₓ, pᵧ, and p₂. They consist of two lobes separated by a nodal plane at the nucleus.

Postulates of Valence Bond Theory (VBT)

  1. Covalent bonds are formed by the overlap of half-filled atomic orbitals containing electrons with opposite spins.
  2. The strength of the bond depends on the extent of overlapping; greater overlap means a stronger bond.

Oxidation Numbers (Underlined Atoms)

  1. H₂SO₄: +6
  2. HNO₃: +5
  3. H₃PO₃: +3

Lyophilic vs. Lyophobic Colloids

  • Lyophilic: “Liquid-loving.” They are stable, reversible, and formed easily (e.g., starch in water).
  • Lyophobic: “Liquid-hating.” They are unstable, irreversible, and require stabilizers to stay in solution (e.g., gold sol).

Tyndall Effect

The phenomenon of scattering of light by colloidal particles when a beam of light passes through a colloidal solution. This makes the path of the beam visible.

Alpha-Decay

It is a type of radioactive decay in which an atomic nucleus emits an alpha particle (a helium nucleus, ⁴₂He). This results in a decrease in the atomic number by 2 and the mass number by 4.

IUPAC Names

  • a) 2,4,5-trimethylhexane
  • b) 2-bromo-3-methylbutane
  • c) 3-methylpentane
  • d) Propylbenzene
  • e) 2-phenylethanol

Structural Isomerism in Alkenes

Alkenes show:

  1. Chain Isomerism: Difference in the carbon skeleton (e.g., but-1-ene and 2-methylprop-1-ene).
  2. Position Isomerism: Difference in the position of the double bond (e.g., but-1-ene and but-2-ene).

Law of Mass Action

It states that at a constant temperature, the rate of a chemical reaction is directly proportional to the product of the active masses (molar concentrations) of the reacting substances.

Bredig’s Arc Method

A method used to prepare colloidal sols of metals (like gold or silver). An electric arc is struck between metal electrodes under water. The intense heat vaporizes the metal, which then condenses into colloidal-sized particles in the cold water.