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Chemistry basics

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Chemistry Basics – Study Notes

(For Meghalaya Patwari / Lekhpal – General Awareness & Current Affairs)

1. Simple Explanation

Chemistry is the science that deals with the composition, structure, properties, and changes of matter.

  • Matter = anything that occupies space and has mass.
  • Atoms = the basic building blocks of matter.
  • Elements = pure substances made of only one type of atom.
  • Compounds = substances formed when two or more elements combine chemically.
  • Chemical reaction = a process where substances transform into new substances, involving rearrangement of atoms.

2. Key Points

Topic Quick Facts
Atomic Structure 1. Nucleus (protons + neutrons)
2. Electrons orbit in shells (K, L, M, …).
3. Atomic number = number of protons.
4. Mass number = protons + neutrons.
Periodic Table 1. Elements arranged by increasing atomic number.
2. Groups (columns) – elements with similar valence electrons.
3. Periods (rows) – elements with successive energy levels.
4. Metals, Non‑metals, Metalloids – classification by properties.
States of Matter 1. Solid – fixed shape, close‑packed atoms.
2. Liquid – fixed volume, free flow.
3. Gas – neither fixed shape nor volume, atoms far apart.
4. Plasma – ionised gas (high temperature).
Chemical Bonds 1. Ionic – transfer of electrons (e.g., NaCl).
2. Covalent – sharing of electrons (e.g., H₂O).
3. Metallic – delocalised electrons in a lattice (e.g., Fe).
4. Hydrogen & Van der Waals – weaker interactions.
Reaction Types 1. Synthesis (A + B → AB)
2. Decomposition (AB → A + B)
3. Single Replacement (A + BC → AC + B)
4. Double Replacement (AB + CD → AD + CB)
5. Combustion (fuel + O₂ → CO₂ + H₂O)
Stoichiometry 1. Moles – 1 mol = 6.02 × 10²³ particles.
2. Avogadro’s number – 6.022 × 10²³.
3. Molar mass – g per mol.
4. Empirical vs. Molecular formula.
Acids & Bases 1. Acids – donate H⁺ (pH < 7).
2. Bases – accept H⁺ or donate OH⁻ (pH > 7).
3. Neutralisation – acid + base → salt + water.
Important Elements 1. Carbon – backbone of organic life.
2. Oxygen – respiration, combustion.
3. Hydrogen – most abundant, fuel source.
4. Nitrogen – essential for proteins, fertilizers.
5. Sulphur, Phosphorus – life‑supporting.
Environmental Chemistry 1. Green Chemistry – design safer chemicals.
2. Pollution control – catalytic converters, wet‑scrubbers.
3. Renewable energy – hydrogen fuel cells, bio‑methane.
Current Affairs (2024‑26) 1. COVID‑19 vaccine production – mRNA technology uses lipid‑nanoparticle chemistry.
2. Plastic recycling – chemical depolymerisation (e.g., PET → terephthalic acid).
3. Battery technology – lithium‑ion chemistry, solid‑state electrolytes.
4. Carbon capture – chemical sorbents (amine‑based).

3. Worked Example – Stoichiometry

Problem:
Calculate the mass of water (H₂O) produced when 10 g of hydrogen gas reacts completely with excess oxygen.

Solution Steps

  1. Write and balance the equation
    [ 2,\text{H}_2(g) + \text{O}_2(g) \longrightarrow 2,\text{H}_2\text{O}(l) ]

  2. Convert the given mass to moles

    • Molar mass of H₂ = 2.02 g mol⁻¹
      [ n_{\text{H}_2} = \frac{10,\text{g}}{2.02,\text{g mol}^{-1}} \approx 4.95,\text{mol} ]

  3. Use stoichiometry
    According to the balanced equation, 2 mol H₂ gives 2 mol H₂O.
    Therefore, moles of H₂O = 4.95 mol.

  4. Convert moles of H₂O to mass

    • Molar mass of H₂O = 18.02 g mol

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