Chemistry basics: General Awareness & Current Affairs Notes for Tripura Police Sub-Inspector (SI)

Chemistry Basics – Study Notes

For Tripura Police Sub‑Inspector (SI) – General Awareness & Current Affairs

These notes give a concise, clear overview of the most important chemistry concepts that appear in the SI exam. Use the revision summary at the end for a quick refresher before the exam.

1. Simple Explanation

Chemistry is the science that studies matter, its properties, composition, structure, and the changes it undergoes.
Matter is anything that has mass and occupies space. It is made of atoms, which are the smallest units that retain the chemical identity of an element.
Chemical reactions involve rearrangement of atoms, leading to new substances with different properties.
Understanding basic chemistry helps in fields like forensic science, environmental monitoring and public health – all relevant to police work (e.g., evidence analysis, toxicology, crime scene investigations).

2. Key Points

Topic	What to Know
Atomic Structure	Protons (positive), neutrons (neutral), electrons (negative). Atomic number = protons. Mass number = protons + neutrons. Isotopes differ in neutron number.
Periodic Table	Organised by increasing atomic number. Groups (columns) share properties; periods (rows) show electron shell completion. Metallic vs non‑metallic trends.
Chemical Bonding	- Ionic: Transfer of electrons (salt). - Covalent: Sharing of electrons (water). - Metallic: Delocalised electrons (conductivity).
Acids & Bases	Acid: donates H⁺; Base: accepts H⁺. pH scale (0–14); pH <7 acidic, >7 basic, =7 neutral. Common acids: HCl, H₂SO₄. Bases: NaOH, KOH.
Neutralisation	Acid + Base → Salt + Water. Example: HCl + NaOH → NaCl + H₂O.
Stoichiometry	Relates reactants/products by mole ratios. Use molar masses to convert grams ↔ moles.
Solutions & Concentration	Molarity (M) = moles solute / liters solution. Molality (m) = moles solute / kg solvent.
Gas Laws	PV = nRT; Boyle’s law (P ∝ 1/V), Charles’ law (V ∝ T), Avogadro’s law (V ∝ n).
Thermodynamics	Heat (q), enthalpy (ΔH), entropy (ΔS). Endothermic absorbs heat; exothermic releases.
Kinetics	Rate of reaction depends on concentration, temperature, catalysts.
Redox Reactions	Oxidation = loss of electrons; Reduction = gain of electrons. Balancing uses oxidation numbers.
Common Industrial Chemicals	- Sulphuric Acid (H₂SO₄) – used in battery manufacturing, detergents. - Potassium Permanganate (KMnO₄) – oxidiser, used in water treatment. - Chlorine (Cl₂) – disinfectant, bleaching.
Environmental Chemistry	- Acid rain (H₂SO₄, HNO₃). - Ozone depletion (CFCs). - Greenhouse gases (CO₂, CH₄).
Forensic Chemistry	Identification of substances: chromatography, spectroscopy, ion-selective electrodes.

3. Worked Example – Stoichiometry

Problem
A crime scene investigation finds 10 g of sodium chloride (NaCl) in a spill. How many grams of sodium (Na) would be present if the chloride were completely reduced to sodium metal in a laboratory experiment?

Solution

Write the balanced equation (simplified conversion): [ \text{NaCl} \rightarrow \text{Na} + \frac{1}{2}\text{Cl}_2 ] (1 mole NaCl produces 1 mole Na.)
Find moles of NaCl
Molar mass of NaCl = 22.99 (Na) + 35.45 (Cl) = 58.44 g mol⁻¹
[ n_{\text{NaCl}} = \frac{10,\text{g}}{58.44,\text{g mol}^{-1}} = 0.171,\text{mol} ]
Moles of Na produced = 0.171 mol (1:1 ratio).
Mass of Na
Molar mass of Na = 22.99 g mol⁻¹
[ m_{\text{Na}} = 0.171,\text{mol} \times 22.99,\text{g mol}^{-1} = 3.93,\text{g} ]

Answer: About 3.9 g of sodium would be obtained.

In practice, such reduction is not feasible with NaCl alone, but the stoichiometry principle holds.

4. Quick Revision Summary

Concept	Key Formula / Idea	Quick Mnemonic
Atomic Number	(Z = \text{protons})	“Zee the protons”
Ionic Bond	Transfer of electrons; charge balance	“Ion exchange”
pH	(\text{pH} = -\log[H^+])	“pH is the log of hydrogen”
Neutralisation	Acid + Base → Salt + H₂O	“AB → SH”
Molarity	(M = \frac{\text{moles solute}}{\text{liters solution}})	“Molarity = moles per litre”