Maharashtra TET – General Awareness & Current Affairs
Chemistry Basics
1. Simple Explanation
Chemistry is the science that studies matter—its composition, structure, properties, and the changes it undergoes.
- Matter: Anything that has mass and occupies space.
- Atoms: The smallest units of an element, made of a nucleus (protons + neutrons) surrounded by electrons.
- Elements: Pure substances made of only one type of atom.
- Compounds: Substances formed when two or more different elements chemically combine.
Understanding these fundamentals helps explain everyday phenomena—from cooking a meal to the functioning of modern technology.
2. Key Points
| Topic | Key Idea | Important Terms | Typical Question Format |
|---|---|---|---|
| Atomic Structure | Each atom has a nucleus (protons, neutrons) and electrons in shells. | Proton, Neutron, Electron, Atomic Number, Mass Number | “What is the mass number of ^23Na?” |
| Periodic Table | Elements are arranged by increasing atomic number; groups (columns) share properties; periods (rows) show electron shell filling. | Group, Period, Alkali Metals, Noble Gases | “Which group does chlorine belong to?” |
| Chemical Bonding | Atoms share or transfer electrons to achieve stable octets. | Ionic, Covalent, Metallic, Polar | “Identify the type of bond in NaCl.” |
| Types of Reactions | Matter transforms by breaking and forming bonds. | Synthesis, Decomposition, Single Replacement, Double Replacement, Combustion | “Write a combustion equation for methane.” |
| Stoichiometry | Quantitative relationships between reactants and products based on balanced equations. | Moles, Mole Ratio, Avogadro’s Number | “How many grams of H₂O form from 2 g of H₂?” |
| Conservation of Mass | Mass of reactants equals mass of products in a closed system. | Law of Conservation | “Explain the importance of balance in a chemical equation.” |
| Acids & Bases | Acids donate H⁺ ions; bases accept them. pH scale measures acidity (0–14). | pH, pOH, Strong/Weak | “What is the pH of a 0.01 M HCl solution?” |
| Electrochemistry | Redox reactions involve electron transfer; galvanic cells produce electrical energy. | Oxidation, Reduction, Electrode, Cathode, Anode | “Identify the anode in a Zn–Cu cell.” |
| Important Symbols & Notation | Standard notation for elements, ions, compounds, and equations. | ^{n+}, O₂, Fe₂O₃, Δ | “What does Δ represent in a reaction?” |
3. Worked Example
Problem:
Calculate the mass of sodium chloride (NaCl) produced when 5 g of sodium metal reacts completely with 3 g of chlorine gas.
Step‑by‑Step Solution
-
Write and balance the chemical equation
[ 2,\text{Na} + \text{Cl}_2 ;\longrightarrow; 2,\text{NaCl} ] -
Convert masses to moles
- Molar mass of Na = 22.99 g mol⁻¹
- Molar mass of Cl₂ = 70.90 g mol⁻¹
[ n_{\text{Na}} = \frac{5;\text{g}}{22.99;\text{g mol}^{-1}} = 0.217;\text{mol} ] [ n_{\text{Cl}_2} = \frac{3;\text{g}}{70.90;\text{g mol}^{-1}} = 0.0423;\text{mol} ]
-
Identify the limiting reagent
From the balanced equation, 2 mol Na react with 1 mol Cl₂.- Required Na for 0.0423 mol Cl₂ = (2 \times 0.0423 = 0.0846;\text{mol})
- Available Na = 0.217 mol > 0.0846 mol → Na is in excess.
- Limiting reagent: Cl₂.
-
Determine moles of product
1 mol Cl₂ gives 2 mol NaCl.
[ n_{\text{NaCl}} = 2 \times n_{\text{Cl}_2} = 2 \times 0.0423 = 0.0846;\text{mol} ] -
Convert moles of NaCl to mass
Molar mass of NaCl = 22.99 + 35.45 = 58.44 g mol⁻¹[ m_{\text{NaCl}} = 0.0846;\text{mol} \times 58.44;\text{g mol}^{-1} = 4.94;\text{g} ]
Answer: