Some Basic Concepts of Chemistry
Class 11 – CBSE (Chemistry)
1. Simple Explanation
Chemistry is the science that studies matter, its composition, structure, properties, and the changes it undergoes.
- Matter is anything that has mass and occupies space.
- It can be a pure substance (element or compound) or a mixture (physical combination of substances).
- Changes in matter are classified as physical (no new substance produced) or chemical (new substances formed).
- Understanding matter requires knowledge of the atom (the smallest unit of an element), the arrangement of atoms in molecules, and how they interact (chemical bonding).
- The Periodic Table arranges elements by increasing atomic number and organizes them into groups (families) and periods (rows) based on their electronic configuration and properties.
2. Key Points
| Concept | Key Points |
|---|---|
| Matter | - Has mass & occupies volume. - Can exist as solid, liquid, gas. - Classified as pure substance or mixture. |
| Pure Substances | Elements – one type of atom. Compounds – two or more elements chemically combined in fixed ratio. |
| Mixtures | Homogeneous (solution) – uniform composition. Heterogeneous – distinct phases. |
| Physical Changes | No new substances; examples: melting, boiling, dissolution. |
| Chemical Changes | New substances formed; examples: combustion, rusting, photosynthesis. |
| Properties | Intrinsic – independent of amount (e.g., density). Extrinsic – depend on amount (e.g., mass). Physical – change without changing composition (e.g., color). Chemical – involve change in composition. |
| Atomic Theory (Dalton) | 1. Matter is made of atoms. 2. Same elements have identical atoms. 3. Chemical reactions involve rearrangement of atoms. |
| Structure of Atom | Proton (⁺) & neutron (neutral) in nucleus; electrons (–) in shells. Atomic number = # of protons. Mass number = # of protons + neutrons. |
| Periodic Table | 18 groups (columns) & 7 periods (rows). Groups share similar valence electron configuration → similar properties. Metals, non‑metals, metalloids. |
| Valency | Number of electrons an atom can gain, lose, or share to form bonds. |
| Chemical Bonding | Ionic – transfer of electrons. Covalent – sharing of electrons. Metallic – delocalized electrons. |
| Equation Balancing | Law of conservation of mass – atoms must balance on both sides. |
3. Worked Example
Problem:
Identify whether the following process is a physical or chemical change and justify your answer.
A piece of iron is placed in a jar of water and left for 24 h. After that, the iron turns reddish‑brown.
Solution
- Observation: Iron changes color from metallic gray to reddish‑brown.
- Check for product formation: A new substance (iron(III) oxide, Fe₂O₃) is produced.
- Mass change: The iron mass increases due to oxygen uptake.
- Conclusion: This is a chemical change (oxidation).
- Equation (simplified):
[ 4,\text{Fe} + 3,\text{O}_2 ;\xrightarrow{\text{water}}; 2,\text{Fe}_2\text{O}_3 ] - Why it’s not physical:
- Physical changes do not create new substances or alter composition.
- Color change alone is not enough; here a new oxide compound is formed.
4. Quick Revision Summary
- Matter = substance with mass & volume.
- Pure vs. Mixture – elements, compounds, homogeneous/heterogeneous.
- Physical change – no new substances.
- Chemical change – new substances; obeys conservation of mass.
- Atomic theory – atoms are indivisible units; reactions rearrange atoms.
- Atomic structure – nucleus (protons + neutrons) + electron shells.
- Periodic Table – groups (similar properties) & periods (energy levels).
- Valency – electron exchange/ sharing quantity.
- Chemical bonds: ionic, covalent, metallic.
- Balancing equations – atoms on both sides must be equal.
Use these points as a mental checklist whenever you encounter a new topic in chemistry. Happy studying!