Introduction:
Chemistry is often called the central
science because it connects physics, biology, and environmental studies.
For students preparing for SSC, UPSC, Railway, and State PSC exams,
chemistry is a scoring subject if understood in simple terms. In this post, we
will cover the basic concepts of chemistry such as matter, laws of
chemical combination, Dalton’s atomic theory, mole concept, and more with real-life
examples. These basics will help you answer questions with confidence in
competitive exams.
Basics of Chemistry |
|
Before 18th century |
Greek Word (Alchemy) Means Magic |
After 18th cnetury |
Egypt Word (Chemea) Means Black Matter |
Father of Chemistry |
Antonie L Lavosier |
Book |
The element of chemistry |
Study of Matter is called Chemistry. |
|
Matter:
Definition: Anything that has mass and occupies space is
called matter.
Examples:
Water, air, table, even our body.
Matter exists in three primary states:
1. Solid – fixed shape and volume (e.g., ice,
wood).
Very high attraction force
between atoms & high density.
2. Liquid – fixed volume but no fixed shape
(e.g., water, milk).
Attraction force less than
solids & density is less than solids.
3. Gas – no fixed shape or volume (e.g.,
oxygen, carbon dioxide).
Attraction force is zero &
least density.
👉 Exam Tip: Definite shape in solid, No definite shape in liquid & gas.
Classification of Matter:
Matter can be classified into pure substances and mixtures.
1. Pure substances
o Elements: Made of one type of atom (e.g., gold, oxygen).
o Compounds: Made of two or more elements chemically combined (e.g., H₂O, NaCl).
2. Mixtures
o Homogeneous mixture: Same composition throughout (e.g., sugar in water).
o Heterogeneous mixture: Different composition (e.g., oil and water).
👉 Example:
Salt dissolved in water is a homogeneous
mixture.
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Physical and Chemical Changes
1. Physical change: No new substance is formed.
o Example: Melting of ice → water.
2. Chemical change: A new substance with different properties is formed.
o Example: Rusting of iron, burning of paper.
👉 Exam Note: Questions often ask to identify whether a process is physical or chemical.
Law of Conservation of Mass:
Proposed by: Antoine Lavoisier.
Law: “Mass
can neither be created nor destroyed in a chemical reaction.”
Example:
Burning of candle – wax decreases, but gases like CO₂ and H₂O are formed. Total mass before and
after remains the same.
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Law of Constant Proportions:
Proposed by: Joseph Proust.
Law: A
chemical compound always contains the same elements in the same proportion by
mass.
Example: Water (H₂O) always has 11.11% hydrogen and 88.89% oxygen by mass, whether taken from a river or lab.
Dalton’s Atomic Theory:
· Proposed by John Dalton (1808).
· Main points:
1. All matter is made of indivisible atoms.
2. Atoms of a given element are identical.
3. Atoms combine in simple ratios to form compounds.
Example: H₂ + O → H₂O (simple ratio 2:1).
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Berzelius Hypothesis
· Proposed by: Jons Jakob Berzelius.
· Hypothesis: Equal volumes of all gases under same conditions of temperature and pressure contain the same number of molecules.
👉 Note: This was later developed into Avogadro’s Hypothesis.
Example: 1 liter of oxygen and 1 liter of hydrogen (at same temp & pressure) contain the same number of molecules.
Change in State of Matter
Matter
can change from one state to another due to changes in temperature or pressure.
1. Solid → Liquid: Melting (Ice →
Water)
2. Liquid → Solid: Freezing, Solidification. Eg:
Clotting of blood.
3. Liquid → Gas: Evaporation/Boiling (Water →
Steam)Eg: Cloud formation
4. Gas → Liquid: Condensation (Steam →
Water) Eg: Fogging a mirror.
5. Solid → Gas: Sublimation (Camphor →
Vapour) Eg: Camphor, Iodine.
6. Gas → Solid: Deposition. Eg: Frost.
👉 Exam Tip: Direct questions are common.
Allotropy
Definition: Existence of an element in two or more forms in the same physical state.
Examples:
· Carbon → Diamond, Graphite, Fullerenes.
· Oxygen → O₂ (oxygen), O₃ (ozone).
Molecular Weight
Definition: Molecular weight is the sum of the atomic masses of all atoms in a molecule.
Example: H₂O →
(2×1) + (16) = 18 g/mol.
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Mole Concept
Definition: One mole of a substance contains 6.022 × 10²³ particles (Avogadro number).
Examples:
· 1 mole of H₂O = 6.022 × 10²³ molecules of water.
· 1 mole of Na = 6.022 × 10²³ atoms of sodium.
👉 Practice Example: Calculate number of molecules in 36 g of water.
· Molar mass of H₂O = 18 g/mol.
· 36 ÷ 18 = 2 moles.
· So, number of molecules = 2 × 6.022 × 10²³ = 12.044 × 10²³ molecules.
FAQs
Q1. What is matter in
chemistry?
Matter is anything that has mass and occupies space, such as water, air, or
metals.
Q2. What is the difference between physical and chemical change?
Physical change does not form a new substance (e.g., melting of ice), while
chemical change forms a new substance (e.g., rusting of iron).
Q3. Who proposed the law of conservation of mass?
It was proposed by Antoine Lavoisier.
Q4. What is allotropy?
It is the property of an element to exist in more than one form (e.g., diamond
and graphite are allotropes of carbon).
Q5. What is one mole in chemistry?
One mole is 6.022 × 10²³ particles of a substance (Avogadro number).
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