QCC Notes
CLASS 11 · CHEMISTRYJEE MAIN × NEETहिंदी
§ 1.3NCERT Class 11 · Chemistry · Chapter 1

Properties of Matter and their Measurement: SI Units

Chemistry is quantitative — every property must be measured in a language everyone agrees on. That language is SI: seven base units, a decimal prefix ladder from picometres to terametres, and the mass–weight distinction that fools someone in every exam hall.

QCC Notes · Class 11 Chemistry NCERT & NTA aligned UPDATED 30 AUG 2026 9 MIN READ
01

How Do We Measure Matter? — Complete Theory

Matter shows two kinds of properties. Physical properties can be measured without changing the substance’s identity — colour, odour, melting point, boiling point, density. Chemical properties appear only by changing identity — acidity, basicity, flammability, how it reacts with oxygen or acids. Boiling water leaves you water; burning methane leaves you CO2 and H2O — that is the line between the two.

Measurement itself needs a reference standard — otherwise “two metres” means whatever the speaker’s arm span is. Science settled this with the International System of Units (SI), adopted in 1960, refined most recently in 2019 when every base unit was redefined in terms of fixed constants of nature (the kilogram via Planck’s constant, the mole via an exact Avogadro constant — the same fact § 1.8 uses).

Table 1 — The seven SI base units
QuantityUnitSymbolChemistry relevance
LengthmetremBond lengths ~10⁻¹⁰ m
MasskilogramkgWeighing reagents (practically in g, mg)
TimesecondsReaction rates (§ kinetics)
Electric currentampereAElectrochemistry
TemperaturekelvinKGas laws, equilibrium — always K in formulas
Amount of substancemolemolThe counting unit of § 1.8
Luminous intensitycandelacdRarely used in chemistry

Chemistry also spans an absurd range of sizes — atomic radii near 100 pm, wavelengths near 500 nm, flasks of 250 mL, factory tanks of 50 m³. SI handles this with decimal prefixes: multiply or divide the base unit by powers of ten. The ladder below places the exam’s favourites with real examples; the bench converts them live.

Volume1 L = 1 dm³ = 10⁻³ m³ Small volume1 mL = 1 cm³ DensityD = m / V

Density is the derived unit that runs the whole chapter: mass per unit volume, D = m/V, in g/cm³ (lab scale) or kg/m³ (SI) — the two differ by exactly 1000. Water is 1.0 g/cm³ = 1000 kg/m³; gold is 19.3 g/cm³. Every stoichiometry problem that starts “a 25.0 mL sample of…” quietly uses volume ↔ mass through density.

02

Visualising the Prefix Ladder

Chaudah gunaah scale, ek ruler — from atomic radii to road distances, and a converter bench to move between any two rungs.

The decimal prefix ladder — chemistry’s size range on one axis FIG. 1 — THE SI PREFIX LADDER (LOG SCALE) 10⁻¹² 10⁻⁹ 10⁻⁶ 10⁻³ 1 (m) 10³ 10⁶ 10⁹ 10¹² pmatomic radius ≈ 100 pm nmgreen light ≈ 500 nm μmcell ≈ 10 μm myour height ≈ 1.7 m kmDelhi–Gurugram ≈ 30 km chemistry lives left of centre — prefixes are survival gear giga, mega & co. follow the same ×1000 rhythm
FIG. 1 — Every step on this axis is a ×1000 jump: pm → nm → μm → mm → m → km. Chemistry’s typical objects (atoms, molecules, flasks) sit left of the metre mark, which is why the exam’s conversions are mostly pm–nm–μm on one side and mL–L on the other.
Try it live

Unit Bench

Length and volume families cover the chapter’s working range; the temperature bench converts through a Celsius hub exactly as the formulas do (K = °C + 273.15 · °C = 5/9(°F − 32)).

03

Solved Examples (Step-by-Step)

Given → factor → substitute → unit-check. Jo conversions yahan dikhte hain, wahi har numerical ke opening moves hain.

EXAMPLE 01Foundation · Temperature

Two-way temperature conversion

Convert (a) 25 °C to kelvin, and (b) 400 K to °C and °F.

  1. (a) FormulaK = °C + 273.15 → K = 25 + 273.15
  2. (a) Result= 298.15 K — lab temperature, the number behind every 25 °C rate constant.
  3. (b) Kelvin → C°C = 400 − 273.15 = 126.85 °C
  4. (b) C → F°F = 9/5 × 126.85 + 32 = 260.33 °F — hub through Celsius, never direct.

298.15 K · 126.85 °C · 260.33 °F

EXAMPLE 02JEE Main · Density + units

Density across unit systems

A metal block has mass 50.0 g and volume 20.0 cm³. Express its density in g/cm³ and in SI units (kg/m³).

  1. FormulaD = m/V
  2. Lab unitsD = 50.0/20.0 = 2.50 g/cm³
  3. Convert1 g = 10⁻³ kg, 1 cm³ = 10⁻⁶ m³ → 2.50 × 10⁻³/10⁻⁶ = 2.50 × 10³ kg/m³
  4. Shortcut checkg/cm³ → kg/m³ is × 1000 — always. 2.50 → 2500 ✓

2.50 g/cm³ = 2500 kg/m³

EXAMPLE 03NEET · Mass vs weight

The astronaut audit

An astronaut’s mass is 60 kg. Find her weight on Earth (g = 9.8 m/s²) and on the Moon (g = 1.63 m/s²), and state her mass on the Moon.

  1. Earthw = m·g = 60 × 9.8 = 588 N
  2. Moonw = 60 × 1.63 ≈ 98 N — about one-sixth of Earth’s.
  3. MassUnchanged: 60 kg — matter content doesn’t move with gravity; the reading in N does.

588 N → 98 N · mass stays 60 kg

05

Key Takeaways

Key card

Eight lines that solve this topic

Physical propertyMeasured without changing identity — colour, mp, bp, density.
Chemical propertyObserved only through change — acidity, flammability, reactivity.
7 SI base unitsm · kg · s · A · K · mol · cd — all else derived.
Prefix ladderT G M k c m μ n p — each step ×1000 in the middle, ×10 at k/c.
Mass vs weightMass kg, constant; weight = m·g in N, location-dependent.
K = °C + 273.15 · °F = 9/5°C + 32Kelvin carries no ° sign; gas-law inputs are always kelvin.
1 L = 1 dm³ = 10⁻³ m³ · 1 mL = 1 cm³The two volume identities behind every solution calculation.
D = m/Vg/cm³ × 1000 = kg/m³; water = 1.0 g/cm³.

Scale anchors: atomic radius ≈ 100 pm · green light ≈ 500 nm · cell ≈ 10 μm · water 1.0 g/cm³  ·  2019 SI: kg via Planck constant, mol via exact NA, K via Boltzmann constant

  1. The identity test sorts every property — unchanged identity = physical; changed identity = chemical.
  2. Seven base units, one prefix ladder — chemistry’s sizes live between pm and L; conversions are ×1000 hops.
  3. Mass travels, weight doesn’t — kg is luggage; N is what the floor feels.
  4. Density is the chapter’s bridge — it converts volumes to masses and starts every solution-stoichiometry problem.
06

FAQs

What is the difference between physical and chemical properties?

A physical property can be measured or observed without changing the substance's identity — colour, odour, melting point, boiling point, density. A chemical property appears only when the substance undergoes a change in composition — acidity, flammability, how it reacts with oxygen or acids.

What are the seven SI base units?

Metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance and candela (cd) for luminous intensity. All other units are derived from these seven.

What is the difference between mass and weight?

Mass is the amount of matter in an object — constant everywhere — measured in kilograms. Weight is the force gravity exerts on that mass (w = m × g), measured in newtons; it varies with location, about one-sixth on the Moon. A 60 kg person has 60 kg mass on Earth and the Moon, but weighs about 588 N here and 96 N there.

How do you convert between litres and cubic metres?

1 L = 1 dm³ = 10⁻³ m³, and 1 mL = 1 cm³. So 10 L equals 0.01 m³, and a 500 mL bottle has a volume of 500 cm³. Density links these to mass: D = m/V, commonly in g/cm³ or kg/m³, which differ by a factor of 1000.

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QCC Notes — Class 11 Chemistry

Strictly NCERT-aligned notes for JEE Main & NEET, prepared by QCC Notes (Padho Likho JEE). Content follows the latest NCERT edition and current NTA exam pattern.

Last updated
30 Aug 2026