SI Units of Mass, Volume, Density and Temperature (Class 11 Chemistry)
Chemistry · Some Basic Concepts Of Chemistry · NEET
In the SI system, mass is measured in kilogram (kg), volume in cubic metre (m³), density in kg/m³ (kg per cubic metre), and temperature in kelvin (K). In the lab, chemists use smaller units: gram (g) for mass, cm³ or mL for volume, and g/cm³ for density. Memory hook: "Kilo-Metre-cube Keeps Kelvin" — the four SI answers are kg, m³, kg/m³, and K.
The four measured quantities with their SI units (top row) and lab units, plus the two key relations: Mass = Density × Volume and the Celsius-to-Kelvin / Celsius-to-Fahrenheit conversions.
Your doubts, answered
What is the SI unit of density — kg/m³ or g/cm³?
The true SI unit of density is kg/m³ (kilogram per cubic metre), because it is built from the SI unit of mass (kg) divided by the SI unit of volume (m³). But this unit is very large for chemistry, so chemists usually write density in g/cm³. Both are correct units; for NEET, if the question says 'SI unit', the answer is kg/m³. To convert: 1 g/cm³ = 1000 kg/m³ (water = 1 g/cm³ = 1000 kg/m³).
Is temperature measured in Celsius or Kelvin in the SI system?
Kelvin (K) is the SI unit of temperature, not Celsius. Celsius (°C) and Fahrenheit (°F) are common scales but not SI. In NEET, always treat K as the base SI unit. To go from Celsius to Kelvin, just add 273.15: K = °C + 273.15. This matters because gas law formulas (PV = nRT) only work with temperature in kelvin.
How do I convert Celsius to Kelvin and Celsius to Fahrenheit?
Two NCERT formulas: (1) Kelvin: K = °C + 273.15. So 25 °C = 298.15 K. (2) Fahrenheit: °F = (9/5)(°C) + 32. So 100 °C = (9/5)(100) + 32 = 212 °F. Water freezes at 0 °C = 273.15 K = 32 °F and boils at 100 °C = 373.15 K = 212 °F. Only kelvin has no minus sign.
Why can temperature be negative in Celsius but not in Kelvin?
The Celsius scale sets 0 at the freezing point of water, so colder things get negative values (like −10 °C). The Kelvin scale starts at absolute zero (0 K = −273.15 °C), the lowest temperature possible where particle motion stops. Nothing can be colder than 0 K, so kelvin values are never negative. This is exactly why the SI scale is kelvin.
Is 1 mL the same as 1 cm³, and how does volume relate to m³?
Yes, 1 mL = 1 cm³ exactly. Volume is (length)³, so the SI unit is m³. Useful conversions from NCERT: 1 L = 1000 mL = 1 dm³, and 1000 cm³ = 1 dm³. Also 1 m³ = 1000 L. In lab problems, density in g/cm³ combined with volume in mL lets you find mass, because 1 cm³ = 1 mL of solution has mass = density × volume.
How do I get mass from density and volume?
Use Mass = Density × Volume (from NCERT). If a solution has density 1.28 g/cm³ and you take 1000 cm³ (1 L), then mass = 1.28 × 1000 = 1280 g. This single relation is the hidden step in most NEET 'density → molality' questions: it lets you find the mass of the whole solution from its volume.
⚠️ The NEET trap ✗ Picking g/cm³ as the SI unit of density, or leaving temperature in °C inside a gas-law formula. ✓ The SI unit of density is kg/m³ (g/cm³ is only the common lab unit). The SI unit of temperature is kelvin, so always convert using K = °C + 273.15 before using PV = nRT. 🧠 SI question → SI answer: mass = kg, volume = m³, density = kg/m³, temperature = K. Convert to kelvin before any calculation.
Real NEET questions
NEET 2019 (Odisha)
The density of a 2 M aqueous solution of NaOH is 1.28 g cm⁻³. The molality of the solution is (molar mass of NaOH = 40 g mol⁻¹)
A · 1.20 m
B · 1.56 m
C · 1.67 m ✓
D · 1.32 m
Solution: This is a direct use of Mass = Density × Volume. Take 1 L (1000 cm³) of solution containing 2 mol NaOH. Mass of solution = density × volume = 1.28 × 1000 = 1280 g. Mass of solute (NaOH) = 2 × 40 = 80 g. Mass of solvent (water) = 1280 − 80 = 1200 g = 1.2 kg. Molality = moles of solute / kg of solvent = 2 / 1.2 = 1.67 m. Answer: (C). The density-and-volume step is the whole SI-units idea being tested.
NEET 2018
In which of the following is the number of water molecules maximum?
A · 0.00224 L of water vapour at 1 atm and 273 K
B · 0.18 g of water
C · 18 mL of water ✓
D · 10⁻³ mol of water
Solution: For option C you must use density of water = 1 g/mL, so 18 mL of water = 18 mL × 1 g/mL = 18 g = 18/18 = 1 mol. Compare all: A = 0.00224/22.4 = 10⁻⁴ mol; B = 0.18/18 = 10⁻² mol; C = 1 mol (largest); D = 10⁻³ mol. So 18 mL of water has the most molecules. Answer: (C). Notice you cannot solve C without the mass–volume–density link (1 mL = 1 cm³, density 1 g/cm³).
Solved Some Basic Concepts Of Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the SI units of mass, volume, density and temperature?
Mass = kilogram (kg), Volume = cubic metre (m³), Density = kilogram per cubic metre (kg/m³), Temperature = kelvin (K). These are the SI answers NEET expects.
What is the lab unit of density chemists actually use?
Chemists use g/cm³ because kg/m³ is too large for small amounts. Water has density 1 g/cm³ = 1000 kg/m³.
What is the formula to convert Celsius to Fahrenheit?
°F = (9/5)(°C) + 32. For example, 37 °C body temperature = (9/5)(37) + 32 = 98.6 °F.
Why do we add 273.15 to convert Celsius to Kelvin?
Because 0 K (absolute zero) equals −273.15 °C. So the kelvin value is always 273.15 more than the Celsius value: K = °C + 273.15.
Is 1 litre equal to 1 dm³?
Yes. 1 L = 1 dm³ = 1000 mL = 1000 cm³. Also 1 m³ = 1000 L.
What does high density mean about particles?
NCERT says higher density means the particles are packed more closely together in the same volume.
