Chemistry · Some Basic Concepts Of Chemistry · NEET
Write the physical formula, then replace every quantity with its SI base unit. Speed = distance ÷ time → metre ÷ second = m/s (m s⁻¹). Volume = length × length × length → m × m × m = m³. Density = mass ÷ volume → kg ÷ m³ = kg/m³ (kg m⁻³). Pressure = force ÷ area → newton ÷ m² = N/m² = pascal. You never memorise the answer; you derive it. That is exactly why they are called 'derived' units.
The SI unit of density is kg m⁻³ (kg/m³), because density = mass/volume = kg ÷ m³. But this number is very large for everyday chemicals (water = 1000 kg/m³). So chemists express density in g cm⁻³ instead, where water = 1 g/cm³. Both are correct; g/cm³ is just a smaller, handier unit. Conversion: 1 g/cm³ = 1000 kg/m³. NCERT itself says the SI unit is 'quite large' so chemists prefer g cm⁻³.
Pressure = force ÷ area. Force is measured in newton (N) and area in m². So 1 pascal = 1 N/m² = 1 N m⁻². Going one step deeper, 1 N = 1 kg m s⁻² (from F = ma). So 1 Pa = 1 kg m s⁻² ÷ m² = 1 kg m⁻¹ s⁻². NEET loves asking pascal in pure base units: remember Pa = kg m⁻¹ s⁻².
The seven SI base units (metre, kilogram, second, kelvin, ampere, mole, candela) are chosen as fundamental — they are not built from anything else. Pascal is made by combining them (kg, m, s), so it is derived. Speed, volume, density and pressure are all derived because their formulas contain base quantities multiplied or divided together. Only the seven fundamental quantities get base units.
No, litre (L) is NOT an SI unit — the SI unit of volume is m³. Litre is a common, allowed unit. Useful equalities: 1 L = 1000 mL, 1 mL = 1 cm³, and 1 L = 1 dm³ = 1000 cm³. So 1 dm³ = 1000 cm³. This matters in NEET density/molarity sums where data is given in g mL⁻¹ or g cm⁻³ (they are the same thing).
Yes. Speed = distance/time and velocity = displacement/time, so both have the SI unit m/s (m s⁻¹). The difference is that velocity is a vector (has direction) and speed is a scalar. For unit purposes in NEET, both are m s⁻¹.
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)
The density of a 1 M solution of a compound X is 1.25 g mL⁻¹. The molality of the solution is (molar mass of X = 85 g mol⁻¹)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Units made by combining the seven base units through a formula — for example speed (m/s), volume (m³), density (kg/m³) and pressure (pascal, Pa).
The pascal (Pa). 1 Pa = 1 N/m² = 1 kg m⁻¹ s⁻². It equals force per unit area.
The SI unit of volume is the cubic metre, m³. Litre is not an SI unit but is commonly used: 1 L = 1 dm³ = 1000 cm³ = 1000 mL.
Yes. Since 1 mL = 1 cm³, density in g mL⁻¹ equals density in g cm⁻³. Both convert to SI as 1 g/cm³ = 1000 kg/m³.
NCERT gives air mass ≈ 1034 g cm⁻². Converting force per area to SI gives about 1.01 × 10⁵ Pa, close to standard atmospheric pressure. It shows pascal = force/area in action.
