12 min readConcept sheetSection A MCQ (1 mark) and objective (2 marks, two parts, often match a quantity with its unit); Section B very short answer (2 marks)

From the metric system to SI: the seven base quantities and their units

The decimal metric system grew into SI, adopted in 1960. SI has seven base quantities, each with one unit, such as the metre, kilogram and second.

SISystème International d'Unités (International System of Units), adopted at the 11th General Conference on Weights and Measures in 1960

Seven base unitsmetre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), candela (cd)

Base quantityA quantity independent of the others, from which all other quantities are built

Kelvin and CelsiusT (in K) = t (in °C) + 273.15; ice melts at 0 °C = 273.15 K

How the world came to one system of units

FPS (foot, pound, second) was the old British system. It is not part of SI.

The metric system and its first standards

In the 1790s, just after the French Revolution, French scientists built a new system based on tens, like our way of counting. The metre was first one ten-millionth of the distance from the North Pole to the equator. A platinum-iridium bar with two lines one metre apart became its working standard. A platinum-iridium cylinder with the mass of 1 litre (1 cubic decimetre) of water became the kilogram. The second came from the turning of the Earth: a day has 24 hours of 60 minutes of 60 seconds. In 1875, many countries signed the Metre Convention to use metric units in trade. Two versions were common: cgs (centimetre, gram, second) and mks (metre, kilogram, second). Since 1983, the metre is the distance light travels in vacuum in 1/299 792 458 of a second.

The seven SI base quantities

Base quantity	Symbol of the quantity	SI unit	Unit symbol
Length	l	metre	m
Mass	m	kilogram	kg
Time	t	second	s
Electric current	I	ampere	A
Thermodynamic temperature	T	kelvin	K
Amount of substance	n	mole	mol
Luminous intensity	Iᵥ (your NIOS lesson writes I)	candela	cd

Mass, amount of substance and luminous intensity

Mass (kilogram)How much matter a body has. A bag of rice may have a mass of 5 kg.

Amount of substance (mole)How many particles (atoms or molecules) there are. One mole of a substance has a mass equal to its molecular mass in grams: 1 mol of water (H₂O) is 18 g.

Luminous intensity (candela)How bright a light source is in a given direction. A candle flame gives roughly 1 candela.

Since 2019, all seven base units are defined from fixed constants of nature. The kilogram is no longer a metal cylinder; it is fixed by the Planck constant. Your NIOS lesson describes the older definitions.

Activity: read the scales on a thermometer

Use a clinical (doctor's) thermometer at home with an adult. A digital one is safer than one with mercury.

1
Look closely at the scale. Find the two sets of marks: degree Celsius (°C) and degree Fahrenheit (°F).
2
Note the range: a clinical thermometer reads only from about 35 °C to 42 °C (about 94 °F to 108 °F).
3
Find the least count: the value of one small division, often 0.1 °C.
4
Take your temperature under the tongue or in the armpit, and read both scales.
5★
Record it. Normal body temperature is about 37 °C, which is 98.6 °F.
Convert with °F = (9/5 × °C) + 32 and K = °C + 273.15. So 37 °C = 98.6 °F = 310.15 K.

★ key step

Activity: melting point of ice and boiling point of water (online lab or kitchen)

Do this in an online lab, or with a laboratory thermometer (range about −10 °C to 110 °C) and an adult's help.

1
Fill a funnel or a glass with crushed ice and bury the thermometer bulb in it.
2
Read the thermometer every 30 seconds.
The reading falls, then stays steady while the ice melts.
3★
Record the steady reading. The melting point of pure ice is 0 °C (273.15 K).
4
Heat water in a pan, with the bulb in the water but not touching the pan.
5
Read it every 30 seconds until it stops rising.
Pure water at normal air pressure boils at 100 °C (373.15 K).
6
Explain any difference from 0 °C and 100 °C.
Impurities such as salt, or a change in air pressure, shift these values. On a high hill, water boils below 100 °C.

★ key step

In real life

India's copy of the old standard kilogram, number 57, is kept at the National Physical Laboratory, New Delhi. The same laboratory keeps Indian Standard Time accurate with atomic clocks.

Exam angle

1 marks: “Name the SI unit of thermodynamic temperature and write its symbol.” kelvin, K.

2 marks: “Name any four SI base quantities and write their units with symbols.” Length: metre (m); mass: kilogram (kg); time: second (s); electric current: ampere (A). Others: temperature: kelvin (K); amount of substance: mole (mol); luminous intensity: candela (cd).

2 marks: “A learner says 'mass' and 'amount of substance' are the same quantity. Explain the difference, with their SI units.” Mass is the quantity of matter in a body, in kilograms (kg). Amount of substance counts its particles, in moles (mol). One mole has a mass equal to the molecular mass in grams, for example 1 mol of water = 18 g.

Remember it

SI seven, in order: 'My Kind Sister Always Keeps Many Candles' = metre, kilogram, second, ampere, kelvin, mole, candela.