Properties of Matter and Their Measurement

You already know that matter exists in different forms and can be classified in several ways. But how do scientists actually describe and compare different substances? They do it through properties, characteristics that are unique to each substance and can be observed or measured. And when it comes to measurement, having a common language of units is essential so that scientists everywhere can understand each other’s data. This topic covers both ideas: the two broad categories of properties, and the international measurement system that the scientific world relies on.

Two Kinds of Properties: Physical and Chemical

Every substance has its own set of characteristic properties. These fall into two broad categories:

• Physical properties — attributes you can measure or observe without changing the substance into something different. The substance stays exactly what it was before and after the observation. Colour, odour, melting point, boiling point, and density are all physical properties. You can measure the boiling point of water, for instance, and the water remains water throughout.

• Chemical properties — attributes that only show up when the substance undergoes a chemical change, meaning its composition and identity actually transform. Combustibility (whether something catches fire), reactivity with acids, and whether a substance behaves as an acid or a base are all chemical properties. To know that hydrogen is combustible, you have to burn it, and once it burns, the hydrogen is gone and a new substance (water) has formed.

The practical difference is straightforward: measuring a physical property leaves the substance untouched, while observing a chemical property requires the substance to react and become something else.

Chemists rely on both types of properties to describe, interpret, and predict how substances behave. This understanding comes from careful measurement and experimentation, which brings us to the next question: how do we actually measure physical properties in a reliable, universal way?

Why Quantitative Measurement Matters

Science depends on numbers. Saying a room is “long” tells you very little, but saying it is 6 m long gives precise, reproducible information that anyone can verify. This is the difference between a qualitative observation (descriptive, no numbers) and a quantitative measurement (a number paired with a unit).

Every quantitative measurement has exactly two parts:

1. A number — telling you how much.
2. A unit — telling you on what scale the measurement was made.

Without the unit, the number is incomplete. “6” on its own could mean 6 metres, 6 feet, or 6 anything. The unit anchors the number to a specific standard.

The Need for a Common System

For a long time, different parts of the world used different measurement systems. The two most widespread were:

• The English System — used primarily in Britain and its former colonies.
• The Metric System — developed in France in the late eighteenth century. It was more convenient because it was built on the decimal system (powers of 10), which made converting between larger and smaller units simple arithmetic.

As science became increasingly international, the lack of a shared system created confusion. A measurement reported in one system had to be converted before anyone using a different system could make sense of it. The scientific community recognised that a single, universally accepted system of units was essential.

The International System of Units (SI)

That universal system arrived in 1960, when the 11th General Conference on Weights and Measures established the International System of Units. In French, this is called Le Systeme International d’Unites, which is why it is abbreviated as SI.

The conference itself, known by its French abbreviation CGPM (Conference Generale des Poids et Mesures), is an inter-governmental treaty organisation. It was created through a diplomatic agreement called the Metre Convention, signed in Paris in 1875. The CGPM continues to oversee and refine the SI system to this day.

The Seven Base Units

The entire SI system rests on seven base units. Each one corresponds to a fundamental physical quantity that cannot be broken down into simpler quantities. Every other measurable quantity in physics and chemistry, such as speed, volume, density, force, and pressure, can be derived from combinations of these seven.

Base Physical Quantity	Symbol	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	$I_v$	candela	cd

Notice that the table covers everything from everyday quantities like length and mass to more specialised ones like luminous intensity (a measure of how bright a light source appears in a particular direction). Together, these seven form the foundation on which all scientific measurement is built.

How Each Base Unit Is Defined

Modern SI definitions are anchored to fundamental constants of nature rather than to physical objects. This means any well-equipped laboratory anywhere in the world can reproduce these units independently. Here is what each definition rests on:

• Metre (m) — Defined by fixing the speed of light in vacuum, $c$, at exactly 299,792,458 $\text{m s}^{-1}$. In simple terms, one metre is the distance light travels in a tiny, precisely defined fraction of a second.

• Kilogram (kg) — Defined by fixing the Planck constant, $h$, at exactly $6.62607015 \times 10^{-34}$ $\text{J s}$ (which equals $\text{kg m}^2 \text{s}^{-1}$). This links the unit of mass to a quantum-mechanical constant rather than a platinum-iridium cylinder sitting in a vault.

• Second (s) — Defined by fixing the caesium frequency, $\Delta V_{Cs}$, at exactly 9,192,631,770 Hz. This frequency is the unperturbed ground-state hyperfine transition of the caesium-133 atom, an atomic clock standard that is extremely stable and reproducible.

• Ampere (A) — Defined by fixing the elementary charge, $e$, at exactly $1.602176634 \times 10^{-19}$ C (coulombs, where C = A s). One ampere is the flow of a specific number of elementary charges per second.

• Kelvin (K) — Defined by fixing the Boltzmann constant, $k$, at exactly $1.380649 \times 10^{-23}$ $\text{J K}^{-1}$. This ties temperature to the average kinetic energy of particles at the molecular level.

• Mole (mol) — Defined as the amount of substance that contains exactly $6.02214076 \times 10^{23}$ elementary entities. This number is the fixed value of the Avogadro constant ($N_A$) and is called the Avogadro number. The entities can be atoms, molecules, ions, electrons, or any other specified particles or groups of particles.

• Candela (cd) — Defined by fixing the luminous efficacy of monochromatic radiation at a frequency of $540 \times 10^{12}$ Hz at exactly 683 $\text{lm W}^{-1}$. This connects the unit of light intensity to a precisely defined relationship between radiated power and the visual response of the human eye.

SI Prefixes: Scaling Units Up and Down

Scientists regularly work with quantities that are enormously large or incredibly tiny. Writing out all the zeros would be impractical, so the SI system provides a set of prefixes that multiply or divide a unit by specific powers of ten. Attaching a prefix to a unit name creates a new, convenient-sized unit without changing the underlying measurement system.

Multiple	Prefix	Symbol		Multiple	Prefix	Symbol
$10^{-24}$	yocto	y		$10^{1}$	deca	da
$10^{-21}$	zepto	z		$10^{2}$	hecto	h
$10^{-18}$	atto	a		$10^{3}$	kilo	k
$10^{-15}$	femto	f		$10^{6}$	mega	M
$10^{-12}$	pico	p		$10^{9}$	giga	G
$10^{-9}$	nano	n		$10^{12}$	tera	T
$10^{-6}$	micro	$\mu$		$10^{15}$	peta	P
$10^{-3}$	milli	m		$10^{18}$	exa	E
$10^{-2}$	centi	c		$10^{21}$	zetta	Z
$10^{-1}$	deci	d		$10^{24}$	yotta	Y

A few examples to make this concrete:

• 1 kilometre (km) = $10^3$ metres = 1000 metres
• 1 millimetre (mm) = $10^{-3}$ metres = 0.001 metres
• 1 micrometre ($\mu$m) = $10^{-6}$ metres = 0.000001 metres
• 1 nanometre (nm) = $10^{-9}$ metres = 0.000000001 metres

The prefix system is one of the great practical strengths of the SI. Instead of inventing entirely new unit names for every scale, you simply attach the right prefix. Whether you are measuring the diameter of an atom (picometres) or the distance between cities (kilometres), the same base unit (metre) carries through, and the prefix tells you the scale.