Measurement and Description of Matter in Chemistry

Describing Matter

Qualitative and Quantitative Observations

In chemistry, matter is described using both qualitative and quantitative methods. Understanding the distinction between these two types of observations is fundamental to scientific measurement and analysis.

• Qualitative Observations: Descriptions that do not involve numbers. Examples include color, temperature (hot or cold), and size (large or small).

• Quantitative Measurements: Observations that involve numbers and units, such as mass, volume, or temperature measured with a thermometer.

Measurement

The SI (International System) of Units

The SI system, also known as the Metric System, is the standard set of units used in science for consistency and clarity. It was developed in 1960 and is maintained by an international agency in Paris. With few exceptions, it is used worldwide.

• SI Base Units: The fundamental units from which all other units are derived.

Quantity	Unit	Symbol
Length	Meter	m
Mass	Kilogram	kg
Time	Second	s
Temperature	Kelvin	K
Amount of substance	Mole	mol
Electric current	Ampere	A
Luminous intensity	Candela	cd

Note: Chemistry students should especially know the units for length, mass, time, temperature, and amount of substance.

SI System of Units: Global Use

• The SI system is legally used in almost every country.

• The meter (m) is slightly longer than a yard (1 yard = 36 inches, 1 meter = 39.37 inches).

Measurement in the United States

• In 1988, the U.S. government designated the metric system as the preferred system for trade and commerce.

• Customary (imperial) units are still widely used in daily life, but SI units are standard in scientific, medical, and military contexts.

Kilogram: Unit of Mass

Definition of Mass

Mass is a measure of the quantity of matter in an object. The SI unit for mass is the kilogram (kg).

• Example: A laboratory balance may show a mass of 5.01 g for a sample.

Temperature Scales

Fahrenheit, Celsius, and Kelvin

Temperature can be measured using different scales. The three most common are Fahrenheit (°F), Celsius (°C), and Kelvin (K).

Scale	Boiling Point of Water	Freezing Point of Water
Fahrenheit	212 °F	32 °F
Celsius	100 °C	0 °C
Kelvin	373 K	273 K

• 1 kelvin degree is equal in size to 1 degree Celsius.

• The Kelvin scale starts at absolute zero (0 K), the lowest possible temperature.

• Conversion between Celsius and Kelvin:

$T(K) = T(°C) + 273.15$

• Example: 25 °C = 25 + 273.15 = 298.15 K