Measurement and Description of Matter in General Chemistry

Describing Matter

Qualitative and Quantitative Observations

In chemistry, matter is described using both qualitative and quantitative approaches. These methods help scientists characterize substances and their properties.

• Qualitative observations: Descriptions based on characteristics such as color, temperature (hot or cold), and size (large or small). These do not involve numbers.

• Quantitative measurements: Observations that involve numerical values, such as mass, volume, or temperature.

• Example: Saying a solution is blue is qualitative; stating its temperature is 25°C is quantitative.

Measurement

SI (International System) Units

Chemistry and other sciences use the SI (Système International) system of units, commonly known as the Metric System. This system provides standardized units for scientific measurements.

• The SI system was developed in 1960 and is maintained by an international agency in Paris.

• With few exceptions, the SI system is legally used in almost every country worldwide.

• The meter (m) is the SI unit of length and is slightly longer than a yard (1 yard = 36 inches, 1 meter = 39.37 inches).

SI Base Units Table

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

Additional info: The most commonly used SI base units in chemistry are meter, kilogram, second, kelvin, and mole.

Measurement in the United States

Although the metric system is the international standard, the United States still uses customary units in many contexts. In 1988, the U.S. government designated the metric system as the preferred system for trade and commerce, but customary units remain common outside scientific, medical, and military fields.

• Customary units: Inches, feet, pounds, gallons, etc.

• SI units: Used in science, medicine, and the military.

Kilogram: Unit of Mass

Definition and Application

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

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

• Mass is a fundamental property used in chemical calculations, such as determining the amount of reactants or products.

Temperature Scales

Fahrenheit, Celsius, and Kelvin

Temperature is a key physical property in chemistry, measured using different scales. The three most common are Fahrenheit, Celsius, and Kelvin.

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

• Kelvin scale: The SI unit for temperature; absolute zero is 0 K.

• Conversion between Celsius and Kelvin:

$T(K) = T(°C) + 273.15$

• Example: 25°C = 298.15 K

Additional info: The Kelvin scale is used in scientific contexts because it starts at absolute zero, the theoretical point where all molecular motion ceases.