1 Matter and Measurement

Matter and its properties

Matter is anything that has mass and occupies space. It includes solids, liquids, and gases, including invisible gases such as air. A pure substance has a consistent composition, whereas a mixture combines substances physically. Pure water is a compound; air is a mixture.

A property describes a characteristic of matter. Properties can be classified according to whether observing them changes the substance’s chemical identity. A physical property can be observed or measured without changing what the substance is. Color, density, hardness, melting point, and electrical conductivity are examples. When ice melts into liquid water, the substance remains water, so melting is a physical change.

A chemical property describes how a substance can change into different substances. Flammability and the tendency of iron to rust are examples. Burning wood and iron rusting are chemical changes because new substances form.

Properties are also classified as intensive or extensive. An intensive property does not depend on the amount of sample; density and temperature are examples. An extensive property depends on the amount, as mass and volume do.

States of matter

The state of matter depends on how its particles are arranged and move.

• Solid: Has a definite shape and definite volume. Its particles stay close together and vibrate around fixed positions.

• Liquid: Takes the shape of its container but has a definite volume. Its particles remain close together while moving past one another.

• Gas: Fills its container and has no definite volume. Its particles are far apart and move freely.

Heating or cooling can cause a physical change of state, which does not by itself change a substance’s chemical identity. Melting changes a solid to a liquid, freezing changes a liquid to a solid, vaporization changes a liquid to a gas, and condensation changes a gas to a liquid. Plasma, an ionized gas, is another state of matter and is found in settings such as stars.

Measurements and units

A measurement consists of a number and a unit. For example, a measurement might be $12.4\ \text{g}$. The unit identifies what is being measured, and the recorded value reflects the measuring instrument’s precision. Use appropriate units and do not report more precision than the instrument supports.

Chemistry commonly uses metric and SI units. Examples include:

• Length: meter ($\text{m}$) and centimeter ($\text{cm}$); a pencil is about $18\ \text{cm}$ long.

• Mass: kilogram ($\text{kg}$) and gram ($\text{g}$); a small sample can have a mass of $5.0\ \text{g}$.

• Volume: liter ($\text{L}$) and milliliter ($\text{mL}$); a beaker can contain $250\ \text{mL}$.

• Temperature: kelvin ($\text{K}$) and degree Celsius ($^{\circ}\text{C}$); water freezes at about $273\ \text{K}$ or $0\ ^{\circ}\text{C}$.

• Time: second ($\text{s}$); a reaction can take $30\ \text{s}$.

SI prefixes and unit conversion

An SI prefix indicates a power-of-ten factor. Common prefixes include kilo- ($\text{k}$, $10^3$), centi- ($\text{c}$, $10^{-2}$), and milli- ($\text{m}$, $10^{-3}$). For example, $1\ \text{km}=1000\ \text{m}$, and $1\ \text{g}=1000\ \text{mg}$. A liter equals $1000$ milliliters, and $1\ \text{mL}$ has the same volume as $1\ \text{cm}^3$.

Convert units by multiplying by a conversion factor equal to one. For example:

The liters cancel, leaving milliliters.

Mass, weight, and density

Mass measures the amount of matter in a sample. Weight is the force of gravity on that mass. An object’s weight can change with gravitational conditions even when its mass does not.

Density relates mass to volume. It is mass per unit volume:

For example, an object with a mass of $20\ \text{g}$ and a volume of $10\ \text{cm}^3$ has a density of $2\ \text{g/cm}^3$. Density is intensive: cutting a uniform sample in half reduces its mass and volume but leaves its density unchanged.