Mass, Volume, Density, and Temperature

The previous topic introduced the SI system and its seven base units. Now it is time to look at some of the most common physical quantities a chemist actually measures in the laboratory: mass, volume, density, and temperature. Each of these has its own units, its own measuring instruments, and its own set of relationships that connect it to other quantities. Understanding them well is essential before you can make sense of any quantitative work in chemistry.

Mass and Weight: Two Different Things

People often use the words “mass” and “weight” as though they mean the same thing, but in science they refer to two distinct quantities:

Mass is the amount of matter present in a substance. It is an intrinsic property, meaning it depends only on how much material is there, not on where you happen to be measuring it. Whether you are standing in your school lab or floating in a space station, the mass of a 50 g sample of sodium chloride stays at 50 g.
Weight, on the other hand, is the force that gravity pulls on that mass. Since gravitational strength varies from place to place (it is slightly different at the equator compared to the poles, and much weaker on the Moon), the weight of the same object can change depending on where it is located.

In short: mass is constant, weight is not. Be careful not to mix the two up.

Measuring Mass in the Laboratory

In a chemistry lab, mass is determined using an analytical balance. This is a precision instrument designed to measure very small masses accurately.

Fig 1.5: Analytical balance

The SI unit of mass is the kilogram ( $\text{kg}$ ), but since chemical reactions typically involve small amounts of substances, chemists work with the gram ( $\text{g}$ ) as their everyday unit. The two are related simply:

$1 \text{ kg} = 1000 \text{ g}$

Volume: Measuring How Much Space a Substance Occupies

Volume tells you how much three-dimensional space a substance takes up. Since it involves three length measurements (length, width, and height), its dimensions are $(\text{length})^3$ .

In SI, the unit of volume is the cubic metre ( $\text{m}^3$ ), but this is far too large for typical laboratory work. Chemists use smaller, more practical units instead:

Unit	Symbol	Relationship
Cubic centimetre	$\text{cm}^3$	$1 \text{ cm}^3 = 1 \text{ mL}$
Cubic decimetre	$\text{dm}^3$	$1 \text{ dm}^3 = 1000 \text{ cm}^3 = 1 \text{ L}$
Litre	$\text{L}$	$1 \text{ L} = 1000 \text{ mL}$

The litre ( $\text{L}$ ) is not officially an SI unit, but it is so widely used for measuring liquid volumes that it has become a standard laboratory unit.

Visualising the Relationship

Think of a cube with each side measuring $10 \text{ cm}$ (which is $1 \text{ dm}$ ). The volume of that cube is:

$10 \text{ cm} \times 10 \text{ cm} \times 10 \text{ cm} = 1000 \text{ cm}^3$

That is exactly $1 \text{ dm}^3$ , which equals $1 \text{ L}$ . And if you zoom in to one tiny corner of that cube and imagine a cube with each side just $1 \text{ cm}$ , its volume is $1 \text{ cm}^3$ , which equals $1 \text{ mL}$ .

Fig 1.6: Different units used to express volume

Laboratory Devices for Measuring Volume

Several pieces of glassware are used to measure liquid volumes in the lab, each suited for a different purpose:

Graduated cylinder — reads volume across a wide range (marked in mL divisions along its height). Good for general-purpose measurements, though not the most precise.
Burette — a tall, narrow tube with a stopcock at the bottom. Used in titrations to deliver precise, measured amounts of liquid.
Pipette — a narrow tube with a bulb in the middle. Designed to transfer a single, fixed volume of liquid with high accuracy.
Volumetric flask — a flat-bottomed flask with a long, narrow neck and a single graduation mark. Used to prepare solutions of a known, exact volume.

Fig 1.7: Some volume measuring devices

Density: Linking Mass and Volume

Mass and volume are connected through a third quantity: density. It tells you how much mass is packed into each unit of volume.

$\text{Density} = \frac{\text{Mass}}{\text{Volume}}$

To find the SI unit of density, simply divide the SI unit of mass by the SI unit of volume:

$\text{SI unit of density} = \frac{\text{kg}}{\text{m}^3} = \text{kg m}^{-3}$

In practice, $\text{kg m}^{-3}$ produces inconveniently large numbers for the amounts chemists typically work with. That is why laboratory density values are almost always reported in $\text{g cm}^{-3}$ instead, with mass measured in grams and volume in cubic centimetres. The resulting numbers are far more practical for day-to-day lab work.

What Density Tells You

Density is essentially a measure of how tightly the particles of a substance are arranged. A substance with a higher density has its particles packed more closely together compared to a substance with a lower density. This is why, for example, a block of iron feels much heavier than a block of wood of the same size: iron particles are packed far more tightly.

Temperature: Three Scales, One Phenomenon

Temperature measures how hot or cold a substance is. Scientists use three scales to express temperature, and knowing how to move between them is an essential skill:

Scale	Unit	Symbol	Freezing Point of Water	Boiling Point of Water
Celsius	degree Celsius	$°\text{C}$	$0°\text{C}$	$100°\text{C}$
Fahrenheit	degree Fahrenheit	$°\text{F}$	$32°\text{F}$	$212°\text{F}$
Kelvin	kelvin	$\text{K}$	$273.15 \text{ K}$	$373.15 \text{ K}$

Among these, the kelvin ( $\text{K}$ ) is the SI unit of temperature.

A few everyday reference points help put these scales in perspective: normal human body temperature is $37°\text{C}$ ( $310 \text{ K}$ or $98.6°\text{F}$ ), and typical room temperature is about $25°\text{C}$ ( $298 \text{ K}$ or $77°\text{F}$ ).

Fig 1.8: Thermometers using different temperature scales

How the Celsius Scale Works

The Celsius scale divides the temperature range between the freezing and boiling points of water into 100 equal parts. Water freezes at $0°\text{C}$ and boils at $100°\text{C}$ . This makes it intuitive for everyday and laboratory use.

Converting Between Scales

Two key formulas let you move between these scales:

Celsius to Fahrenheit:

$°\text{F} = \frac{9}{5}(°\text{C}) + 32$

Celsius to Kelvin:

$\text{K} = °\text{C} + 273.15$

An Important Feature of the Kelvin Scale

One thing that sets the Kelvin scale apart is that it has no negative values. The Celsius scale can go below zero (for instance, $-10°\text{C}$ on a cold winter day), but the Kelvin scale starts at absolute zero ( $0 \text{ K}$ ), the lowest temperature that is physically possible. There is nothing colder than $0 \text{ K}$ , so negative Kelvin values simply do not exist.

This is why the Kelvin scale is preferred in scientific calculations: it avoids the complications that negative temperatures can introduce in mathematical formulas.