Which one weighs more, a kilogram of feathers or a kilogram of bricks? Though many people will say that a kilogram of bricks is heavier, they actually weigh the same! However, many people are caught up by the concept of density, which causes them to answer the question incorrectly. A kilogram of feathers clearly takes up more space, but this is because it is less "dense." But what is density, and how can we determine it?

Introduction

Density (ρ) is a physical property found by dividing the mass of an object by its volume. Regardless of the sample size, density is always constant. For example, the density of a pure sample of tungsten is always 19.25 grams per cubic centimeter. This means that whether you have one gram or one kilogram of the sample, the density will never vary. The equation is as follows:

Density=MassVolume(1)

or just

ρ=mv(2)

Based on Equation 2, it's clear that density can, and does, vary from element to element and substance to substance due to differences in the relation of mass and volume. Let us break it down one step further. What are mass and volume? We cannot understand density until we know its parts: mass and volume. The following two sections will teach you all the information you need to know about volume and mass to properly solve and manipulate the density equation.

Mass

Mass concerns the quantity of matter in an object. The SI unit for mass is the kilogram (kg), although grams (g) are commonly used in the laboratory to measure smaller quantities. Often, people mistake weight for mass. Weight concerns the force exerted on an object as a function of mass and gravity. This can be written as

Weight=mass×gravity(3)

Weight=mg

Hence, weight changes due to variations in gravity and acceleration. For example, the mass of a 1 kg cube will continue to be 1 kg whether it is on the top of a mountain, the bottom of the sea, or on the moon, but its weight will differ. Another important difference between mass and weight is how they are measured. Weight is measured with a scale, while mass must be measured with a balance. Just as people confuse mass and weight, they also confuse scales and balances. A balance counteracts the effects of gravity while a scale incorporates it. There are two types of balances found in the laboratory: electronic and manual. With a manual balance, you find the unknown mass of an object by adjusting or comparing known masses until equilibrium is reached.

Volume

Volume describes the quantity of three dimensional space than an object occupies. The SI unit for volume is meters cubed (m3), but milliliters (mL), centimeters cubed (cm3), and liters (L) are more common in the laboratory. There are many equations to find volume. Here are just a few of the easy ones:

Volume = (length)3 or (length)(width)(height) or (base area)(height)

Density: A Further Investigation

We know all of density's components, so let's take a closer look at density itself. The unit most widely used to express density is g/cm3 or g/mL, though the SI unit for density is technically kg/m3. Grams per centimeter cubed is equivalent to grams per milliliter (g/cm3 = g/mL). To solve for density, simply follow the equation d = m/v. For example, if you had a metal cube with mass 7.0 g and volume 5.0 cm3, the density would be

ρ=7g5cm3=1.4g/cm3(4)

Sometimes, you have to convert units to get the correct units for density, such as mg to g or in3 to cm3.

Density can be used to help identify an unknown element. Of course, you have to know the density of an element with respect to other elements. Below is a table listing the density of a few elements from the Periodic Table at standard conditions for temperature and pressure, or STP corresponding to a temperature of 273 K (0° Celsius) and 1 atmosphere of pressure.