## Encyclopedic Entry

Densely packed in the "Pink City."

Photograph by Kedryn Samson, MyShot

Elbow Room in Africa
Namibia, one of the least densely populated countries in the world, has just 2.6 people per square kilometer (6.6 per square mile.)

Density is the number of things—which could be people, animals, plants, or objects—in a certain area. To calculate density, you divide the number of objects by the measurement of the area.

The population density of a country is the number of people in that country divided by the area in square kilometers or miles. The country of Singapore is one of the most densely populated countries in the world. It has a population of 4,839,400 and an area of 687 square kilometers (265 square miles), so its density is 7,044 people per square kilometer (18,262 per square mile).

Of course, not all the people in the country are spread out evenly. Cities have a greater population density than rural areas.

Density can also be used to refer to the number of plants or animals in a certain area. Sometimes, animal or plant populations are too dense. This leads to overpopulation.

Deer have overpopulated areas of the Midwestern United States, for example. A common reason is a lower density of their natural predators, such as pumas or bears. As a result of the deer’s dense population, the competition for land and food is heightened and many deer die of starvation. Vegetation does not have time to develop, so food becomes scarce. A habitat can only support a limited number of each type of organism, so an overpopulation of deer can drive those deer to other habitats, including human habitats such as towns.

Density can refer to the number of molecules in a substance. This can apply to gases, liquids, and solids.

Air quality is defined by the number of pollutants in a certain area. Air quality therefore measures the density of air pollutants, such as smoke and emissions. Densely populated cities often have poor air quality because of the air pollution density.

Density is used to measure the salinity of seawater. Water densely packed with salt has a high salinity. Water that has few salt molecules has low salinity.

Rocks and minerals are also measured for density. Many igneous rocks, or rocks formed from volcanic eruptions, have a low density. They are full of pockets of air. One type of igneous rock, pumice, has such a low density that it can actually float on water. The water is more dense than the rock.

## Vocabulary

Term Part of Speech Definition Encyclopedic Entry

air quality

Noun

measurement of pollutants and other harmful materials in the air.

bear

Noun

mammal with a very large body, relatively short limbs, and an elongated snout.

calculate

Verb

to reach a conclusion by mathematical or logical methods.

city

Noun

large settlement with a high population density.

country

Noun

geographic territory with a distinct name, flag, population, boundaries, and government.

deer

Noun

mammal whose male members have antlers.

density

Noun

number of things of one kind in a given area.

Encyclopedic Entry: density

emission

Noun

discharge or release.

eruption

Noun

release of material from an opening in the Earth's crust.

habitat

Noun

environment where an organism lives throughout the year or for shorter periods of time.

Encyclopedic Entry: habitat

heighten

Verb

to increase, especially in height.

igneous rock

Noun

rock formed by the cooling of magma or lava.

mineral

Noun

inorganic material that has a characteristic chemical composition and specific crystal structure.

molecule

Noun

smallest physical unit of a substance, consisting of two or more atoms linked together.

organism

Noun

living or once-living thing.

overpopulation

Noun

situation where the amount of organisms in an area is too large for the ecosystem to support.

pollutant

Noun

chemical or other substance that harms a natural resource.

population density

Noun

the number of people living in a set area, such as a square mile.

predator

Noun

animal that hunts other animals for food.

puma

Noun

mammal, relative to a cat, native to the Americas. Also called a cougar or mountain lion.

pumice

Noun

type of igneous rock with many pores.

reduction

Noun

lowering.

rock

Noun

natural substance composed of solid mineral matter.

rural area

Noun

regions with low population density and large amounts of undeveloped land. Also called "the country."

Encyclopedic Entry: rural area

salinity

Noun

saltiness.

scarce

rare.

seawater

Noun

salty water from an ocean or sea.

smoke

Noun

gases given off by a burning substance.

starvation

Noun

dying from lack of food.

vegetation

Noun

all the plant life of a specific place.

### Credits

#### Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

#### Illustrators

Tim Gunther
Mary Crooks, National Geographic Society

Kara West
Jeannie Evers

Nancy Wynne

#### Producer

Caryl-Sue, National Geographic Society

#### Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.