• density
    Densely packed in the "Pink City."

    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.

  • 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 Adjective

    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.