Homogeneous and Heterogeneous Mixtures

In this article, we will discuss the definition of homogeneous and heterogeneous mixtures, and then look at some examples of each! It is very important to understand the differences, since they can influence many aspects of materials and processes. Heterogeneity impacts chemical reaction rates, efficiency of chemical processes, strength of concrete, adhesion of paint, and the texture of foods and beverages, just to name a few. Read on to learn more!

Further Reading

What is a mixture?

In chemistry, a mixture is a substance made up of 2 or more substances that are not chemically combined, but physically combined. This means there are no chemical bonds between the different substances in a homogeneous or heterogeneous mixture. You can think of a salad as an example of a mixture— lettuce, tomatoes, cucumbers, and Parmesan cheese are the substances, and each substance retains its own chemical composition and identity.

Definitions of homogeneous and heterogeneous mixtures

A homogeneous mixture (from the root “homo” meaning same) has uniform composition throughout. This means there are no “clumps” of one substance in any one area. Furthermore, in a homogeneous mixture, all substances exist in one state of matter. Liquids can be homogeneously mixed with liquids, solids with solids, and so on.

On the other hand, a heterogeneous mixture (from the root “hetero” meaning different) has non-uniform composition, meaning that there may be distinct regions with more or less of one component. This types of mixture will have “clumps” of just one substance in certain portions of the substance. Substances in a heterogeneous mixture can exist in different states of matter at once – solid with liquid or liquid with gas, for example.

Visual representation of a homogeneous mixture (a few purple dots spread evenly throughout a sea of green dots) and a heterogeneous mixture (purple dots concentrated in some areas, green dots concentrated in other areas).

Examples of homogeneous mixtures

  • Saltwater
    If salt dissolves in water, it disperses evenly throughout the water. Note that seawater can be heterogeneous if pieces of particulate matter are present, as in nature.
  • Coffee, milk
    These drinks consist of many chemicals dissolved in water, spread evenly like in saltwater. However, when milk curdles, it becomes a heterogeneous mixture.
  • Cement, glue
    These are homogeneous mixtures of chemicals that set (harden) on drying or exposure to other special conditions. They may have other things added which could make them heterogeneous (see “Concrete” below)
  • Bronze, steel
    These are alloys, made by mixing copper and tin (for bronze) or iron and carbon (for steel). Because the resulting mixtures do not have distinguishable regions of each component, they are homogeneous.
  • Air
    Air is a mixture of gases spread evenly throughout the atmosphere. Because gas molecules are distant from one another, they always mix evenly and do not form heterogeneous mixtures.
  • Unopened soda
  • Vinegar, brass, steel, wine are other examples

Examples of heterogeneous mixtures

  • Sand
    Sand usually consists of many different types and sizes of particles, including different minerals and pockets of air in between grains (or water if wet sand)
  • Oil and water
    Most oils do not mix well with water, so they have heterogeneous regions of mostly oil and mostly water
  • Salad
    Salads contain many distinguishable components: vegetables, cheese, chicken, dressing, or others
  • Granite
    Granite, a common type of rock, consists of grains of multiple minerals, like quartz, mica and feldspar. The grains are distinguishable, so this is a heterogeneous mixture.
  • Concrete
    Concrete has pieces of gravel dispersed in it which are distinguishable from the surrounding material (cement) that holds them together. Sometimes these particles are as large as small stones.
  • Vegetable soup
    Similar to a salad, a soup is not homogeneous because there are many distinguishable parts. Even if you blend it up, it will not be homogeneous on a molecular level.
  • Opened soda
    An open soda is a heterogeneous mixture because gases in the drink begin to come out of solution when it depressurizes. This results in the formation of gas bubbles, which are distinguishable from the surrounding drink.
  • Blood
    Challenge question – can you explain why blood is a heterogeneous mixture?
  • Soil, ice in soda, sand mixed with gravel, orange juice with pulp are other examples

Switching behavior (UCST/LCST)

Some systems are only homogeneous mixtures under some conditions. One example is a polymer solution, which is homogeneous as long as the polymer chains are dissolved by the solvent. If the temperature or the solvent changes, these solutions can suddenly precipitate as the polymer chains coil up and aggregate. The result is a heterogeneous mixture of solid polymer and liquid solvent.

The temperature at which this conversion takes place is known as a critical solution temperature. If the polymer dissolves at higher temperatures, this point will be an “upper” critical solution temperature (UCST), but if it dissolves at lower temperatures, it will have a “lower” critical solution temperature (LCST).

Many molecular liquids also mix with each other under some conditions, but will phase-separate under other conditions. In these cases, a homogeneous mixture of two molecules will become a heterogeneous mixture like water and oil. Droplets of one liquid are then dispersed in the other liquid, so there are regions of different compositions.