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3.5: Pure Substances and Mixtures

  • Page ID
    177889
  • Learning Objective

    • Distinguish between pure substances and mixtures
    • Identify mixtures as homogeneous or heterogeneous

    Pure Substances

    When we speak of a pure substance, we are speaking of something that contains only one kind of matter. This can either be one single element or one single compound, but every sample of this substance that you examine must contain exactly the same thing with a fixed, definite set of properties.

    Pure Substance Element or Compound? Consists of:
    Lead (Pb) element lead atoms
    Oxygen gas (O2) element oxygen molecules*
    Water (H2O) compound water molecules
    Ammonia (NH3) compound ammonia molecules

    *Note: pure oxygen gas consists of molecules but it is still considered an element, rather than a compound, as the molecules are made up of a single type of element. Compounds are made up of one or more element.

    Mixtures

    If we take two or more pure substances and mix them together, we refer to this as a mixture. Mixtures can always be separated again into component pure substances, because bonding among the atoms of the constituent substances does not occur in a mixture. Whereas a compound may have very different properties from the elements that compose it, in mixtures the substances keep their individual properties. For example sodium is a soft shiny metal and chlorine is a pungent green gas. These two elements can combine to form the compound, sodium chloride (table salt) which is a white, crystalline solid having none of the properties of either sodium or chlorine. If, however, you mixed table salt with ground pepper, you would still be able to see the individual grains of each of them and, if you were patient, you could take tweezers and carefully separate them back into pure salt and pure pepper.

    Heterogeneous mixture

    A heterogeneous mixture is a mixture in which the composition is not uniform throughout the mixture. Vegetable soup is a heterogeneous mixture. Any given spoonful of soup will contain varying amounts of the different vegetables and other components of the soup.

    Homogeneous mixture/ Solution

    A homogeneous mixture is combination of two or more substances that are so intimately mixed that the mixture behaves as a single substance. Another word for a homogeneous mixture is solution. Thus, a combination of salt and steel wool is a heterogeneous mixture because it is easy to see which particles of the matter are salt crystals and which are steel wool. On the other hand, if you take salt crystals and dissolve them in water, it is very difficult to tell that you have more than one substance present just by looking—even if you use a powerful microscope. The salt dissolved in water is a homogeneous mixture, or a solution (Figure \(\PageIndex{3}\)).

    Figure \(\PageIndex{3}\): Types of Mixtures © Thinkstock On the left, the combination of two substances is a heterogeneous mixture because the particles of the two components look different. On the right, the salt crystals have dissolved in the water so finely that you cannot tell that salt is present. The homogeneous mixture appears like a single substance.

    Example \(\PageIndex{3}\)

    Identify the following combinations as heterogeneous mixtures or homogenous mixtures.

    • soda water (Carbon dioxide is dissolved in water.)
    • a mixture of iron metal filings and sulfur powder (Both iron and sulfur are elements.)

    a silver iron filing and yellow sulfur mixture

    Figure \(\PageIndex{4}\): A mixture of iron filings and sulfur powder  (Asoult, Fe-S mixture 03, CC BY 4.0)

    Solution

    1. Because carbon dioxide is dissolved in water, we can infer from the behavior of salt crystals dissolved in water that carbon dioxide dissolved in water is (also) a homogeneous mixture.
    2. Assuming that the iron and sulfur are simply mixed together, it should be easy to see what is iron and what is sulfur, so this is a heterogeneous mixture.

    Exercise \(\PageIndex{3}\)

    Are the following combinations homogeneous mixtures or heterogeneous mixtures?

    1. the human body
    2. an amalgam, a combination of some other metals dissolved in a small amount of mercury

    Answers

    1. heterogeneous mixture
    2. homogeneous mixture

     

    Categorizing Matter

    Figure \(\PageIndex{1}\): Relationships between the Types of Matter and the Methods Used to Separate Mixtures

    Ordinary table salt is called sodium chloride. It is considered a substance because it has a uniform and definite composition. All samples of sodium chloride are chemically identical. Water is also a pure substance. Salt easily dissolves in water, but salt water cannot be classified as a substance because its composition can vary. You may dissolve a small amount of salt or a large amount into a given amount of water. A mixture is a physical blend of two or more components, each of which retains its own identity and properties in the mixture. Only the form of the salt is changed when it is dissolved into water. It retains its composition and properties.

    Phase

    A phase is any part of a sample that has a uniform composition and properties. By definition, a pure substance or a homogeneous mixture consists of a single phase. A heterogeneous mixture consists of two or more phases. When oil and water are combined, they do not mix evenly, but instead form two separate layers. Each of the layers is called a phase.

    Example \(\PageIndex{1}\)

    Identify each substance as a compound, an element, a heterogeneous mixture, or a homogeneous mixture (solution).

    1. filtered tea
    2. freshly squeezed orange juice
    3. a compact disc
    4. aluminum oxide, a white powder that contains a 2:3 ratio of aluminum and oxygen atoms
    5. selenium

    Given: a chemical substance

    Asked for: its classification

    Strategy:

    1. Decide whether a substance is chemically pure. If it is pure, the substance is either an element or a compound. If a substance can be separated into its elements, it is a compound.
    2. If a substance is not chemically pure, it is either a heterogeneous mixture or a homogeneous mixture. If its composition is uniform throughout, it is a homogeneous mixture.

    Solution:

    1. A) Tea is a solution of compounds in water, so it is not chemically pure. It is usually separated from tea leaves by filtration.
      B) Because the composition of the solution is uniform throughout, it is a homogeneous mixture.
    2. A) Orange juice contains particles of solid (pulp) as well as liquid; it is not chemically pure.
      B) Because its composition is not uniform throughout, orange juice is a heterogeneous mixture.
    3. A) A compact disc is a solid material that contains more than one element, with regions of different compositions visible along its edge. Hence a compact disc is not chemically pure.
      B) The regions of different composition indicate that a compact disc is a heterogeneous mixture.
    4. A) Aluminum oxide is a single, chemically pure compound.
    5. A) Selenium is one of the known elements.

    Exercise \(\PageIndex{1}\)

    Identify each substance as a compound, an element, a heterogeneous mixture, or a homogeneous mixture (solution).

    1. white wine
    2. mercury
    3. ranch-style salad dressing
    4. table sugar (sucrose)
    Answer a:
    homogeneous mixture (solution)
    Answer b:
    element
    Answer c:
    heterogeneous mixture
    Answer d:
    compound

    Example \(\PageIndex{2}\)

    How would a chemist categorize each example of matter?

    1. saltwater
    2. soil
    3. water
    4. oxygen

    SOLUTION

    1. Saltwater acts as if it were a single substance even though it contains two substances—salt and water. Saltwater is a homogeneous mixture, or a solution.
    2. Soil is composed of small pieces of a variety of materials, so it is a heterogeneous mixture.
    3. Water is a substance; more specifically, because water is composed of hydrogen and oxygen, it is a compound.
    4. Oxygen, a substance, is an element.

    Exercise \(\PageIndex{2}\)

    How would a chemist categorize each example of matter?

    1. coffee
    2. hydrogen
    3. an egg
    Answer a:
    a homogeneous mixture (solution), assume it's filtered coffee
    Answer b:
    element
    Answer c:
    heterogeneous mixture.

    Summary

    Matter can be classified into two broad categories: pure substances and mixtures. A pure substance is a form of matter that has a constant composition and properties that are constant throughout the sample. Mixtures are physical combinations of two or more elements and/or compounds. Mixtures can be classified as homogeneous or heterogeneous. Elements and compounds are both examples of pure substances. Compounds are substances that are made up of more than one type of atom. Elements are the simplest substances made up of only one type of atom.

    Key Takeaways

    • Pure substances are composed of a single element or compounds.
    • Combinations of different substances are called mixtures.
    • Homogeneous mixtures are mixtures of two or more compounds (or elements) that are not visually distinguishable from each other.
    • Heterogeneous mixtures are mixtures of two or more compounds (or elements) that are visually distinguishable from one another.

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