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4.2: Classifying Matter- the Macroscopic Level

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    Learning Objectives
    • Given a macroscopic definition classify the type of matter.

    Matter can be classified using a hierarchy (Figure \(\PageIndex{1}\)). In this section, we will think about this hierarchy in terms of macroscopic properties. In the next section, we will revisit this hierarchy based on nanoscopic structure.

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

    A pure substance always has the same composition and same physical properties regardless of the source of the sample. For example, pure water is always composed of two atoms of hydrogen for every one atom of oxygen. This means its composition is always the same. Also, the physical properties of pure water will always be the same: at "normal" pressures it will boil at \(100^\text{o} \text{C}\), freeze at \(0^\text{o} \text{C}\), and have a density of 1.00 g/ml.

    Contrast this with salt water. The amount of salt in the water can vary, meaning the composition of salt water is not always the same. Additionally, the density, freezing print, and boiling point will change depending how much salt is in the water. Variable composition and variable properties means that salt water is not a pure substance. In fact, it is composed of two pure substances: water and sodium chloride. Any matter composed of multiple pure substances is a mixture. Mixtures can be separated into pure substances by means of physical separations.

    There are two types of pure substances: compounds and elements. Compounds can be broken into simpler pure substances by chemical means. When a current with a sufficient voltage  is passed through water, for example, it can be broken into oxygen and hydrogen. This process is called hydrolysis and is one example of a chemical separation. Since water is a pure substance that can be broken into simpler pure substances by chemical means, water is a compound.

    A pure substance that can not be broken down into simpler substances by chemical means is an element. You might already know from a previous science course that the periodic table contains all the known elements. The oxygen and hydrogen that form when water undergoes hydrolysis are examples of elements. They cannot be broken down into simpler substances by any means. Today, there are about 118 elements in the known universe. In contrast, scientists have identified tens of millions of different compounds to date.

    Mixtures can also be further divided: some are homogeneous and some are heterogeneous.

    A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. The salt water described above is homogeneous because the dissolved salt is evenly distributed throughout the entire salt water sample. Often it is easy to confuse a homogeneous mixture with a pure substance because they are both uniform. The difference is that the composition of the substance is always the same. The amount of salt in the salt water can vary from one sample to another. All solutions are considered homogeneous because the dissolved material is present in the same amount throughout the solution.

    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.

    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), assuming it is filtered coffee
    Answer b:
    element
    Answer c:
    heterogeneous mixture

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