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Classifying Matter

>> Parts of this equation/concept include:

A -	Physical Versus Chemical Properties
B -	Heterogenous Mixture, Homogenous Mixture, Compound, or Element >> Metal, Nonmetal, or Metalloid

Chemical properties are characterized by a change in identity, whereas physical properties are not. Any property that can be seen or measured is probably a physical property. Any property that describes changes in the substance is a chemical property. Changes in physical state (liquid, solid, or gas) involve physical properties, not chemical ones. Water has different names, depending on its physical state (ice is solid water, steam is gaseous water), but it is still the same substance.

>> Example 1

Classify the following as physical or chemical properties of water.

1. density = 1.00 g/mL
2. forms ice at 0°C
3. makes hydrogen gas when combined with sodium metal
4. is colorless
5. is formed when wood is burned

Solution:

1. This is a physical property; density can be measured without changing a substance to anything else.
2. This is a physical property; freezing is just changing the physical state, not the identity of the substance.
3. This is a chemical property; hydrogen gas is something different from water. Another tip-off is that another substance was required to achieve the change. Another substance is often, but not always, present in a chemical change.
4. This is a physical property; it is observed without altering the substance.
5. This is a chemical property; that it was not present to begin with suggests an identity change.

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The first separation is between mixtures and pure substances. The variable composition of mixtures is usually the best indication that it is a mixture rather than a pure substance. Are there different ways to combine the components so that the mixture can still be labeled as the same thing? Sometimes because there are so many possible combinations that the one used is specified. Mixtures can be physically separated. Methods of physical separation particular to chemistry include filtration, distillation, and chromatography. Pure substances are often harder to identify because they are usually defined by what they are not. They are not mixtures. They cannot be physically separated.

Once you have identified a mixture, you should determine whether it is heterogeneous or homogeneous. Heterogeneous mixtures are not uniform across the sample. Does the substance have different characteristics in different locations? A homogeneous mixture should be the same no matter where you sample it.

Once you have identified a pure substance, you can determine whether it is an element or a compound. A compound can be chemically separated. A clue that it is a chemical rather than a physical separation is that the ratio of components will always be the same (composition is not variable). Usually, it is easier to see the elements, since each every element is listed in the periodic table. If it is named and that name is on the list, it is an element.

If you occasionally mistake compounds for elements and homogeneous mixtures for compounds, don't panic. Historically, you are in good company. People believed water to be an element for hundreds of years. However, when you are given more information (often in the solution to the problem), you should be convinced of the logic of the answer.

>> Example 2

Classify the following as heterogeneous mixture, homogeneous mixture, compound, or element.

1. 10-k gold
2. neon
3. fog
4. steel
5. salt

Solution:

1. 10-k gold is a homogeneous mixture; "24-k" gold is known as "pure gold," but is too soft to be used for most purposes. Jewelry commonly comes as 10-, 14-, or 18-karat gold (variable composition).
2. Neon is an element. It is number 10 on the periodic table.
3. Fog is a mixture of water and air. Whether it is homogeneous or heterogeneous depends on how closely you look at it. When caught in a fog bank you might observe that everything looks the same, therefore it is a homogeneous mixture. When you look closely, you may actually be able to differentiate between the air and water droplets, making it a heterogeneous mixture. If you can get a wider view you might notice that it is denser in some places than in others; thus it is a heterogeneous mixture. Sometimes things don't fit neatly into categories.
4. Steel is a homogeneous mixture. There are many types of steel that are easily modified for different properties by changing the ratios of the components.
5. Salt is a compound. It can be separated into sodium and chlorine (elements), but always in the same ratio. You may have heard salt referred to as NaCl, which are the symbols for sodium and chlorine. Compounds can always be shown as formulas of their component elements. (So if you can write a molecular formula for it and the formula has more than one element, it is a compound.)

>> Metal, Nonmetal, or Metalloid

Metals, nonmetals, and metalloids are subcategories or elements-and although mixtures and compounds contain elements, these subcategories apply only to the elements themselves.

One way to categorize the element as a metal, metalloid, or nonmetal is by its properties. Physical properties of metals include luster, malleability, and electrical conductivity. A chemical property of metals is that they form cations. Nonmetals are not malleable or conductive, nor do they have metallic shine. If it forms any type of ion, a nonmetal is more likely to be an anion. Metalloids tend to have physical properties similar to metals but chemical properties similar to nonmetals.

The easiest way to categorize elements is to use the position of the elements on the periodic table. A staircase-like line starts to the left of boron (B) and extends down between polonium (Po) and astatine (At). Elements to the left of the staircase are metals, those to the right are nonmetals, and those touching it are metalloids.

>> Example 3

Classify the following as metals, nonmetals, or metalloids.

1. a shiny substance that tends to become an anion
2. calcium (Ca)
3. sulfur (S)
4. neon (Ne)
5. arsenic (As)

Solution:

1. Metalloid. Shiny is a physical property of metals, making anions a chemical property of nonmetals.
2. Metal. It is in the second column of the periodic table.
3. Nonmetal. It is in the third row of the periodic table, toward the right side.
4. Nonmetal. It is a gas on the last column of the periodic table.
5. Metalloid. It touches the staircase.

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