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Chemistry LibreTexts

1.4: Chemistry - A Study of Matter and Its Changes

  • Page ID
    152137
  • Learning Objectives

    • Separate physical from chemical properties.
    • Label a change as chemical or physical.

    Chemistry is the study of matter—what it consists of, what its properties are, and how it changes. Matter is anything that has mass and takes up space—that is, anything that is physically real. Some things are easily identified as matter—the screen on which you are reading this book, for example. Others are not so obvious. Because we move so easily through air, we sometimes forget that it, too, is matter. Because of this, chemistry is a science that has its fingers in just about everything. Even a description of the ingredients in a cake, and how those ingredients change when the cake is baked, is chemistry!

    Physical and Chemical Properties

    All matter has physical and chemical properties. Physical properties are characteristics that scientists can measure without changing the composition of the sample under study, such as mass, color, and volume (the amount of space occupied by a sample). Chemical properties describe the characteristic ability of a substance to react to form new substances; they include its flammability and susceptibility to corrosion. All samples of a pure substance have the same chemical and physical properties. For example, pure copper is always a reddish-brown solid (a physical property) and always dissolves in dilute nitric acid to produce a blue solution and a brown gas (a chemical property).

    Physical Property

    A physical property is a characteristic of a substance that can be observed or measured without changing the identity of the substance. Silver is a shiny metal that conducts electricity very well. It can be molded into thin sheets—a property called malleability. Salt is dull and brittle and conducts electricity when it has been dissolved into water, which it does quite easily. Physical properties of matter include color, hardness, malleability, solubility, electrical conductivity, density, melting point, and boiling point. In Table \(\PageIndex{1}\), notice that corn oil has a lower mass to volume ratio than water. This means that when added to water, corn oil will “float.”

      Table \(\PageIndex{1}\) Densities of Common Substances
    Substance Density at 25°C (g/cm3)
    blood 1.035
    body fat 0.918
    whole milk 1.030
    corn oil 0.922
    mayonnaise 0.910
    honey 1.420

    Hardness helps determine how an element (especially a metal) might be used. Many elements are fairly soft (silver and gold, for example) while others (such as titanium, tungsten, and chromium) are much harder. Carbon is an interesting example of hardness. In graphite, (the "lead" found in pencils) the carbon is very soft, while the carbon in a diamond is roughly seven times as hard.

    Figure \(\PageIndex{1}\) A pencil (top) and a diamond ring (bottom). Both are a form of carbon, but exhibit very different physical properties.

    Melting and boiling points are somewhat unique identifiers, especially of compounds. In addition to giving some idea of the compound's identity, important information can be obtained about the purity of the material.

    Chemical Property

    A chemical property of matter describes its "potential" to undergo some chemical change or reaction by virtue of its composition; as well as what elements, electrons, and bonding are present to give the potential for chemical change. It is quite difficult to define a chemical property without using the word "change". Eventually, students of chemistry should be able to look at the formula of a compound and state a chemical property. For example, hydrogen has the potential to ignite and explode, given the right conditions—this is a chemical property. Metals in general have the chemical property of reacting with acids. Zinc reacts with hydrochloric acid to produce hydrogen gas—this is a chemical property.

    Figure \(\PageIndex{2}\) Heavy rust on the links of a chain near the Golden Gate Bridge in San Francisco; it was continuously exposed to moisture and salt spray, causing surface breakdown, cracking, and flaking of the metal. (CC BY-SA 3.0; ).

    A chemical property of iron is that it is capable of combining with oxygen to form iron oxide, the chemical name of rust (Figure \(\PageIndex{2}\)). The more general term for rusting and other similar processes is corrosion. Other terms that are commonly used in descriptions of chemical changes are burn, rot, explode, decompose, and ferment. Chemical properties are very useful in identifying substances. However, unlike physical properties, chemical properties can only be observed as the substance is in the process of being changed into a different substance.

    Table \(\PageIndex{2}\): Contrasting Physical and Chemical Properties
    Physical Properties Chemical Properties
    Gallium metal melts at 30 oC Iron metal rusts.
    Mercury is a very dense liquid. A green banana turns yellow when it ripens.
    Gold is shiny. A dry piece of paper burns.

    Example \(\PageIndex{1}\)

    Which of the following is a chemical property of iron?

    1. Iron corrodes in moist air.
    2. Density = 7.874 g/cm3
    3. Iron is soft when pure.
    4. Iron melts at 1808 K.

    Solution

    "Iron corrodes in air" is the only chemical property of iron from the list.

    Exercise \(\PageIndex{1A}\)

    Which of the following is a physical property of matter?

    1. corrosiveness
    2. pH (acidity)
    3. density
    4. flammability
    Answer
    3.

    Exercise \(\PageIndex{1B}\)

    Which of the following is a chemical property?

    1. flammability
    2. melting point
    3. boiling point
    4. density
    Answer
    1.

    Physical and Chemical Changes

    Change is happening all around us all of the time. Just as chemists have classified elements and compounds, they have also classified types of changes. Changes are either classified as physical or chemical changes. Chemists learn a lot about the nature of matter by studying the changes that matter can undergo. Chemists make a distinction between two different types of changes that they study—physical changes and chemical changes.

    Physical Change

    Physical changes are changes in which no bonds are broken or formed. This means that the same types of compounds or elements that were there at the beginning of the change are there at the end of the change. Because the ending materials are the same as the beginning materials, the properties (such as color, boiling point, etc) will also be the same. Physical changes involve moving molecules around, but not changing them. Some types of physical changes include:

    • Changes of state (changes from a solid to a liquid or a gas and vice versa).
    • Separation of a mixture.
    • Physical deformation (cutting, denting, stretching).
    • Making solutions (special kinds of mixtures).

    As an ice cube melts, its shape changes as it acquires the ability to flow. However, its composition does not change. Melting is an example of a physical change. A physical change is a change to a sample of matter in which some properties of the material change, but the identity of the matter does not. When liquid water is heated, it changes to water vapor. However, even though the physical properties have changed, the molecules are exactly the same as before. Each water molecule still contains two hydrogen atoms covalently bonded to one oxygen atom. When you have a jar containing a mixture of pennies and nickels, and you sort the mixture so that you have one pile of pennies and another pile of nickels, you have not altered the identity of either the pennies or the nickels—you've merely separated them into two groups. This would be an example of a physical change. Similarly, if you have a piece of paper, you don't change it into something other than a piece of paper by ripping it up. What was paper before you started tearing is still paper when you're done. Again, this is an example of a physical change.

    Figure \(\PageIndex{3}\): Ice melting is a physical change. When liquid water (\(H_2O\)) freezes into a solid state (ice), it appears changed; However, this change is only physical, as the composition of the constituent molecules is the same: 11.19% hydrogen and 88.81% oxygen by mass. (Public Domain; Moussa).

    Physical changes can further be classified as reversible or irreversible. The melted ice cube may be refrozen, so melting is a reversible physical change. Physical changes that involve a change of state are all reversible. Other changes of state include vaporization (liquid to gas), freezing (liquid to solid), and condensation (gas to liquid). Dissolution is also a reversible physical change. When salt is dissolved into water, the salt is said to have entered the aqueous state. The salt may be regained by boiling off the water, leaving the salt behind.

    Chemical Change

    Chemical changes occur when bonds are broken and/or formed between molecules or atoms. This means that one substance with a certain set of properties (such as melting point, color, taste, etc.) is turned into a different substance with different properties. Chemical changes are frequently harder to reverse than physical changes.

    One good example of a chemical change is burning a candle. The act of burning paper actually results in the formation of new chemicals (carbon dioxide and water, to be exact) from the burning of the wax. Another example of a chemical change is what occurs when natural gas is burned in a furnace. This time, on the left there is a molecule of methane, \(\ce{CH_4}\), and two molecules of oxygen, \(\ce{O_2}\); on the right are two molecules of water, \(\ce{H_2O}\), and one molecule of carbon dioxide, \(\ce{CO_2}\). In this case, not only has the appearance changed, but the structure of the molecules has also changed. The new substances do not have the same chemical properties as the original ones. Therefore, this is a chemical change.

    Figure \(\PageIndex{4}\): The burning of wax to generate water and carbon dioxide is a chemical reaction. (CC-SA-BY-3.0; Andrikkos )

    We can't actually see molecules breaking and forming bonds, although that's what defines chemical changes. We have to make other observations to indicate that a chemical change has happened. Some of the evidence for chemical change will involve the energy changes that occur in chemical changes, but some evidence involves the fact that new substances with different properties are formed in a chemical change.

    Observations that help to indicate chemical change include:

    • Temperature changes (temperature increase or decrease).
    • Light given off.
    • Unexpected color changes (a substance with a different color is made, rather than just mixing the original colors together).
    • Bubbles are formed (but the substance is not boiling—you made a substance that is a gas at the temperature of the beginning materials, instead of a liquid).
    • Different smell or taste (do not taste your chemistry experiments, though!).
    • A solid forms if two clear liquids are mixed (look for floaties—technically called a precipitate).

    Example \(\PageIndex{2}\)

    Label each of the following changes as a physical or chemical change. Give evidence to support your answer.

    1. Boiling water.
    2. A nail rusting.
    3. A green solution and colorless solution are mixed. The resulting mixture is a solution with a pale green color.
    4. Two colorless solutions are mixed. The resulting mixture has a yellow precipitate.

    Solution

    1. Physical: boiling and melting are physical changes. When water boils, no bonds are broken or formed. The change could be written as: \(\ce{H_2O} \left( l \right) \rightarrow \ce{H_2O} \left( g \right)\)
    2. Chemical: The dark grey nail changes color to form an orange flaky substance (the rust); this must be a chemical change. Color changes indicate chemical change. The following reaction occurs: \(\ce{Fe} + \ce{O_2} \rightarrow \ce{Fe_2O_3}\)
    3. Physical: because none of the properties changed, this is a physical change. The green mixture is still green and the colorless solution is still colorless. They have just been combined. No color change, or other evidence of chemical change, occurred.
    4. Chemical: the formation of a precipitate and the color change from colorless to yellow indicates a chemical change.

    Exercise \(\PageIndex{2}\)

    Label each of the following changes as a physical or chemical change.

    1. A mirror is broken.
    2. An iron nail corroded in moist air.
    3. Copper metal is melted.
    4. A catalytic converter changes nitrogen dioxide to nitrogen gas and oxygen gas.
    Answer 1:
    physical change
    Answer 2:
    chemical change
    Answer 3:
    physical change
    Answer 4:
    chemical change

    Summary

    • A physical property is a characteristic of a substance that can be observed or measured without changing the identity of the substance. Physical properties include color, density, hardness, and melting and boiling points.
    • A chemical property describes the ability of a substance to undergo a specific chemical change. To identify a chemical property, we look for a chemical change.
    • Physical changes are changes that do not alter the identity of a substance.
    • Chemical changes are changes that occur when one substance is turned into another substance.
    • Chemical changes are frequently harder to reverse than physical changes. Observations that indicate a chemical change occurred include color change, temperature change, light given off, formation of bubbles, and formation of a precipitate.

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