2.8: Physical and Chemical Properties and Changes
- To separate physical from chemical properties.
- Label a change as chemical or physical.
- List evidence that can indicate a chemical change occurred.
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).
Change is happening all around us all of the time. Changes are classified as either 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 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. Physical properties of matter include color, hardness, malleability, solubility, electrical conductivity, density, melting points, and boiling points.
For the elements, color does not vary much from one element to the next. The vast majority of elements are colorless, silver, or gray. Some elements do have distinctive colors: sulfur and chlorine are yellow, copper is (of course) copper-colored, and elemental bromine is red. However, density can be a very useful parameter for identifying an element. Of the materials that exist as solids at room temperature, iodine has a very low density compared to zinc, chromium, and tin. Gold has a very high density, as does platinum. Pure water, for example, has a density of 0.998 g/cm 3 at 25°C. The average densities of some common substances are 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.”
| 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.
Melting and boiling points are somewhat unique identifiers, especially of compounds. In addition to giving some idea as to the identity of the compound, important information can be obtained about the purity of the material.
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 we heat the liquid water, it changes to water vapor. But even though the physical properties have changed, the molecules are exactly the same as before. We still have each water molecule containing two hydrogen atoms and one oxygen atom covalently bonded. 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 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 are done. Again, this is an example of a physical change.
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). Dissolving is also a reversible physical change. When salt is dissolved into water, the salt is said to have entered the aqueous solution. The salt may be regained by boiling off the water, leaving the salt behind.
Chemical Properties
Chemical properties of matter describe its potential to undergo some chemical change or reaction by virtue of its composition. The elements, electrons, and bonds that are present give the matter potential for chemical change. It is quite difficult to define a chemical property without using the word "change". Eventually, after studying chemistry for some time, you should be able to look at the formula of a compound and state some 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 an acid. Zinc reacts with hydrochloric acid to produce hydrogen gas—this is a chemical property.
A chemical property of iron is its capability 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.
| Physical Property | Chemical Property |
|---|---|
| Gallium metal melts at 30 o C. | 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. |
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) from the burning of the wax. Another example of a chemical change is what occurs when natural gas is burned in your furnace. This time, we have a molecule of methane, CH 4 , and two molecules of oxygen, O 2 , convert to two molecules of water, H 2 O, and one molecule of carbon dioxide, 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.
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:
-
Release of Energy (light, heat, or sound)
- Temperature changes (either the temperature increases or decreases)
- Light is given off
- Permanent color changes (a substance with a different color is made, rather than just mixing the original colors together)
-
Release of a gas
- 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)
- The gas released has a different smell.
- A solid forms if two clear liquids are mixed (look for floaties/haziness —technically called a precipitate)
Which of the following is a chemical property of iron?
- Iron corrodes in moist air
- Density = 7.874 g/cm 3
- Iron is soft when pure.
- Iron melts at 1808 K.
Solution
"Iron corrodes in moist air" is the only chemical property of iron from the list.
Which of the following is a physical property of matter?
- corrosiveness
- pH (acidity)
- density
- flammability
- Answer
- c
Which of the following is a chemical property?
- flammability
- melting point
- boiling point
- density
- Answer
- a
Label each of the following changes as a physical or chemical change. Give evidence to support your answer.
- Boiling water
- A nail rusting
- A green solution and colorless solution are mixed. The resulting mixture is a solution with a pale green color.
- Two colorless solutions are mixed. The resulting mixture has a yellow precipitate.
Solution:
- Physical: boiling and melting are physical changes. When water boils, no bonds are broken or formed. The change could be written: H 2 O(l)→H 2 O(g)
- 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: Fe+O 2 →Fe 2 O 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 spread together. No color change occurred or other evidence of chemical change.
- Chemical: the formation of a precipitate and the color change from colorless to yellow indicate a chemical change.
Label each of the following changes as a physical or chemical change.
- A mirror is broken.
- An iron nail corroded in moist air
- Copper metal is melted.
- A catalytic converter changes nitrogen dioxide to nitrogen gas and oxygen gas.
- Answer a:
- physical change
- Answer b:
- chemical change
- Answer c:
- physical change
- Answer d:
- chemical change
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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. Physical changes may happen during an observation of a physical property. A physical change does not alter the identity of a substance. To identify a chemical property, we look for a chemical change. Chemical changes are changes that occur when one substance is turned into another substance. Observations that indicate a chemical change has occurred include color change, energy change, evolution of a gas, formation of a precipitate, etc.
Contributions & Attributions
This page was constructed from content via the following contributor(s) and edited (topically or extensively) by the LibreTexts development team to meet platform style, presentation, and quality:
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Henry Agnew (UC Davis)