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Learning Objectives
To separate physical from 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.
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/cm3 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.
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 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?
- Iron corrodes in moist air.
- Density = 7.874 g/cm3
- 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.
Exercise \(\PageIndex{1A}\)
Which of the following is a physical property of matter?
- corrosiveness
- pH (acidity)
- density
- flammability
- Answer
- c
Exercise \(\PageIndex{1B}\)
Which of the following is a chemical property?
- flammability
- melting point
- boiling point
- density
- Answer
- a
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. A chemical change always produces one or more types of matter that differ from the matter present before the change. The formation of rust is a chemical change because rust is a different kind of matter than the iron, oxygen, and water present before the rust formed.
I've spent years immersed in the realm of physical and chemical properties of matter, from understanding the fundamental characteristics to delving into their implications across various substances.
Physical properties, such as mass, color, volume, and conductivity, are measurable without altering the composition of a substance. Take silver, for example—it's not just a shiny metal but an excellent conductor of electricity and remarkably malleable, allowing it to be molded into thin sheets. Contrast that with salt, which appears dull and brittle but exhibits conductivity when dissolved in water.
When it comes to elements, while color variation is minimal, density serves as a defining parameter. For instance, elements like iodine possess significantly lower densities compared to zinc or chromium. The density variations among different substances, like water and corn oil, elucidate their behavior when combined—corn oil, being less dense, floats on water.
Hardness is another pivotal factor, influencing the utility of elements, especially metals. This spectrum ranges from softer elements like silver and gold to substantially harder ones like titanium or tungsten. The intriguing case of carbon showcases extreme contrasts in hardness between graphite and diamonds, despite both being forms of carbon.
Melting and boiling points serve as distinct markers, aiding in identifying compounds and assessing their purity. Chemical properties, on the other hand, encompass a substance's potential to undergo chemical changes or reactions based on its composition. Hydrogen's propensity to ignite or metals' reaction with acids, like zinc with hydrochloric acid producing hydrogen gas, exemplifies these properties.
Consider iron—it's not merely a metal with a specific density or melting point. Its capability to combine with oxygen, forming iron oxide (commonly known as rust), signifies a chemical property. Rusting, a form of corrosion, exemplifies how chemical properties differentiate substances as they transform into entirely new forms.
So, distinguishing between physical and chemical properties boils down to observing changes: physical properties remain observable without changing the substance, whereas chemical properties reveal themselves through transformations into new substances. For instance, while a metal's melting point or density represents a physical property, its ability to rust or react with other elements showcases chemical properties.
In summary, understanding physical and chemical properties allows us to discern characteristics and behaviors of matter, whether it's the conductivity of silver, density variations among substances, or the transformative nature of chemical reactions like rust formation.
