Physical versus Chemical Properties
During a typical chemistry experiment, we observe the properties of matter before and after a change and use these observations to reach some conclusion about the nature of that change. Physical properties are characteristics that, when observed, do not cause the substance to change identity. An example of a physical property is mass, one of the most common measurements in the chemistry lab. When we weigh an object, we do not expect it to be converted into something else just because we put it on the balance. Even if we change the mass by adding or removing some matter from our sample, it remains the same substance. Mass, volume, and density (mass per unit volume) are all physical properties.
Some of the changes we observe in the chemistry lab coincide with an alteration in the substance’s identity. These are called chemical changes or reactions. During reactions new elements/compounds are formed from the starting materials. Chemical properties are the characteristics of matter that, when observed, are associated with a change in identity. Combustibility is an example of a chemical property. If we test whether or not a substance is combustible, we have to burn it and change it into something new. Acidity is another example of a chemical property. In order to tell if a solution is acidic, we test it with an indicator, like litmus or phenolphthalein. A small amount of the acid reacts with the indicator, changing them both.
We need to be cautious about the degree of a change when dealing with mixtures. Recall from a previous page in this outline that the properties of a mixture are a blend of the properties of its components. Most laboratory acids are mixtures of an acidic compound in water. When we dilute an acid, the acidity decreases, not because a new substance is formed, but because the relative proportions of acid and water are different.
What types of observations might you make to determine if a chemical reaction has taken place? What chemical properties are involved?
When a chemical reaction occurs, new substances are formed. Both the chemical and the physical properties of the new substance will be different from those of the original. Although chemical properties are the ones associated with the types of reactions a substance can undergo, physical properties are often easier to observe. These are some of the types of evidence we look for when deciding if a reaction has taken place:
Evolution of gas (bubbles)--State is a physical property, since you can change the state of a substance without changing its identity. However, different substances may be different states at room temperature and pressure. If a liquid begins to bubble without being heated, it is probably undergoing a chemical reaction.
Formation of a precipitate (a cloudy suspension of insoluble solid in a liquid mixture)--When a solid dissolves in water, it forms a transparent mixture called a solution. If a solid doesn't dissolve, the mixture will look cloudy, like muddy water. Solubility, the maximum amount of a substance that can be dissolved in a given amount of liquid, is a physical property.
Color changes--When reactions occur, energy is absorbed or released. Sometimes we can 'see' this energy change, since color is an indication of the frequency of light that a substance absorbs/transmits. Looking at an object doesn't change it into something else, so color is a physical property.
Spontaneous temperature changes--The energy changes that accompany reactions can also be in the form of transfer of heat which we observe as temperature change. Temperature is not a property of matter in the same sense as these other examples.
Changes in pH--Acidity and alkalinity are chemical properties as noted above.
When our parents were chemistry students, they were taught how to 'safely' smell chemicals to check for chemical changes. We do not use this test because there isn't a 'safe' way to smell toxic or corrosive substances. Odor is a chemical property, and the sense of smell is the interaction of molecules with chemical receptors in our nose and mouth.
Speaking of mouth. . . . When our grandparents were chemistry students, they were taught what different organic compounds tasted like in order to identify them. Taste is a chemical property that is never used in a chemistry lab.
Extensive versus Intensive Properties of Matter
Extensive properties are characteristics of matter that depend on the size of the sample rather than its identity. Mass and volume, are examples of extensive properties. When we weigh two different samples of the same substance, we get two different mass measurements; the larger mass is associated with the bigger sample. Extensive properties are useful when we try to determine the composition of a mixture or the efficiency of a process, such as a titration experiment or finding the per cent yield.
If we measure the mass and volume of an object then divide the mass by the volume, the result is the object’s density, which gives us an indication of how tightly packed the particles that make up that object are. Density is an intensive property of matter. Intensive properties are the characteristics of a sample of matter that do not depend on its size. When you measure the density of aluminum, you will find it is 2.70 grams per cubic centimeter, no matter what sized piece of aluminum you use. Intensive properties are most useful when trying to identify a substance. Density, color, and boiling point are examples of intensive properties that can be used to identify unknown chemical compounds.
Note that physical properties of matter can be either extensive (like mass) or intensive (like density). That is because physical/chemical and extensive/intensive are two different classification systems.
The purpose of an experiment is to make aspirin. Near the end of the procedure, you are instructed to take the melting point of your product. What type of property is melting point, and what will it tell you?
Melting point is an intensive property; a snowflake and an iceberg both melt at 0 C. The melting point of aspirin is 284 C. The melting point of salicylic acid, the starting material used to make aspirin, is 157 C. Though the two compounds look the same, you can tell which is which by the melting point. You can also tell if the product you made is pure or if it is a mixture. Recall that the physical properties of a mixture are a blend of the properties of its components. The melting point of a sample that has impurities will be somewhat lower that the melting point of a pure sample; the greater the amount of impurities, the farther the melting point will be. If your sample has a melting point of 275 C, it must be aspirin because it can't be salicylic acid. Unfortunately, it isn't very pure, either.