Physical and Chemical Properties and Changes
Properties of matter distinguish one substance from another.
Physical properties are characteristics of matter that can be observed and measured without changing the chemical composition of the substance. They are characteristics of substances not associated with changes in the chemical identity of the substance. Color and density are physical properties. One can determine the color of gold by just looking at it, and the density of gold can be determined by measuring the mass and volume of a given amount of it and dividing the mass by the volume. Neither of these determinations involve any change in the chemical character of gold.
The determination of some physical properties, such as melting and boiling point temperatures, are accompanied by physical changes, which are changes that involve changes in the state of the substance. The changes of solid water (ice) to liquid water to gaseous water (steam) are physical changes. The water in all three states is composed of H2O molecules, so the chemical identity of the water stays the same while the physical changes take place. When table sugar (sucrose, C12H22O11) dissolves in coffee, the sugar molecules are separated from each other, but they are still C12H22O11, so the dissolving of sugar in coffee is a physical change.
Chemical changes (or chemical reactions) are changes for which one or more pure substances (elements or compounds) are transformed into one or more different pure substances, and chemical properties are descriptions of chemical reactions a substance undergoes. When hydrogen gas burns, it combines with oxygen to form water. The hydrogen, H2, and oxygen, O2, molecules have been transformed into water molecules, H2O, so this is a chemical change. The fact that hydrogen reacts with oxygen to form water is a chemical property. Changes in color or odor often accompany chemical changes. Carefully heating sugar on the stove can melt the sugar, which is a physical change, but if you heat it too much, it will burn, leading to a change in color and a smell we associate with burning. The burning of sugar involves chemical changes.
Intensive properties are properties that are independent of the amount of substance. Gold has the same color whether you have a tiny flake or a large gold bar, and the temperatures of the flake and bar are the same if they have been sitting the same room all day. Color and temperature are intensive properties. Extensive properties are properties that are dependent on the amount of substance. There is a lot more matter in the gold bar than the small flake of gold, so the masses and volumes of the flake of gold and the gold bar are very different. Mass and volume are examples of extensive properties.
Methane, CH4, and the chlorofluorocarbon CFC-12, CF2Cl2, can both do environmental damage. Using the descriptions below for each substance, identify some of the physical and chemical properties of methane and CFC-12, identify each property as an intensive or extensive property, and identify the changes mentioned as chemical or physical changes.
1. Methane, CH4, is an odorless and colorless compound with a boiling point temperature of -161.6 °C, so it is a gas at normal temperatures and pressures. Natural gas is mostly methane, and the worldwide consumption of natural gas in 2019 was about 3.9 trillion cubic meters. Methane has a flash point of -188 °C, showing that it is a highly flammable substance. (The flash point is the lowest temperature at which the vapors of a volatile substance ignite in the presence of an ignition source.) When methane burns, it reacts with oxygen to form carbon dioxide and water. Liquid petroleum contains some dissolved methane. When the petroleum is pumped from the ground, the dissolved methane can escape from the petroleum to form gaseous methane in the atmosphere. Methane molecules in the troposphere (Earth’s lower atmosphere) can absorb infrared photons released by Earth as it cools. The methane molecules then reemit the photons, leaving the methane unchanged. This makes it possible for each methane molecule to absorb and reemit many infrared photons. Because the emitted photons are released in all directions equally, some of them are directed back to Earth, leading to a warming of our planet.
2. Because the chlorofluorocarbon CFC-12, CF2Cl2, has a boiling point of -29.6 °C, it is a gas at normal temperatures and pressures, but can be converted into a liquid at room temperature by subjecting it to the pressures found in typical aerosol cans and refrigerators. The liquid, which has an ether-like odor, has a density of 1.486 g/mL at -29.6 °C. Because of the ease with which CFC-12 can be changed from a gas to a liquid and because CFC-12 is very stable and therefore unlikely to react with other substances, in the 1970s and 1980s, it was a very common propellant in aerosol cans and refrigerant for refrigerators. In the 1970s, the worldwide production of CFCs was about a million tons per year. Because it is so unreactive and because it has a low solubility in water of 0.386 grams of CFC-12 per liter of water at 20 °C, CFC-12 in the atmosphere is not likely to be changed chemically in the troposphere or to dissolve in the water in clouds and be rained out. This allows it to remain in the atmosphere for decades and eventually migrate into the stratosphere where it is bombarded by high energy photons that provide the energy to break carbon-chlorine bonds and release chlorine atoms, which combine with ozone molecules, O3, and form chlorine monoxide, ClO, and oxygen molecules, O2. In a series of steps, the chlorine monoxide reacts to reform chlorine atoms. Because the chlorine atoms are regenerated in each cycle, they can each destroy many ozone molecules, and because ozone protects us from high energy photons coming into the stratosphere from space, this is a problem. The Montreal Protocol that went into effect in 1989 called for phasing out the production of all chlorofluorocarbons, including CFC-12.