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Answers to the Problem for Physical and Chemical Properties and Changes

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.

Physical Properties

Odorless (Intensive)

Colorless (Intensive)

Boiling point = -161.6 °C (Intensive)

Methane is a gas. (Intensive)

3.9 trillion cubic meters of natural gas consumed in 2019. (Extensive)

Methane dissolves in petroleum (Intensive)

Methane molecules can absorb and reemit infrared photons (Intensive)

Chemical Properties

Flash point = -188 °C (Intensive)

Methane is flammable. (Intensive)

Physical Changes

Methane changes from liquid to gas when it boils.

When methane dissolves in petroleum

When methane escapes from petroleum

When methane molecules absorb and reemit infrared photons

Chemical Changes

Change from methane and oxygen to carbon dioxide and water

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 CFC-12 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.

Physical Properties

Boiling point = -29.6 °C (Intensive)

CFC-12 is a gas a normal temperatures and pressures (Intensive)

CFC-12 can be converted into a liquid by increasing the pressure to the pressures found in aerosol cans and refrigerators (Intensive)

Liquid has ether-like odor (Intensive)

Density of liquid at -29.6 °C = 1.486 g/mL (Intensive)

One million tons produced per year in the 1970s (Extensive)

Solubility in water of 0.386 g/L at 20 °C (Intensive)

Chemical Properties

High energy photons remove chlorine atoms from CFC-12 molecules. (Intensive)

Physical Changes

CFC-12 changes from liquid to gas when boils

When CFC-12 dissolves in water

Chemical Changes

Change CF2Cl2 to CF2Cl and Cl