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We will continue to discuss the important components that link physics and chemistry by looking at gas laws and the characteristics of gases.
We have begun to review the juncture between physics and chemistry on a macro scale, including the motion, collisions, temperature, and pressure.
Because gas particles have a relatively large distance between them, the volume can vary depending on temperature and pressure. Solids and liquids don’t have enough distance between the particles to change volume appreciably. Gas laws provide a means to predict pressure, volume, or temperature given the other two variables. Firefighters use this principle when entering a burning room to quickly extinguish the flames and check for victims. In a hot, closed room, a small amount of water will quickly expand as it turns to steam, to squeeze out the available oxygen which knocks down the flames. Water can expand over a thousand times in volume: popcorn pops because the tiny bit of water in the kernel expands with such force that the kernel pops. Pressure, temperature, and volume are important variables to understand when working with gases.
With a gas, an increase in pressure will result in a decrease in volume. Take a balloon to the bottom of the pool and watch what happens. The resulting increase in pressure will result in an decrease in volume. That’s why when a balloon rises, it eventually will burst because the pressure decreases as it climbs higher in the atmosphere and the volume expands. This type of relationship is called an inverse one because as one variable increases the other variable decreases. Robert Boyle, a British chemist, first described this relationship. Please explore these ideas further in the interactive module on Gas Laws.
With a gas, an increase in temperature will result in an increase in volume. Take a balloon outside in the sunshine. Watch what happens to the size of the balloon. Now take it and put it in the refrigerator. What happens? This relationship is called a direct relationship because both variables do the same thing. As one variable increases the other increases and vice versa. A French chemist named Jacques Charles who loved to fly hot air balloons first described this relationship. This graph always produces a straight line characteristic of any direct proportion.
In addition to these variables, it was determined by the Italian Amedeo Avogadro, that under similar conditions, equal volumes of gases contain the same number of molecules. This principle led to the measurement discussed previously, the mole. One mole of any gas at standard temperature (273 K) and standard pressure (1 atmosphere) will occupy 22.4 liters of volume and contain 6.02 · 1023 molecules. This very large number, 6.02 · 1023, is also called Avogadro’s number.
The particles of a gas move rapidly and are far apart, relatively speaking. Because of these two facts the interaction of the particles with each other is minimal compared to liquids and solids and therefore acts similarly to each other no matter what gas we are observing. Chemists use what is called an ideal gas equation that is a useful predictor. It was determined that the ratio of the pressure and volume to the number of moles and temperature always resulted in the same ratio. This ratio is called the ideal gas law constant, or R. The relationship is written:
pV = nRT
Where p = pressure, V = volume, n = number of moles, and T = temperature. The ideal gas law constant, R, is 0.08206 L-atm/mol-K. Using this constant proportion, the properties of most gases can be calculated given three of the four variables. This estimation is not as accurate at lower temperatures or higher pressures because of the interaction between the particles at closer distances and lower energies. Nonetheless, it is an extremely useful equation that can help predict everyday gas interactions.