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    Home / Science lessons / The Pressure & Gas Laws
    • The Pressure & Gas Laws

      Introduction to Gas & Pressure

      Everything that has mass, from an elephant to a sugar molecule, is considered to be matter if it can exist as a solid, liquid, or gas. Gases are formed by small molecules in constant rapid, random motion; they have a much lower density (concentration of molecules) than either solids or liquids. You can observe all three states of matter in water: from a liquid, water (H2O) will turn into solid ice when it freezes or will evaporate as gas when it's heated to boiling.

      One of the important properties of gas is that it expands when it gets hot and contracts when it cools. This means that when a gas is heated, the individual molecules that form it will move faster and spread further apart from each other. The gas also gets lighter as it gets hotter, since the molecules are less densely concentrated.

      The expanding property of gas is what enables hot air balloons to inflate and fly. Hot air going into the balloon has a less dense concentration of molecules, making the inflated balloon lighter than the cooler, denser air around it. That denser outer air buoys up the balloon and allows it to float.

      Conversely, the affect of cooling air will bring the balloon back down—the air molecules inside the balloon move closer together when cooled, making the air in the balloon no longer lighter than the surrounding air. That's why a cold, damp cloud or fog can be dangerous for someone in a hot air balloon.

      (Although it might seem like air is weightless, it's actually formed by gases and has mass like any other matter. Usually the air is made up of 78% nitrogen, 21% oxygen, and 1% carbon dioxide and noble gases. The density of air is 0.075 pounds per cubit foot at 70 °F and one atmosphere of pressure.)

      The Pressure & Gas Laws

      Pressure (P) can be described mathematically as "force (F) divided by the area over which that force is applied," or P=F/A. Pressure is measured in pascals (Pa), named in honor of the 17th-century scientist and philosopher Blaise Pascal, who showed that confined liquids transmit pressure equally in all directions. Pressure can also be measured in pounds per square inch.

      You can use this formula to find the pressure exerted on your feet by your body; divide your weight in pounds by the number of square inches that your feet cover. If you weigh 100 pounds and the surface area of your feet is approximately 50 inches, then the pressure is around two lbs/in2. (To find total surface area of your foot, multiply average length by average width for each foot and then add those together.) The larger the area, the less pressure per square inch. There are advantages sometimes to having big feet!

      There are several scientific laws to describe the behavior of gases. Boyle's Law says that under constant temperature, the product of pressure (P) times volume (V) must remain constant, or PV=constant. This means that if one increases, the other must decrease (if P↑ then V↓). Charles' Law says that under constant pressure, the quotient of volume (V) divided by temperature (T) is constant, or V/T=constant. This means that if one increases, so must the other, and vice versa (if T↑, V must also). These laws are summed up in the Combined Gas Law: PV/T=constant. In other words, P1V1/T1= P2V2/T2, or the total of PV/T at one point will be the same as the total of PV/T at another point.

      If you have a medicine dropper or a basting tube with a rubber bulb at one end, you can demonstrate Boyle's Law. Partially fill a glass with water, then set the open end of the dropper or tube in the glass. Next, squeeze the rubber bulb at the end; as you exert pressure, the volume inside the tube decreases. What happens when you release the bulb? The immediate decrease in pressure causes the volume to rapidly fill up, pulling the water into the tube. To empty the tube, squeeze the bulb again. This increases the pressure and causes the volume to decrease, causing the water to squirt out.

      Barometric Pressure

      Galileo's secretary and scientific successor, Evangelista Torricelli, helped prove the existence of air pressure through a famous experiment that he did with a mercury-filled glass tube. Previous natural philosophers had shown that water will rise in a pump suction or siphon up to around 35 feet and then stop. They correctly postulated that a vacuum formed at the top, between the water level and the suction.

      That, however, presented a problem. Centuries earlier, Aristotle had defined space as "an extension of the substance filling it," and this idea was backed up by 17th-century Aristotelian philosophers who held that a vacuum could not logically exist. Some of them reasoned that "God was everywhere," so all space would be filled at least by Him. But mechanical philosophers like Galileo (and later Pascal) were convinced that a vacuum could and did exist, even in a world made by an omnipresent God. This is consistent with the doctrine that God is spirit and not made up of physical matter.

      Some of Galileo's contemporaries built simple barometers, using water in long tubes, to try to prove the existence of a vacuum. But in 1643, Torricelli took the barometer idea a step further than merely proving the existence of a vacuum. He realized that air—contrary to contemporary thought—was not weightless and that the rise and fall of liquid in a barometric tube was related to air pressure.

      To show this, he developed an experiment using liquid mercury or "quicksilver." Since mercury is about 14 times as heavy as water, Torricelli was able to use a much shorter tube than the earlier water barometers required. He filled the tube with mercury and then inverted it into a dish of mercury. Some of the mercury drained from the tube, but then it stopped at a certain level—14 times less than the level water stopped at, to be exact.

      Torricelli postulated that air exerted pressure on the mercury in the dish, pushing hard enough on the surface area to keep most of the mercury inside the tube. Thus, it was not the attraction of the vacuum at the top of the tube which held the mercury inside (as other scientists thought), but rather, the phenomenon was the result of the pressure exerted by air. Thus, the barometer could be used to measure air pressure. Later generations of scientists, most notably Blaise Pascal, developed the barometer further.

      You can demonstrate air pressure's effects through an experiment similar to Torricelli's. Stick a plastic straw in a glass of juice or other colored drink, and suck enough liquid into the straw to fill it about halfway up. When you suck on the straw, a partial vacuum is created in the top of the straw. Air pressure on the liquid in the glass forces the juice up the straw and into your mouth. Now, hold your finger over the top of the straw and slowly pull the straw out of the glass. While your finger is pressed over the top, a partial vacuum is maintained in the top of the straw. The air pressure is greatest underneath the straw and will keep the liquid from dripping out.

      Flashback in History: Finding the Noble Gases

      In the 1890s, a special group of elements known as the noble gases was discovered. Scottish scientist Sir William Ramsey was in on the discovery of five of these elements. He and Lord Rayleigh jointly discovered the first of the noble gases, argon (Ar), in 1894; a decade later Ramsey won the Nobel prize for chemistry and Rayleigh won the Nobel Prize for physics for their work. The name Argon means "lazy one"—the gas was called this because it didn't react with any other element or compound that it was tested with.

      Ramsey and M.W. Travers worked together to find neon (Ne), krypton (Kr), and xenon (Xn), and Ramsey also discovered helium (He). The other noble gas, radon (Rn), was discovered in 1900 by a German, Friedrich Ernst Dorn.

      In spite of their name, these elements are not very stirring to the senses: they are all odorless, tasteless, and colorless. Because their outer electron shell is already filled, they have no negative or positive charge and rarely will react with other elements and form compounds—behavior that sets them apart from most other elements.

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