Chapter 14 – Gases & Atmosphere

Chapter 14 – Gases & Atmosphere Mr.Yeung

Observe • Write your own notes on the following demonstrations: • Weight of the newspaper • Getting Eggy back into the beaker • Collapsing can • Historical development of pressure

Air pressure • Atmospheric pressure (or air pressure) is the weight of air resting on the earth's surface. • In a pile of books the weight means that pressure is greatest at the bottom of the pile. • Gravity pulls the atmosphere towards the ground, so just as in a pile of books, the pressure is greater nearer the surface. • But air pressure can go against gravity as well

Weight of the newspaper • The newspaper was much harder to lift than the printer paper. • As the ruler lifted the printer paper, air rush in under the rising paper and thereby quickly allowed the air pressure to equalize on all sides. • Essentually, the weight of the air above the paper had no effect on the difficulty in lifting the paper. • As the ruler lifted the newspaper, the edges of the newspaper remained in contact with the desk. Very little air was allowed to rush in and equalize the pressure on the bottom side of the newspaper. • Since there is less air below the paper the pressure is less as well. Now the weight of all the air above the paper now becomes more evident.

History of pressure • Galileo (1564) • Torricelli (1608) • Boyle (1627) • Otto von Guericke (1643) • Blaise Pascal (1648) • Charles’

Back in the day… • During the late 1500's the mining industry in Europe had become very important • In order to reach the valuable ore, mines needed to be drained. • Pumping out the water that would seep into the shafts was a terribly difficult and expensive activity. • The efficiency of these pumps was moderate and the best of designs could not "lift" water more than 34 feet. • This limit caused them to place pumps in series or to use buckets on chains to haul it up. All of this at great cost in materials and energy.

Back in the day • If water could be pumped higher with less energy and materials, then profits could be increased and more ore could be recovered

Why? • Why do you think that the water cannot be pumped up over 34ft?

Galileo • Galileo was the court mathematician for the Duke of Tuscany during the early 1600s. • Developed the suction pump for the mines • But was perplexed as to why there was a limit to the height water could be raised

Torricelli • Was Galileo’s assistant • Took over Galileo’s position after he died • Designed an experiment similar to the suction of the water in the mines • But since 34ft is too long, he used mercury instead • Mercury is about 14 times heavier than water

Torricelli • Torricelli designed an experiment in which he would fill a glass tube 1 meter long and sealed at one end with mercury. • Placing his finger over the opening he would invert the tube in a bowl of mercury with the sealed end up and measure the resulting height of the column. • The mercury would fall to about 24cm height and an empty space or vacuum was created in the top of the tube.

Torricelli • So he developed the first barometer (measures atmospheric pressure) • He observed the level of the vacuum changed over time due to the air pressure • The air pressure from the atmosphere is pushing down on the mercury dish on the outside preventing the drainage of the mercury

“Chemistry does not suck” • We cannot think of pressure due to the ‘suction’ • Instead it is the pressure of the atmosphere that is PUSHING it out • In a syringe, when you are getting the liquid, it is the pressure that is pushing it in… not SUCKING it in • THAT IS A BIG MISCONCEPTION

Answer to the question • Why the water cannot be lifted 34ft is NOT due to suction of the pump • But rather the atmospheric pressure can only apply enough pressure to PUSH the water up 34ft • So what do you think will happen if you were up a mountain… • The water will go up less than 34ft due to less pressure

Further developments • Otto von Guericke (1643) • Made a pump that created a vacuum so strong that a team of 16 hourses could not pull two metal spheres apart. • Reasoned that the hemispheres were held together by mechanical force of the atmospheric pressure rather than vacuum

Blaise Pascal • Pascal took Torricelli’s ‘barometer’ and traveled up and down a mountain • He saw the pressure increased as he went down the mountain • He developed the SI Unit of pressure, the Pascal, which was named after him

Later… • Boyle’s law - Pressure and volume are inversely proportional at a fixed temperature. • Pressure increases when volume decreases • P1V1 = P2V2 • Pressure 1 Pressure 2 • Volume 1 Pressure 2 • Charles’ law • Increase with temperature, increases pressure.

So… • That was the story of pressure! • “We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight". Torricelli

Summary • Atmospheric pressure • Historical build up of atmospheric pressure

Chapter 14 – Gases & Atmosphere