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Measuring Temperature and Pressure:
Temperature
Temperature is how hot or cold something is. Temperature is really a measure of how fast the atoms and molecules
that make up a substance are moving (average kinetic energy) (this movement is sub-microscopic; you cannot see it).
A thermometer measures the temperature of something, showing how hot or cold it is. There are many different
types of thermometers that measure the temperature of different things.
Watch the video clip: http://www.youtube.com/watch?v=2FDvlUE7bc4 (2 ½ minutes)
Temperature Scales
Temperature is measured in many different scales, including Fahrenheit, Celsius, and Kelvin scales.
The units of the Celsius and Fahrenheit scales are called degrees (the symbol for degree is °).
The units of the Kelvin scale are called Kelvins.
A change of one degree of temperature Celsius is the same as a change of one Kelvin; a change of one
degree Fahrenheit is a smaller change in temperature. Each temperature scale is useful for different
things.
Fahrenheit is a measure of temperature that is abbreviated F. Water freezes at 32°F and boils
at 212°F. This scale was developed by Gabriel Daniel Fahrenheit in 1714. The Fahrenheit scale is
used in the USA but not in many other countries. The degrees in Fahrenheit are smaller than
those in Celsius, making weather-related temperatures more easily written (without having to use
a decimal point).
Celsius (also called Centigrade) is a measure of temperature that is abbreviated C. Water
freezes at 0°C and boils at 100°C. This scale was developed by Anders Celsius in 1742.
Watch this video clip: http://www.youtube.com/watch?v=XKB99pFnYlI
To help understand the degree Celsius temperature scale, think:
Thirty is hot
Twenty is nice
Ten is cool
Zero is ice
Kelvin is a measure of temperature that is abbreviated K. Kelvin is a temperature scale
designed so that 0 K is defined as absolute zero, and the size of one unit is the same as
the size of one degree Celsius. [Absolute zero is a hypothetical temperature at which all
molecular movement stops. All actual temperatures are above absolute zero. Absolute
zero would occur at -273.16°C, -459.69°F, or 0 K.] Water freezes at 273.16K; water boils
at 373.16K. Many scientists use this scale (because all the temperatures are positive,
making calculations simpler) or the Celsius scale. This temperature scale was designed by
Lord Kelvin (William Thomson).
The temperature of 0 K cannot actually be reached; however, with great cooling,
temperatures very close to it have been reached. Experiments have shown that when a
gas is cooled down, its volume decreases. At constant pressure, the volume decrease for a
gas, is proportional to the temperature decrease.
Since it is practically very difficult to reach very low temperatures, we have to
extrapolate the graph to cross the temperature axis. Of course, at the point of
intersection, Velocity = 0. This means that the gas is so cold that its electrons are not
spinning around the nuclei and do not need any space for their vibration. In other words,
the no need for space means Zero Volume. The temperature (-273°C) is chosen to be the
Zero on the Kelvin scale.
If we plot a graph of the volume of a sample of gas versus the temperature we get
something that looks like the following:
From the extrapolated line, we can
determine the temperature at which an ideal
gas would have a zero volume. Since ideal
gases have infinitely small atoms the only
contribution to the volume of a gas is the
pressure exerted by the moving atoms
bumping against the walls of the container.
If no volume then there must be no kinetic
energy left. Thus, absolute zero is the
temperature at which all kinetic energy
(motion) has been removed. NOTE: This does
not mean all energy has been removed,
merely all kinetic energy.
Standard Temperature = 0*C or 273 K
This makes the zero on the Celsius scale to correspond to 273 on the Kelvin scale;
therefore,
K = °C + 273
Example:
Water boils at a
temperature of 100°C.
What would be the
corresponding
temperature for the
Kelvin scale?
K = °C + 273
K = 100°C + 273
K = 373
373K
A substance has a
melting point of 625 K.
At what Celsius
temperature would this
substance melt?
°C = K – 273
°C = 625 K – 273
°C = 352
352 °C
Conversion Formulas
If you know the temperature in one temperature scale you can use a formula to figure out what
it is equivalent to in other scales. The conversion formulas are in the table that follows.
Celsius to Fahrenheit (and
Fahrenheit to Celsius):
*F = 1.8*C + 32
*C= (*F - 32)/1.8
Kelvin to Celsius (and Celsius to Kelvin):
*C = K - 273
K = *C + 273
(Converting between Celsius and Kelvin is easy
(A degree Celsius is 1.8 times because the size of a degree Celsius is the
bigger than one degree
same as the size of a kelvin.)
Fahrenheit.)
IN CHEMISTRY THIS YEAR WE WILL ONLY BE USING THE CELCIUS AND THE KELVIN
SCALES!!!!!!
************************IMPORTANT************************
When working with GASES you must ALWAYS use the Kelvin scale.
Pressure
Pressure = force/area
Pressure is the application of force over a particular area. In mathematical terms, pressure is equal to
force divided by area. With more force comes more pressure, and with more area comes less pressure.
This is why a person wearing a high-heeled shoe exerts more pressure on the ground than an elephant
does. An elephant weighs a lot more than a person, but its feet have more surface area than the pointed
heel of a shoe.
How can someone lie on a bed of nails and not get hurt?
Watch the video clip: http://www.youtube.com/watch?v=MrBnTmW21po (5 minutes)
Watch the Video clip for an explanation of why this ‘works’:
http://www.youtube.com/watch?v=vo2iE94iAoA
If you step on the point of a nail, your foot exerts a tremendous amount of pressure on the nail's tiny point. As a
result, the point can go straight through your foot. But a bed of nails has lots of points that are close together -there's a lot of surface area for the body to cover. Hundreds of nails support the weight of your body instead of
just one. Typically, your body doesn't exert enough pressure on any one nail for it to break the skin. You can even
place a second bed on top of you and break a cinderblock on it. The resulting force is distributed over all of the
nails, so you shouldn't be injured.
The trick is getting on and off the bed. If you sit on the bed, only a few nails are supporting your weight. The same
is true if you put your hand down to steady yourself. And if one nail is taller than the rest, it will receive the brunt
of your body's pressure and puncture your skin.
Pressure of Gases
Pressure in a container, like a balloon or a tire, is due to the collisions of particles of gas against the
sides of the container.
What is Atmospheric Pressure?
An understanding of the concept of pressure is required to understand atmospheric pressure. Pressure is defined
as the force per unit area applied in a direction perpendicular to the object. The pressure of a static fluid is equal
to the weight of the fluid column above the point the pressure is measured. Therefore, the pressure of a static
(non-flowing) fluid is dependent only of the density of the fluid, the gravitational acceleration, the atmospheric
pressure and the height of the liquid above the point the pressure is measured. The pressure can also be defined
as the force exerted by the collisions of particles. In this sense, the pressure can be calculated using the
molecular kinetic theory of gasses and the gas equation. Atmospheric pressure is defined as the force per unit
area exerted against a surface by the weight of air above that surface in the Earth’s atmosphere. When going to
the high altitudes, the air mass above the point decreases, thereby reducing the atmospheric pressure. The
atmospheric pressure at the mean sea level is taken as the standard atmospheric pressure. The pressure is
measured in Pascal (unit P) or mmHg (millimeters of mercury). The atmospheric pressure at the mean sea level is
taken as 101.325 kPa .
How to measure air pressure………
Consider the inverted tube filled with mercury such that its open end is immersed in a pool of mercury. A standard
rule in fluid dynamics states that the pressure of a liquid is constant at any given height. So the pressure inside
and outside the tube at the height of the surface must be equal and opposite that of the air pressure. Thus, the
pressure due to the weight of the column of mercury must be equal to the pressure of the air. By measuring the
height of the column we have a direct measure of the air pressure.
Note that a gas at STP (T= 0ºC., P = 101.325 kPa) will support a column of mercury of height 760.0 mmHg. Hence,
we sometimes take the short-cut way of expressing pressure as a height of a column of mercury that would be
supported by the gas pressure being measured. At STP, the pressure of an ideal gas is 760 mmHg.
There are many different units that are used to measure pressure. Below are a just a few:
760 mm Hg = 760 torr = 30 in Hg = 1 atm = 101.3 kPa = 14.7 psi = 1.013 bar
It is easy to make conversions between the various units using dimensional analysis (the factor label method).
Example:
Convert 0.875 atm to mmHg.
Convert 0.955 atm to kPa.
Convert 740.0 mmHg to kPa.
66.5 mmHg
96.77 kPa
98.66 kPa
What is Barometric Pressure?
A barometer is a device, which consists of a glass tube that is closed at one end and filled with a high density liquid
and vacuum between the top of the liquid and the tube, and the other end of the tube is submerged in an open
container containing the same liquid. When mercury is used as the liquid this apparatus is termed as mercury
barometer. Since the pressure of the vacuum is zero and the pressure at the liquid surface is P, the pressure
difference is also P. This pressure difference is responsible for holding the liquid column. Therefore, the force
from the pressure difference is equal to the weight of the column. The pressure that is measured using the
barometer is the barometric pressure. This is equal to the atmospheric pressure, if the open end is in the
atmosphere.
The open-tube manometer is another device that can be used to measure pressure. The open-tube manometer is
used to measure the pressure of a gas in a container. The pressure of the gas is given by h (the difference in
mercury levels) in units of torr or mmHg. Atmospheric pressure pushes on the mercury from one direction, and the
gas in the container pushes from the other direction. In a manometer, since the gas in the bulb is pushing more
than the atmospheric pressure, you add the atmospheric pressure to the height difference:
Pgas > Patm Gas pressure = atmospheric pressure + h (height of the mercury)
Pgas < Patm Gas pressure = atmospheric pressure - h (height of the mercury)
Closed-tube manometers look similar to regular manometers except that the end that’s open to the atmospheric
pressure in a regular manometer is sealed and contains a vacuum.
Closed tube
Open Tube
Open Tube
Examples:
Pgas = 755mmHg + 24mmHg = 779mmHg
Pgas = 763mmHg - 35mmHg = 728mmHg
What is the difference between Atmospheric Pressure and Barometric Pressure?
• Atmospheric pressure at the mean sea level is a constant defined.
• Barometric pressure is the pressure measured using the barometer.
Good video and data about kelvin
http://chemmovies.unl.edu/chemistry/dochem/dochem076.html