Unit 3: Gas Laws Flashcards Preview

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Flashcards in Unit 3: Gas Laws Deck (18):

Air Pressure

The force exerted by the weight of a column of air above a given point.



An instrument used to measure barometric pressure.



Matter with no definite volume or shape.



The amount of force that pushes on an area. When the molecules of a gas collide with the walls of their container, they exert pressure. That pressure can be measured with the calculation: P = F / A

Where pressure (P) is the ratio of the force (F) applied to the surface area (A).


Physical Properties of Gas

- Temperature
- Volume
- Pressure


Pressure Formula

P = F / A

Where pressure (P) is the ratio of the force (F) applied to the surface area (A).



Volume measures the space that matter takes up. It is a measurable physical property of gas. Measuring the volume of a gas is easy: You measure the volume of its container.


How are pressure and volume related to temperature and each-other?

- If the volume of a container increases, the pressure in the container decreases. In the same way, if the volume of a container decreases, the pressure in the container increases.
- If the temperature of a gas increases, volume and pressure increase. If the temperature of a gas decreases, volume and pressure decrease.


Combined Gas Law

The combined gas law makes use of the relationships shared by pressure, volume, and temperature: the variables found in other gas laws, such as Boyle's law, Charles' law and Gay-Lussac's law.

This can be stated mathematically as PV/T=k

P is the pressure,
V is the volume,
T is the temperature measured in kelvins,
k is a constant (with units of energy divided by temperature).


Boyle's Law

Imagine you are a diver, and you begin your dive with lungs full of air. As you go deeper under water, the pressure you experience in your lungs increases. When this happens, the air inside your lungs gets squished, so the volume decreases.

This is an example of Boyle's law in action, which states that the higher the pressure (P), the lower the volume (V), as shown in this image. Here, k is any constant number.


Charles' Law (Law of Volumes)

Charles's law (also known as the law of volumes) is an experimental gas law that describes how gases tend to expand when heated. A modern statement of Charles's law is: When the pressure on a sample of a dry gas is held constant, the Kelvin temperature and the volume will be directly related.


Gay-Lussac's Law (Amontons' Law)

Gay-Lussac's law, Amontons' law or the pressure law was found by Joseph Louis Gay-Lussac in 1809. It states that, for a given mass and constant volume of an ideal gas, the pressure exerted on the sides of its container is directly proportional to its absolute temperature.



The kelvin (symbol: K) is a unit to measure temperature. It is one of the seven SI base units. It is defined by two factors: zero kelvin is absolute zero (when molecular motion stops), and one kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (0.01 °C).


Absolute Zero

Absolute zero is when molecular motion stops.


Triple Point

The triple point is the temperature and pressure at which three phases (gas, liquid, and solid) of a particular substance may coexist in thermodynamic equilibrium.



Thermodynamics is a branch of physics concerned with heat and temperature and their relation to other forms of energy and work.


Triple Point of Water

For water the triple point temperature is exactly 273.16 K (0.01 C).


Why volume of gas changes when gas heated?

Case 1: Expansion of gas

Heat is the form of energy. If you supply heat to gas, energy of the gas molecule increases.

Now with this energy, kinetic energy of the gas molecule increases and they move with increased speed in all possible direction.

With increased velocity, range of the gas molecule increases and they can travel to long distance similarly when you have extra petrol, your journey can get longer and you can move to far distant place

So with this energy, as every molecule is traveling away from every other molecule, the boundary of the molecule (gas placed in a closed container) is no more able to hold gas molecules in the container, because volume is increased which means pressure on the gas molecule due to every other molecule is decreased as every molecule is now pressurizing walls of the container.

“Hence we can safely say volume of the gas is increased when temperature of the gas is increased”

Case 2: Compression of gas

Now consider the second case, when heat is taken out from a certain volume of gas. As heat is taken out, energy of the gas molecule decreases and hence they can not move to long distance as they don’t have enough energy to move. It is like petrol tank of your vehicle is punctured and most of the petrol has been leaked and now with left over petrol you are trying to move to the distance where a mechanic shop is near by or you have to drop your vehicle in between.

Hence all the gas molecule now move very little, and near to each other. Thus pressure on the gas molecule due to every other gas molecule increases (Because resistance to the motion due to other gas molecule) and thus pressure to the walls of the container due to gas decreases.

"Hence we can safely say volume of the gas decreases when temperature of the gas is decreased”

Hence we have Ideal gas equation PV=nRT, where R and n are constant for any given gas, and we have gas laws like

1. Boyle’s law (At constant temperature, pressure on gas molecule is inversely proportional to volume of gas),

2. Charles law (At constant pressure, volume of the gas is directly proportional to the temperature of the gas),

3. Gay-Lussac’s law (At constant volume of the gas, temperature of the gas is directly proportional to the pressure on the gas molecules)