# at constant temperature, when the volume of a gas is decreased, what happens to its pressure?

### James

Guys, does anyone know the answer?

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## Gas Laws

## Gas Laws

The content that follows is the substance of lecture 18. In this lecture we cover the Gas Laws: Charles',Boyle's,Avagadro's and Gay Lussacs as well as the Ideal and Combined Gas Laws.

### Laws of Gas Properties

There are 4 general laws that relate the 4 basic characteristic properties of gases to each other. Each law is titled by its discoverer. While it is important to understand the relationships covered by each law, knowing the originator is not as important and will be rendered redundant once the combined gas law is introduced. So concentrate on understanding the relationships rather than memorizing the names.

**Charles' Law-**gives the relationship between volume and temperature

**if the pressure and the amount of gas are held constant**:

1) If the Kelvin temperature of a gas is increased, the volume of the gas increases. (P, n Constant)

2) If the Kelvin temperature of a gas is decreased, the volume of the gas decreases. (P, n Constant)

This means that the volume of a gas is **directly** proportional to its Kelvin temperature. Think of it this way, if you increase the volume of a gas and must keep the pressure constant the only way to achieve this is for the temperature of the gas to increase as well.

Calculations using Charles' Law involve the change in either temperature (T2) or volume (V2) from a known starting amount of each (V1 and T1):

**Boyle's Law -**states that the volume of a given amount of gas held at constant temperature varies inversely with the applied pressure when the temperature and mass are constant.

The reduction in the volume of the gas means that the molecules are striking the walls more often increasing the pressure, and conversely if the volume increases the distance the molecules must travel to strike the walls increases and they hit the walls less often thus decreasing the pressure.

Like Charles' Law, Boyle's Law can be used to determine the current pressure or volume of a gas so long as the initial states and one of the changes is known:

**Avagadro's Law-**Gives the relationship between volume and amount of gas in moles when pressure and temperature are held constant.

If the amount of gas in a container is increased, the volume increases. If the amount of gas in a container is decreased, the volume decreases. This is assuming of course that the container has expandible walls.

The relationship is again directly proportional so the equation for calculations is

**Gay Lussac's Law**- states that the pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature.

If you heat a gas you give the molecules more energy so they move faster. This means more impacts on the walls of the container and an increase in the pressure. Conversely if you cool the molecules down they will slow and the pressure will be decreased.

To calculate a change in pressure or temperature using Gay Lussac's Law the equation looks like this:

To play around a bit with the relationships, try this simulation.

### The Ideal Gas Law:

A combination of the laws presented above generates the Ideal Gas Law:

The addition of a proportionality constant called the Ideal or Universal Gas Constant (R) completes the equation.

As you can see there are a multitude of units possible for the constant. The only constant about the constant is that the temperature scale in all is KELVIN.

When using the Ideal Gas Law to calculate any property of a gas, you must match the units to the gas constant you choose to use and you always must place your temperature into Kelvin.

To use the equation, you simply need to be able to identify what is missing from the question and rearrange the equation to solve for it.

A typical question would be given as 6.2 liters of an ideal gas are contained at 3.0 atm and 37 °C. How many of this moles of the gas are present?

Because the units of the gas constant are given using atmospheres, moles, and Kelvin, it's important to make sure you convert values given in other temperature or pressure scales. For this problem, convert °C temperature to K using the equation:

T = °C + 273 T = 37 °C + 273 T = 310 K

Now, you can plug in the values. Solve for the number of moles

n = PV / RT

n = ( 3.0 atm x 6.2 L ) / ( 0.08206 L atm /mol K x 310 K)

n = 0.75 mol

Here are some practice problems using the Ideal Gas Law: Practice

### The Combined Gas Law

I said above that memorizing all of the equations for each of the individual gas laws would become irrelevant after the introduction of the laws that followed. The law I was referring to is the Combined Gas Law:

## When the volume of a gas is decreased at constant temperature the pressure increases because the molecules

Click here👆to get an answer to your question ✍️ When the volume of a gas is decreased at constant temperature the pressure increases because the molecules

Question

When the volume of a gas is decreased at constant temperature the pressure increases because the molecules

**A**

## strike unit area of the walls of the container more often

**B**

## strike the unit area of the walls of the container with higher speed

**C**

## strike the unit area of the wall of the container with lesser speed

**D**

## move with more kinetic energy

Medium Open in App Solution Verified by Toppr

Correct option is A)

We first note that the speed and hence the kinetic energy of the molecules of an ideal gas depends solely on temperature and since here temperature remains constant, the pressure cannot increase due to factors mentioned in options B,C and DNow A is correct and we explain why.

Consider a small area on the wall of the container. Previously when volume was high, it had a lower probability of being hit by a molecule but now space is less, so it will be hit by more molecules so, more pressure will be generated on this area.

Hence pressure increases

Solve any question of Kinetic Theory with:-

Patterns of problems

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## Volume and pressure in gases – the gas laws

Learn about temperature scales, pressure in gases and the gas laws with BBC Bitesize GCSE Physics.

Bitesize GCSE

## Temperature and gas calculations

Part of

Physics (Single Science)Solids, liquids and gases

## Volume and pressure in gases – the gas laws

Volume and pressure in gases – the gas laws Boyle’s law

Decreasing the volume of a gas increases the pressure of the gas. An example of this is when a gas is trapped in a cylinder by a piston. If the piston is pushed in, the gas particles will have less room to move as the volume the gas occupies has been decreased.

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As the pressure applied to a piston is doubled, the volume inside a cylinder is halved

Because the volume has decreased, the particles will collide more frequently with the walls of the container. Each time they collide with the walls they exert a force on them. More collisions mean more force, so the pressure will increase.

When the volume decreases, the pressure increases. This shows that the pressure of a gas is inversely proportional to its volume.

This is shown by the following equation - which is often called **Boyle’s law**. It is named after 17th century scientist Robert Boyle.

P1V1 = P2V2 where:

P1 is the initial pressure

V1 is the initial volume

P2 is the final pressure

V2 is the final volume

It can also be written as:

pressure1 × volume1 = pressure2 × volume2

Note that volume is measured in metres cubed (m3) and pressure in pascals (Pa).

It means that for a gas at a constant temperature, pressure × volume is also constant. So increasing pressure from pressure1 to pressure2 means that volume1 will change to volume2, providing the temperature remains constant.

Question

A sealed syringe contains 10 × 10-6 m3 of air at 1 × 105 Pa. The plunger is pushed until the volume of trapped air is 4 × 10-6 m3. If there is no change in temperature what is the new pressure of the gas?

## Charles’ law

Charles’ law describes the effect of changing temperature on the volume of a gas at constant pressure. It states that:

volume1=volume2×temperature1temperature2

V1=V2×T1T2 where:

V1 is the initial volume

V2 is the final volume

T1 is the initial temperature

T2 is the final temperature

Note that volume is measured in metres cubed (m3) and temperature in kelvin (K).

This means that if a gas is heated up and the pressure does not change, the volume will. So for a fixed mass of gas at a constant pressure, volume ÷ temperature remains the same.

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The volume of a gas rises as its temperature is raised

Balloons shrink when placed inside a beaker of cold liquid nitrogen

## More Guides

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Temperature and gas calculations

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