Gases Have Mass and Volume

Introduction

Gases are the easiest state of matter to forget about because most of them are invisible. Yet the air around us presses on every surface, fills every container and can be weighed. Showing that gases have mass and volume completes the evidence that they are genuine matter, and prepares you for the particle model, which explains why gases behave so differently from solids and liquids.

Core explanation

Gases have mass. A sealed flask can be weighed, then pumped full of extra air and weighed again: the mass increases. In the reverse experiment, a flask from which the air has been pumped out has a smaller mass than when full of air. At room temperature and normal pressure, 1 litre (1 dm³) of air has a mass of about 1.2 g. Other gases differ: 1 litre of carbon dioxide has a mass of about 1.8 g, and 1 litre of helium only about 0.17 g.

Gases take up space. A gas fills any container completely and evenly, so its volume is the volume of the container. Blowing up a balloon shows that the air inside pushes outward and occupies space. If you seal the end of a syringe full of air and push the plunger, it moves only a little way before the trapped air resists: the air is taking up room.

Gases are compressible. Unlike solids and liquids, a gas can be squeezed into a much smaller volume. Pushing the plunger of a sealed syringe halves the volume if you roughly double the pressure. This is because the particles of a gas are far apart, with lots of empty space between them.

Temperature and pressure matter. A gas expands when it is heated and contracts when cooled, and its volume decreases when the pressure on it increases. A balloon taken from a warm room into a freezer visibly shrinks. For this reason, gas volumes are always quoted with the temperature and pressure at which they were measured.

Measuring gas volumes. Chemists collect gases produced in reactions in a gas syringe , or by bubbling them into an upturned measuring cylinder full of water (for gases that do not dissolve much in water, such as hydrogen and oxygen).

Step-by-step reasoning

To show that air has mass using a football:

1. Weigh a deflated football on a balance reading to 0.1 g. 2. Pump it up firmly and weigh it again. 3. Subtract the first mass from the second. 4. The increase (a few grams) is the mass of the extra air pumped in — proof that air has mass.

Visual explanation

Imagine a sealed syringe containing 100 cm³ of air. Pushing the plunger to 50 cm³ crowds the gas particles closer together, but the same particles are still there, so the mass is unchanged. In the particle simulation, gas particles move quickly and randomly, spread through the whole box and bounce off the walls; the bouncing produces pressure.

Real-world analogy

A gas in a container is like a group of energetic children let loose in a gym: they run everywhere, spread into every corner and bump into the walls. Squeeze them into a smaller room and they still fit, just more crowded, and they hit the walls more often.

Real-world example

Scuba divers carry tanks of compressed air. About 2000 litres of air at normal pressure are squeezed into a 10-litre steel tank at around 200 times atmospheric pressure. The full tank is noticeably heavier than the empty one — divers must account for the mass of the air when planning their buoyancy.

Why?

Why does a gas fill its whole container while a liquid only fills the bottom? Gas particles move fast and are only very weakly attracted to one another, so nothing holds them together; they spread out until they hit the walls. Liquid particles attract each other strongly enough to stay together in a pool at the bottom.

Common misconception

"Hot air balloons rise because hot air has no mass." Hot air does have mass; it rises because it has a lower density than the cooler air around it — the same volume contains fewer particles. Page 47 explains this in detail.

Worked example

Question: A 2.0 dm³ flask is found to contain air with a mass of 2.4 g. What is the mass of 1 dm³ of this air?

Reasoning: Divide the mass by the volume: 2.4 g ÷ 2.0 dm³.

Answer: 1.2 g per dm³ (1.2 g/dm³).

Quick check

1. Give one piece of evidence that a gas takes up space. Answer: A balloon inflates when air is blown in (or: water cannot enter an upturned glass of trapped air).

Exam focus

Questions often ask why a gas can be compressed but a solid cannot. Answer in terms of particles: gas particles are far apart with large spaces between them, while particles in a solid are already touching. Also remember to quote gas volumes with conditions of temperature and pressure.

Advanced insight

The relationships between gas pressure, volume and temperature are described by the gas laws (Boyle's law, Charles's law) and combined in the ideal gas equation pV = nRT. These allow chemists to calculate how much gas a reaction produces — you will meet them in the states-of-matter and physical chemistry units.

Summary

Gases are matter: they have mass (about 1.2 g per litre for air) and take up space, filling their containers completely. They are easily compressed because their particles are far apart. Their volume changes with temperature and pressure, so conditions must always be stated. Gas volumes are measured with gas syringes or by collecting over water.

Practice questions

1. A football's mass increases from 410.0 g to 414.5 g when pumped up. What mass of air was added? Answer: 4.5 g. 2. Why must a gas volume always be quoted with a temperature and pressure? Answer: Gas volume changes significantly with temperature and pressure, so the same amount of gas can have different volumes. 3. Explain, using particles, why a gas is easy to compress. Answer: Gas particles are far apart with large empty spaces between them, so they can be pushed closer together. 4. Name two pieces of apparatus used to measure the volume of a gas produced in a reaction. Answer: A gas syringe, and an upturned measuring cylinder (or burette) filled with water.