Gases & Reactions

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Gas Particles

In gases, the particles of matter move about freely and fast. The particles are well spread out, so gases and light have no fixed volume or shape.

The gas particles may be independent atoms (e.g. argon gas) or molecules, which are small group fo atoms bonded together (e.g. chlorine gas).

In this page, you will study the properties and rections of three gases - oxygen, hydrogen and carbon dioxide gases.

Oxygen Gas

Oxygen is a reactive gas that forms oxide compounds with most other elements. For example, calcium and oxygen form calcium oxide; sulfur and oxygen form sulfur oxide.

Oxygen comprises 20.9% of the air on Earth and is the most abundant element in the Earth's crust based on mass. Oxygen is:

colourless
odourless (has no smell)
tasteless
very slightly soluble in water
very important for combustion reactions

The oxygen gas molecule is made from two atoms of oxygen joined (O₂) - it is a diatomic molecule.

Oxygen can be made in the lab by breaking down hydrogen peroxide (hair bleach) into oxygen and water. A catalyst called manganese dioxide is added. (A catalyst as a chemical that speeds up a reaction but does not get used up – it is not a reactant.)

hydrogen peroxide → oxygen + water

H2O2 → O2 + H2O

(The above equation has not been balanced.)

When chemicals originated in oxygen gas reaction cold combustion (burning) occurs. In combustion the product is an oxide. For example:

magnesium + oxygen → magnesium oxide

Mg + O2 → MgO

(The above equation has not been balanced.)

Oxygen makes up 46.1% of the Earth's crust and is primarily found in oxides and silicates (because it is so reactive). The most common silicate minerals in Earth's crust are quartz, feldspar, mica, amphibole, These silicate minerals are fundamental components of many common rock types like granite and sandstone.

3D model of oxygen gas (O₂). To change the size, scroll up or down. Red = oxygen atom.

To check for the presence of oxygen gas, we use the glowing splint test.

Collect a sample of oxygen in a test tube and use a stopper or your thumb to prevent the gas from escaping.
Light a splint and then blow out the flame to leave a glowing ember on the end.
Quickly plunge the glowing splint into the test tube of oxyggen as the stopper is removed.
The glowing splint relights the flame because the extra oxygen present makes the embers burn faster and ignite.

Hydrogen Gas

Hydrogen makes up approximately 90% of all the atoms in the universe and is the most plentiful of the elements that exist naturally. Most of this hydrogen is in stars, forming in outer space, and it is an ionised (or charged) form, known as plasma.

On Earth, hydrogen gas is:

colourless
odourless (has no smell)
neutral to litmus (neither acidic nor basic)
mostly insoluble in water
the lightest gas known - about 15 times less dense than any other element
very flammable, burning explosively when mised with air and ignited

Hydrogen exists as molecules of two hydrogen atoms bonded together (H₂) - it is a diatomic molecule.

Hydrogen gas is highly combustible (burns very readily), even explosive. Hydrogen gas reacts with oxygen to release a very large amount of heat. As it is a very light gas, hydrogen was used in passenger airships. But after disasters, modern airships use helium gas. It also produces water as a product - this is how 'hydrogen' got its name, which is Greek for 'water creator'.

hydrogen + oxygen → water + heat

H2 + O2 → H2O + heat

(The above equation has not been balanced.)

Hydrogen gas can be made in the lab by adding dilute hydrochloric acid to zinc metal.

zinc + hydrochloric acid → hydrogen + zinc chloride

Zn + HCl → H2 + ZnCl2

(The above equation has not been balanced.)

Pictured is the Orion Nebula, which is the only H II region visible to the naked eye. An H II region is interstellar matter consisting of ionized hydrogen atoms.

3D model of hydrogen gas (H₂). To change the size, scroll up or down. White = hydrogen atom.

The test for hydrogen gas involves inserting a burning splint into the gas. Hydrogen gas will pop.

Collect a sample of hydrogen in a test tube and use a stopper or your thumb to prevent the gas from escaping.
Light a splint and hold the flame near the mouth of the test tube while the stopper is removed.
A squeaky pop sound is made as the hydrogen gas ignites and explodes.

Carbon Dioxide Gas

Carbon dioxide makes up only 0.038% of the Earth's atmosphere. The gas is made when living things release energy from their food by the process of respiration, or when fuels burn to release energy by the process of combustion. However, it does not support burning.

Carbon dioxide is:

colourless
odourless
slightly soluble in water
much heavier (more dense) than air

Carbon dioxide is a compound rather than an element. Carbon dioxide gas exists as molecules with two oxygen atoms bonded to a single carbon atom (CO₂).

Living things produce carbon dioxide gas in the presence of respiration. Respiration releases the energy stored in sugars.

sugar + oxygen → carbon dioxide + water + heat

Carbon dioxide is produced when fuels are burnt. This reaction is called combustion and releases heat.

fuel + oxygen → carbon dioxide + water + heat

Carbon dioxide can be made in the lab by adding hydrochloric acid to calcium carbonate (marble chips).

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

To check for the presence of carbon dioxide gas we use the limewater test.

Half fill a clean test tube with fresh limewater (calcium hydroxide solution).
Bubble the gas produced from the carbonate and acid reaction into the limewater.

The limewater will immediately turn milky to confirm the gas is carbon dioxide. This is due to many tick specks of solid calcium carbonate forming. If you continue to bubble the gas through the milky limewater, the limewater will eventually turn clear again, but this step is not requried to identify carbon dioxide.

Burning wood is an example of a chemical reaction in which wood in the presence of heat and oxygen is transformed into carbon dioxide, water vapor, and ash.

3D model of carbon dioxide gas (CO₂). To change the size, scroll up or down. Grey = carbon atom; red = oxygen atom.

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