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Tests for Identifying Common Gases

Chapter 18 /  Lesson 3   Transcript
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  • 0:04 Background
  • 0:43 Hydrogen
  • 1:24 Oxygen
  • 2:07 Carbon Dioxide
  • 3:44 Chlorine
  • 4:17 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Have a mystery gas you need to identify? You have come to the right place. This lesson discusses the tests needed to identify hydrogen, oxygen, carbon dioxide, and chlorine gas.

Background

We've all been there; you have a bunch of unknown gases trapped in test tubes, and you need to figure out what you're dealing with. Okay, maybe we haven't all been there, but there are some nifty ways you can identify gases by using burning splits, limewater, or litmus paper. Most of the gases we'll focus on are colorless and odorless, so it's not possible to identify them by appearance.

In order to perform the tests, you'll need the following supplies:

  • Test tube of mysterious gas (with a stopper, so the gas doesn't escape)
  • Wooden splints
  • Matches
  • Goggles
  • Limewater (we'll go over the prep for this later)
  • Rubber tubing, glass rod, and a one-holed stopper
  • Litmus paper

Hydrogen

The first gas we will focus on is hydrogen, which is a non-toxic, colorless, odorless gas that's the most abundant element in the universe. Before we test for hydrogen, remember to wear your goggles for all of these tests. They'll protect you if the test tube breaks.

Here are the steps you need to take in order to test for hydrogen gas:

  • Light the wooden splint
  • Remove the stopper from the test tube and place the burning splint near the opening of the test tube
  • If hydrogen is present, you will hear a popping sound
  • Extinguish the splint

So, why the popping sound? When exposed to the flame, the hydrogen gas in the test tube reacts with the oxygen gas in the air. This reaction releases energy and that results in the popping sound.

Oxygen

We are going to use the splint again to test for oxygen, which is a non-toxic, colorless, odorless gas that you need to breathe in order to survive.

The directions for testing oxygen are a little different than for hydrogen, so pay attention:

  • Light the splint and then blow it out so it is glowing, but not an open flame
  • Remove the stopper from the test tube
  • Place the glowing splint into the test tube
  • If the splint reignites, the test is positive for oxygen
  • Extinguish the splint

Oxygen is responsible for combustion (burning of the splint), but normal atmospheric oxygen levels (around 21%) aren't enough to reignite the glowing splint. The test tube contains a higher percentage of oxygen, so when the glowing splint is placed inside, it's able to reignite.

Carbon Dioxide

The third gas we are going to test is carbon dioxide, which is colorless, odorless and you're breathing it out right now. It's also the stuff that makes your soda fizzy. There are a couple of easy ways to identify carbon dioxide gas.

Let's go over the first:

  • Light a splint
  • Remove the stopper from the test tube
  • Place the lit splint into the test tube
  • If the flame goes out, it could be carbon dioxide

While the first method is simple, if the gas in the test tube is oxygen, the ignited splint could cause the test tube to explode, so there's another method that uses limewater.

Limewater can be made by mixing water with calcium hydroxide (about 1 teaspoon calcium hydroxide for every gallon of water). Let the solution sit for 24 hours and then pour the solution through a coffee filter.

In order to perform the test do the following:

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