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Observing a Candle (2 Favorites)

LAB in Observations, Phase Changes, Combustion. Last updated February 5, 2019.


Summary

In this lab, students accumulate observations of a candle including a look at the combustion reaction's reactants and products. In addition, in the style of Faraday's Chemical History of a Candle, they learn a little about how wax melts, vaporizes, and burns.

Grade Level

High school

Objectives

By the end of this lesson, students should be able to

  • Make better observations.
  • Explain how a candle burns.
  • Know about combustion reactions.

Chemistry Topics

This lesson supports students’ understanding of

  • Observations
  • Combustion

Time

Teacher Preparation: 30 minutes

Lesson: 2 hours

Materials

For each group:

  • Candle
  • Matches
  • Safety goggles
  • Ruler
  • Watch glass
  • 400-mL beaker
  • Limewater solution

Safety

  • Always wear safety goggles when working in a chemistry lab.
  • Always be aware of an open flame. Do not reach over it, tie back hair, and secure lose clothing.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

  • Explain to students that they are practicing of scientific observation in this experiment. Observation is not the same as seeing. If five people see the same movie, there will be five different summaries. Some of the people will be more observant than others. Rather than depending on memory, you can write down observations so they are more accurate. And this is good scientific practice.
  • You should prepare the limewater at least 24 hours before you plan on using it. It may take a while for the Ca(OH)2 to dissolve, so if you need to filter out undissolved Ca(OH)2, you will have time to do that.
  • The “jumping flame trick” (part 6) requires that you light a candle, blow it out, and then hold a lit match over the smoke (not touching the wick). The smoke has some vaporized wax, and will initiate a flame to reform on the wick.
  • Students often remember this lab even at the end of the year. It's a great way to get students to appreciate how chemistry is relevant even to things as familiar as a candle.

For the Student

Lesson

Background

You will observe a burning candle in this experiment. When you light a candle, you initiate a chemical reaction called combustion. This reaction is expressed as follows:

hydrocarbons + oxygen ⇾ carbon dioxide + water + heat/light

or

CnH2n+2 + O2 ⇾ CO2 + H2O + energy

You can tell that a chemical reaction occurs because heat and light are released. Combustion reactions require three things to take place: fuel (hydrocarbons), oxygen, and ignition. Hydrocarbons are molecules made up of hydrogen and carbon and substances such as gasoline, fuel oil, and propane are examples of hydrocarbons.

In this experiment you will use a candle, which is also a hydrocarbon. You will use oxygen in the atmosphere, and you will supply ignition with a match.

It may seem odd to think that burning the candle produces water. It is hard to see the water that results from burning the candle because it is a gas. Carbon dioxide is also a gas and it is more dense than air, so it “sinks” and can be separated from air easily.

When CO2 is added to a calcium hydroxide, Ca(OH)2, solution (limewater), it reacts to form insoluble calcium carbonate, CaCO3. This substance is white and when the reaction occurs, it makes the solution turn cloudy because of the insoluble CaCO3.

Prelab Questions

1. What happens to a candle when you light it?

2. How can you prove that a candle needs oxygen to burn?

3. How can you prove that a candle produces carbon dioxide when it burns?

4. How can you prove that a candle produces water when it burns?

5. What happens when you hold a piece of glass in different parts of the flame? What do these results say about the process of burning wax in a candle?

6. Is it possible to light a candle without directly touching the flame and the wick? Why or why not?

Purpose

You will make a series of observations to hone your observation skills for future experiments. You will learn something new about an object you assume is familiar.

Materials

  • Candle
  • Matches
  • Safety goggles
  • Ruler
  • Watch glass
  • 400-mL beaker
  • CO2 indicator solution in a beaker

Safety

  • Always wear safety goggles when working in a chemistry lab.
  • Tie back loose hair and clothing.

Procedure

Before proceeding with any procedure steps your group has designed, get approval from your teacher. Once you have made observations, if you feel you don’t quite understand what you’ve observed, check in with your teacher to discuss before moving on to the next part.

PART I: Observations of the Candle

  1. Record some quantitative observations about the candle before you light it. Record things such as length, mass, diameter, length of wick, or anything else that occurs to you. Try getting the mass of the candle at specific time intervals while burning: does its mass change over time?
  2. Record some qualitative observations about the candle before you light it. Record such things as wax color, color of wick, smell, new/old/damaged, or anything else that occurs to you.
  3. Record the sequence of events that occurs as you light the candle. Try to be as detailed as possible. These observations will be qualitative and should be as organized as possible.

PART II: Candles Use Oxygen as They Burn

  1. When the chemical change called combustion happens, oxygen must be present. Oxygen is a gas that makes up about 20% of Earth’s atmosphere (by volume). Can you prove that oxygen from the air is required for the candle to burn? As a group, suggest a procedure. Have your teacher approve the procedure before carrying it out.
  2. Write down what you observe in your experiment, whether it seems relevant or not.

PART III: Candles Produce Carbon Dioxide as They Burn

  1. Combustion produces carbon dioxide (CO2). Recall from the background a way to detect CO2 using a solution. Devise and carry out an experiment to test whether a candle releases CO2.
  2. Write down what you observe in your experiment, whether it seems relevant or not.

PART IV: Candles Produce Water as They Burn

  1. Combustion reactions also produce water (H2O). Because the candle also releases heat, if water is present you will have to provide a way to cool it down to see it. Can you prove that candles do release H2O? Devise and carry out an experiment.
  2. Write down what you observe in your experiment, whether it seems relevant or not.

PART V: Flames Have Parts

  1. There are at least three distinct regions in a candle flame. One: the blue-rimmed clear region very close to the wick. Two: the dim-orange-fading-to-bright-yellow region that produces light. Three: the clear region just above the visible flame. Observe these flame regions. Devise and carry out an experiment to observe what is in these regions. Write down the steps of your experiment below and show them to your teacher before proceeding.
  2. Write down what you observe in your experiment, whether it seems relevant or not.

PART VI: Flames Can Be Surprising

Your teacher will show you the “jumping flame trick.”

Analysis

Part II-V:

1. Write a story about what happens to the solid wax as it melts, enters the flame, burns, and leaves the flame.

Part VI:

1. Describe how to perform the jumping flame trick in your own words.

2. What makes the trick possible?

3. What burns in a candle: the wick, the solid wax, the melted wax, or vaporized wax? Justify your answer.