MnSTEP Teaching Activity Collection > MnSTEP Activities > Observation of a Candle

Observation of a Candle

Pam Fier-Hansen
Marshall High School
400 Tiger Drive
Marshall, MN 56258
Adapted from Glencoe's Chemistry: Concepts and Applications Chemistry Lab 1
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Summary

In this activity, students observe a candle from a chemist's point of view. From these observations and some simple chemical tests, they will try to determine what must be present in order for a candle to burn and what the products of combustion are.

Learning Goals

1. Carefully observe and describe the characteristics of a candle flame.
2. Determine what part of the candle is burning.
3. Infer the products of the combustion

Context for Use

This is a general chemistry lab which I use at the beginning of the semester to review the concept of chemical and physical change. Students are asked to determine what the products of the reaction are by making observation of simple chemical tests. The lab requires about 2 class periods or 1 block to complete. It does not require any special lab equipment and could be easily adapted to physical science.

Subject: Chemistry:General Chemistry:Chemical Reactions
Resource Type: Activities:Lab Activity
Grade Level: High School (9-12)

Description and Teaching Materials

Observation of a Candle


You have seen a candles burn on many occasions, but you have probably never considered the burning of a candle from a chemist's point of view. In this lab, you will try to determine what is necessary for a candle for a candle to burn, and what products are formed when the candle burns.


Objectives:
1. Carefully observe and describe the characteristics of a candle flame.
2. Determine what is necessary for a candle to burn.
3. Determine the products of the combustion of a candle.


Materials: (pair of students)
Large wax candles 2
Matches 1 book
Shallow plastic or metal 1
10 ml of phenol red solution 25 ml
250 ml beaker 1
250 ml Erlenmeyer flasks 1
Rubber stopper to fit the flask 1
Wire gauze square 1
Tongs 1


Safety:
Keep all flammable materials away from matches and burning candles.
Wear safety goggles.

Procedure:
1. Light a candle and let 3-4 drops of wax fall in to the center of the pan. Press the candle into the melted wax and hold it upright until the wax solidifies

2. Light the candle. Carefully observe the flame. List at least 8 observations of the flame. Look at the colors and the shape of the flame. Where does the burning take place? What is burning?

observations
1
2
3
4
5
6
7
8

3. When a chemical change takes place, a different substance is formed. When a physical change takes place, the substance changes shape or phase, but the substance remains the same. Classify each of the following as a chemical (C) or physical (P) change.
a. _____ wax melting
b. _____wick burning and turning in to soot
c. _____wax burning and forming carbon dioxide
d. _____breaking a glass
e. _____baking cookies
f. _____digesting food

4. Light a second candle and hold the flame 2-4 cm from the flame of the first candle. Gently blow out the first candle flame and then move the other flame in to the smoke from the first flame. Do you have to touch the wick in order to get the candle to relight? Write your observations below. What does this tell you about the part of the candle that is burning?

Observations

5. Relight the standing candle and blow out the second candle. Using a tongs, hold the wire gauze over the flame, perpendicular to the flame. Slowly lower the gauze on to the flame. Do not touch the wax. If the flame appears to go out, quickly move the wire gauze to the side. Record your observations. What do you thick is happening? What does this say about the part of the candle that is burning?

Observations


6. Place approximately 150 ml of ice water in a 250 ml beaker. Dry the outside of the beaker. Hold the beaker about 4-5 cm above the flame. Look for the formation of a new compound on the bottom of the beaker. Note: you may see the formation of black soot on the bottom of the beaker. That is not the compound that we are looking for. If you see soot, you are probably holding the candle too close to the flame. Record your observations. What do you think the compound is? Where do you think that the compound came from?

Observations


7. Pour water in to the pan that the candle is in to a depth of 1cm. Quickly lower the mouth of the Erlenmeyer flask over the candle so that the mouth of the candle is below the surface of the water. Hold the flask in place for approximately one minute. After 1-2 minutes, lift the flask out of water and quickly place the rubber stopper in the mouth of the flask. Record your observations below.

Observations


8. Add 10 ml of phenol red solution to the flask. Stopper the flask and swirl the solution for about 1 minute. Record your observations below. What does this tell you about the gas that was formed?

Observations





Analyze and Conclude:
1. Do your results indicate that the candle burns as a solid, liquid or a vapor?

List at least two observations/activities that helped you determine this and explain how it helped you decide this.




2. What are two products from the combustion of a candle?

List at least two observations/activities that helped you determine this and explain how it helped you decide this.




2. What gas in our atmosphere promotes combustion?


3. Some fire extinguishers have a compressed gas in them. Do you think that gas would be oxygen, hydrogen or carbon dioxide?


What did you observe that helped you decide this?


4. What changes occurred in the water level when you put the Erlenmeyer over the flask.


Propose an explanation for why this occurred.


Extension:
1. When the flask was placed over the candle, the flame went out and the water rose. Propose an explanation as to why this occurred.


2. Design an experiment to test your explanation.
You might begin by repeating this experiment and making careful observations. Which happened first? How high did the water rise in the flask? What factors affect how high the water rises? Design an experiment to test your hypothesis.

Supplies: candles of different heights, multiple candles Student Handout for Observation of a Candle lab (Microsoft Word 62kB Aug25 09)

Teaching Notes and Tips

The students should observe that the flame is not touching the solid candle or the melted wax. It appears to be only touching the wick. I ask them if they think that the candle would burn as long if it were only the wick that was burning. What is the purpose of the wax? When the students place the first flame into the smoke of the second candle, they should see the second candle relight. It is not necessary to touch the wick in order for it to relight. This should lead them to conclude that it is the wax vapor that is burning.
When the students place the beaker containing the ice water above the candle, they should see condensation on the bottom of the beaker. If they hold it too close, they will not see the condensation and will see soot form. Students will usually recognize that the condensation is made of water, but they have difficulty recognizing that it comes from the combustion of the candle.
When the Erlenmeyer flask is placed over the top of the candle in the water, student should observe that the water candle goes out and the water rises. The students test the gas that was collected with phenol red solution (an acid-base indicator). They should observe that the solution changes from red to yellow. I demonstrate that this might indicate the formation of carbon dioxide by blowing through a straw into a solution of phenol red.
When students are asked to propose why the water rises in the flask, they come up with a variety of different ideas. Some of these include ideas such as, "all the oxygen was used up and so it created an empty space" or it 'gets sucked into the flask". You can remind them that carbon dioxide was created and it also takes up space. The new part of this is the extension to the lab. In this inquiry activity, I ask students to try to determine what factors affect how high the water rises and then revise their hypothesis. An explanation as to why this occurs is that the air inside the flask is heated by candle. This causes the air inside the flask to expand and some of it escapes when the flask is placed over it. When the candle goes out and the air cools, it contracts. Since the pressure outside the flask is greater than inside the flask, the water is pushed up into the flask. The collapsing can demo would be a good follow-up activity to this lab.

Assessment

Assessment of this activity will be based on the lab report that they hand in. Key concepts that I want them to understand is the importance of making and recording good observations, the difference between chemical and physical change, and making inferences from their observations.

Standards

Standards Match: Chemistry 2.1.3-2.1.4 Chemical reactions and state of matter.

References and Resources

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