candle (1K)

Observing a Candle

Note to Teachers:

This lab is an inquiry-based lab. The idea is for students to come up with their own procedure. They must decide what will be held constant, what will be varied, and how to make their measurements. The notion is to have students learn science by doing science the way scientists do it. Students may either write narrative answers to the questions as posed in the objective section (complete with graphs) or write a full lab report. I expect that it will require anywhere from 1 to 3 full hours of classroom time, depending on the level of complexity you require from the students. Students may extend their investigations at home in order to get a better grade. If you try it, please get in touch (my email address is on the home page) and let me know how it worked for you.

There is a section of this web page near the end which will not print. It gives information required to make the lab successful. Be sure to read it. This section will also not print.


In this lab you will light a candle and perform various tests to discover some things about burning and about candles. You will learn the practice of scientific observation. Observation is not the same as seeing. If five people see the same movie and then are each asked to tell about it you will hear five different stories. Some of the people are more observant than others or have better memories. You can make your memory better by carefully writing down your observations and you can become more observant by practicing.

When you complete your work on this lab you will be able to answer the following questions based on your observations:

  1. What happens to the candle when you light it?
  2. Does the candle need oxygen in order to burn?
  3. Can you prove that the candle produces carbon dioxide when it burns?
  4. Can you prove that the candle produces water when it burns?
  5. What happens when you hold a piece of glass in the flame and what is the identity of the substance you can collect this way?
  6. Is it possible to light a candle without touching the flame directly to the wick? Why or why not?
You can see a sample lab report for this lab if you would like (Word doc).


safety goggles
lab notebook & pen
watch glass
400 mL beaker
at front of room: supply of CO2 indicator solution
   (e.g., limewater: Ca(OH)2 in water)

water in a 200 mL beaker
50 mL beaker (for indicator)


You will be observing a candle and what happens when you light one. To understand what you are observing a little background would help. When you light a candle you initiate a type of chemical reaction called a combustion reaction. This reaction can be written in chemical shorthand as:

hydrocarbons + oxygen (O2) Arrowsngl carbon dioxide (CO2) + water (H2O) + heat/light
CnH2n+2 + O2 Arrowsngl CO2 + H2O + heat/light

You can tell that a chemical reaction is occurring because of that heat and light: a sure sign. Combustion reactions require three things: fuel (hydrocarbons), oxygen, and a source of ignition. Hydrocarbons are molecules made up of hydrogen and carbon and are in fact what make up such things as gasoline, fuel oil and propane. Candles are made of hydrocarbon wax. Oxygen is supplied by the atmosphere and you supply the ignition (a match). One point of some importance is that different phases of matter burn at different rates. Solids burn more slowly than liquids and liquids burn more slowly than gases.

Combustion can be an imperfect process. That is, some of the hydrocarbons may not burn completely. When that happens several carbon-containing products can form besides carbon dioxide. First, carbon monoxide (a highly toxic gas) can form. This is only dangerous in cases of burning charcoal indoors or using a gas-powered generator in a closed space. Second, incomplete combustion can result in pure carbon: the hydrogen is burned away (it combines with oxygen to become water) and the carbon stays behind unburnt.

It may seem odd to think that burning the candle produces water but it is a fact even so. Water is a compound of hydrogen and oxygen and when those two elements combine the most common compound is H2O. It is hard to see the water that results from burning the candle because it is a gas (steam) and it is invisible.

A few words about carbon dioxide. It is a gas that is more dense than air and so it can be collected from containers by pouring. When CO2 is added to water containing calcium hydroxide (Ca(OH)2) it reacts with the
Ca(OH)2 to form insoluble calcium carbonate (CaCO3). This substance is white and when the reaction occurs it makes the water turn cloudy. Water with Ca(OH)2 dissolved in it is called limewater.

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In this lab you will make a series of observations. The purpose of doing so is to hone your observational skills for the labs you will do in the future and to learn something about an object you may have taken for granted.


Tie back loose hair and clothing. Before lighting a match don the safety goggles and keep them on during all activities performed while the candle is lit. Before handling the indicator solution don the gloves provided. Treat chemicals you don’t know anything about with as much caution as you treat bleach or gasoline. If you are not wearing shoes with closed toes and heels then you should be!


The procedure for this lab is mostly up to you. After your teacher gives you an introduction to the lab it is your decision about how to proceed. Some things to keep in mind:

Tips and Pointers

Here are a few things you need to know how to do in order to be successful in answering the objective questions.

Note to Teachers: Here is how to do the trick: Limewater

Make limewater by mixing about 1 g of Ca(OH)2 into 0.5 L of degassed water. Degas the water by boiling for about ten minutes. The actual maximum amount of Ca(OH)2 that will dissolve is around 0.8 g/L according to the Ksp of the dissociation: 5.02 × 10-6. Filter any remaining solid before using the solution in the lab.


Answer each of the questions in the Objectives section of this handout. In your answer to each question include:

Scientific explanations require that you refer to your observations as collected during class. Scientific information is best when it can be made quantitative: length, volume, time, etc. Report these data as part of your observations and use them to support your explanations.

You will be graded on the quality of your writing, the profesionalism of your work’s appearance, the design and execution of your experiments and the degree of your understanding of the underlying science.

Comments on past student lab reports can be found here.
Some common errors in writing the report for this lab are explained here.
Last updated: Jul 07, 2008       Home