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