candle (1K)

in Observing a Candle Reports

Making Connections

Many students failed to make connections between observations and conclusions. This is the most common mistake in any lab report. The science you learn from labs should come from what you actually did and observed in the lab! Some guidelines for writing scientific explanations are on the back of this sheet. This connection-making was required for all of the questions you were assigned to answer in the brief report. An example of a good (if somewhat wordy) explanation follows:

Candles, Combustion, and H2O

The scientific theory claims that H2O is a product of the combustion of a candle. To investigate that claim my research group held a glass beaker over the candle with the opening just at the level of the flame. Very quickly a fog formed on the inside of the beaker. When I ran my finger through the fog it wiped off easily. After a short time the heat of the candle flame warmed the glass and the fog disappeared from the hottest parts of the glass. Since the fog acted like condensation it most likely is condensation. (Condensation is when water vapor--a gas--cools to form tiny droplets of liquid water). Since it formed inside the overturned beaker it appears to be coming from the candle. Therefore water is produced as the candle burns.


There were a host of scientific misconceptions that really ought to be cleared up:

In order to prove that the candle produces carbon dioxide (CO2) or water (H2O) it is not necessary to snuff out the candle. The experimental techniques used work quite well when the candle is allowed to burn. The only experiment which required the snuffing of the flame was the one which showed the need for oxygen. By limiting the supply of oxygen to what was in an enclosed space, and seeing the flame go out, you could show that oxygen is required for burning.

Carbon dioxide (CO2) and water (H2O) are produced continuously while the candle burns. They stop being made when you put it out.

The smoke you see when you blow out the candle is not made of carbon dioxide (CO2) and water (H2O). Both of these chemicals are invisible gases at the temperature of the flame.

The smoke is made of unburned vaporized wax. The heat of the flame serves to melt and then vaporize the wax of the candle. When the flame goes out some wax is still hot enough to be vaporized. Until the candle cools this vaporized wax continues to stream out.

Soot is not the same thing as CO2! Soot is the solidifed fuel which is burning in the flame. The black stuff that collects on a piece of glass held in a flame is made of tiny particles of carbon. This carbon is the stuff that is glowing to make the flame visible and is also the fuel that is burning (combining with oxygen). Soot is unburnt fuel. When the fuel burns, carbon dioxide is one of the chemical products.

Liquid wax is not ‘wet’. Wax does not contain any water. Wax can be melted but the liquid you observe still contains no water: it is still only wax (which is made of compounds of hydrogen and carbon). The water in this lab comes as a result of the chemical reaction between the hydrogen part of these compounds with oxygen from the air. After all, water is a compound of hydrogen and oxygen (H2O).

Limewater is a mixture of lime (Ca(OH)2, like in limestone, or the lime the occurs in teapots) and water. Such a mixture is called a solution. The solution of lime and water is useful because it can show the presence of CO2. In the presence of CO2 the lime dissolved in the water reacts to form calcium carbonate (CaCO3) otherwise known as ordinary chalk. Chalk is, as you know, white. So when the reaction occurs the water becomes cloudy: small particles of chalk have formed and are floating in the water. Since this only happens when CO2 is around this is a clear indication that CO2 has gotten into the water. The reaction does not cause the entire solution to become solid chalk. The chalk is the cloudy stuff like dust that makes the water turn white.

page break

Writing Scientific Explanations


Science is at least in part an attempt to explain how the world works. As students of science it is important to learn how to write scientific explanations. Simple scientific explanations are the first step in building theories that model how the real world works.

Scientists collect information about the world and using what they know they try to explain how things work. Or, if they can, why they work the way they do. Using what you learn in science class you can learn to write correct scientific explanations. Given a situation to explain you use a three-step process to write a good explanation. First, you make a claim about the situation that you think is true. Next, you provide one or more pieces of evidence that can be used to support your claim. Finally, you show the reasoning that connects your claim with the evidence. In other words, you clarify the connection between your claim and the evidence.

Step One: Making a Claim

Step Two: Provide Evidence

Step Three: Provide Reasoning

In short: make a claim, report observations and make a logical connection between them!

This document refers to the lab that can be found here.
Last updated: Jun 17, 2015 Home
var sc_project=6424697; var sc_invisible=1; var sc_security="6d94fb45";