Misconception.
There is a misconception that matter is changed to energy or destroyed in some other way when it undergoes combustion or any other chemical change. So when wood burns in a fire or a candle burns the composite materials that make up the fuel are changed into heat or just "burn away".

Basis
This misconception is related to the misunderstanding that gas has no mass. Also anyone that has burned a candle or lit a fire has notice that the material seems to disappear as it burns and gives off heat.

Explanation
Generally when we are talking about fire or burning, we are referring to combustion. Combustion is a chemical reaction where Oxygen usually combines with Hydrogen or Carbon or Both to produce Heat Carbon Dioxide and/or water. For example, when a candle burns, paraffin wax which is comprised of Carbon and Hydrogen is converted into Carbon Dioxide and water. Occasionally the reaction doesn't completely happen, and some smoke is released. Also fragrances that are added are carried into the air by the hot Carbon Dioxide and water vapor. The Carbon Dioxide, Water vapor and other items if collected and weighed would equal the mass of the candle before it was burned.

Experimentation
Here is a possible experiment to show the conservation of matter in chemical reactions producing a gas.
Caution this experiment uses expanding gases in enclose chambers (like a bomb) Do not use a glass container.
Experiment base.jpg
A

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A. Very small amounts of baking Soda and vinegar are placed in separating containers within the same sealed container. These containers are weighed.
Experiment mix.jpg



B. Agitate the container until the mixtures combine. (Warning combining these substances will create a lot of gas and could make the container explode. Use small amounts of reactants and mix with caution. Do not use a glass container). If the container is sealed it will not change weight.
Experiment Final.jpg
C. Carefully remove the cap and allow gas (CO2) to escape. Replace the cap and weigh again. The gas in the container should now have around the same pressure and weight as air, and the total weight will have decreased. The pressurized gas that had mass has now dispersed and mixed with the air in the room. Like a fuel that has been burned, there is now less visible matter, but we can account for the change.