Question Video: Identifying the Statement That Correctly Describes the Change in Mass When Burning a Log | Nagwa Question Video: Identifying the Statement That Correctly Describes the Change in Mass When Burning a Log | Nagwa

# Question Video: Identifying the Statement That Correctly Describes the Change in Mass When Burning a Log Science • First Year of Preparatory School

When a wooden log burns on a fire, a lot of carbon dioxide is produced as well as heat and light. Which of the following statements about the ash that remains after the wood has burnt is correct? [A] The ash has more mass than the original log. [B] The ash has less mass than the original log. [C] The ash has the same mass as the original log.

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### Video Transcript

When a wooden log burns on a fire, a lot of carbon dioxide is produced as well as heat and light. Which of the following statements about the ash that remains after the wood has burnt is correct? (A) The ash has more mass than the original log. (B) The ash has less mass than the original log. Or (C) the ash has the same mass as the original log.

In this question, we are trying to determine the relationship between the mass of the wood before it is burnt and the mass of the remaining ash. We can imagine a log of wood. The wood is a starting material of the chemical reaction. And it contains a lot of carbon. We are told that the wood burns on a fire. In order for a material to burn, there must be oxygen present. Oxygen is composed of O2 molecules. The wood and the oxygen in the air are the reactants of the chemical reaction. When the wood burns, the carbon chemically reacts with the oxygen in the air. While the log burns, heat and light are given off. Heat and light are both different forms of energy.

Energy does not have mass. So we know that giving off heat and light will not affect the mass. We also know that a lot of carbon dioxide is given off as well. Carbon dioxide is made up of CO2 molecules. Carbon dioxide is a product from this reaction. As the wood continues to burn, more and more carbon dioxide is produced and less and less carbon remains in the wood. When the wood is done burning, ash remains. Ash is a brittle solid that is often black or gray in color. Ash is the carbon from the wood that did not react. This reaction, like all chemical reactions, will satisfy the law of conservation of mass.

The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction. Because of this, we know that the mass of the carbon before and after the reaction must be the same. Most of the carbon in the wood becomes a part of the carbon dioxide gas. Only a small amount of carbon remains as ash. So, the ash will have less mass than the original log since much of the matter from the original log became carbon dioxide.

We can see that the statement that correctly describes the mass of the ash is answer choice (B). Therefore, the statement that correctly describes the ash that remains after the wood has burnt is answer choice (B). The ash has less mass than the original log.

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