Question

answer the following questions:

1. how can you defend the claim that mass is conserved in physical changes?
2. how can you defend the claim that mass is conserved in chemical reactions?
3. when i burn a log, why is the mass of the ashes left behind less than the mass of the log?

Explanation:

1. In physical changes like melting or freezing, no new substances are created. The same atoms or molecules are present before and after the change. For example, when ice (solid water) melts to form liquid water, the number of water molecules remains the same. So, the mass is conserved as mass is a measure of the amount of matter, and the amount of matter (number of molecules/atoms) doesn't change in a physical change.
2. In chemical reactions, according to the law of conservation of mass, atoms are neither created nor destroyed. They are just rearranged. For example, in the reaction \(2H_{2}+O_{2}

ightarrow2H_{2}O\), we can count the number of hydrogen and oxygen atoms on both sides. On the reactant side, for hydrogen: \(2\times2 = 4\) atoms (from \(2H_{2}\)), and for oxygen: \(2\) atoms (from \(O_{2}\)). On the product side, in \(2H_{2}O\), hydrogen: \(2\times2=4\) atoms and oxygen: \(2\) atoms. So, the total mass of reactants (sum of masses of \(H_{2}\) and \(O_{2}\)) is equal to the total mass of products (\(H_{2}O\)) because the number and type of atoms are conserved.

1. When a log burns (\(C + O_{2}

ightarrow CO_{2}\) is a simplified part of the combustion process), it undergoes a chemical reaction. The log is mainly composed of carbon - based compounds. When it burns, some of the reactants (like carbon in the log reacting with oxygen) form gaseous products (\(CO_{2}\), \(H_{2}O\) vapor etc.). These gases escape into the atmosphere. The ashes are the non - combustible (or less combustible) part of the log. So, the mass of the log is equal to the mass of the ashes plus the mass of the gases that have escaped. That's why the mass of the ashes is less than the mass of the log.

Answer:

1. In physical changes, no new substances are formed. The same atoms/molecules are present, so mass (a measure of matter amount) is conserved.
2. In chemical reactions, atoms are rearranged but not created/destroyed. Counting atoms on reactant and product sides (e.g., in \(H_{2}+O_{2}

ightarrow H_{2}O\) balanced as \(2H_{2}+O_{2}
ightarrow2H_{2}O\)) shows mass conservation.

1. When a log burns, some of it reacts to form gases (\(CO_{2}\), \(H_{2}O\) vapor etc.) that escape. The log's mass = ashes' mass+ gases' mass, so ashes' mass < log's mass.