How pure substances combine

Before heating, a magnet attracts the iron because it is a free element in a mixture. After heating, the magnet no longer works, meaning the iron's magnetic property has changed.

In a mixture, elements keep their own properties — so iron stays magnetic. When a chemical reaction occurs, atoms bond to form a new compound with entirely different properties. Iron sulfide (FeS) is that new compound.

The iron atoms are not destroyed — they are rearranged into a new compound. This is why the mass is conserved but the magnetic property disappears: the iron atoms are now chemically bonded to sulfur atoms.

The correct answer is that a chemical reaction has occurred, forming iron sulfide, a compound with different properties. A compound's properties are not simply a combination of its elements' properties — they are new and different.

The question asks why the heated product is no longer magnetic, even though the original mixture contained iron.

The Law of Conservation of Mass states that atoms are never created or destroyed in a chemical reaction — they are only rearranged. So the option saying iron atoms are destroyed is incorrect.

Melting is a physical change, not a chemical one. A physical change does not alter the chemical properties of iron — melted iron is still iron and would still be magnetic.

This explanation is about a physical barrier, but a thin coating of sulfur would not stop a magnet from attracting iron underneath. Also, this does not account for the formation of a new substance.

The remaining option — that a chemical reaction formed iron sulfide with different properties — correctly explains the observation using the concept that compounds have new properties unlike their constituent elements.

The key observation is bubbles forming rapidly where there were no bubbles before. This is one of the recognised signs of a chemical reaction.

One of the main signs of a chemical reaction is gas being released — bubbles appearing that were not there before indicate a gaseous product has been formed. This means atoms have been rearranged into new substances.

The correct answer is that bubbles indicate gas production, which is evidence that a new substance has been made in a chemical reaction. Baking soda reacting with vinegar produces carbon dioxide gas as a product.

The question tests whether students can identify bubbles as evidence of a chemical reaction and explain why they indicate a new substance.

This is incorrect — mixing two liquids (such as water and juice) does not automatically produce bubbles. Bubbles only appear when a gas is produced, which is a sign of a chemical reaction.

The question describes bubbles, not a temperature drop. Also, dissolving is a physical change — the original substances can be recovered, and no new substance is formed. This option mixes up different concepts.

Boiling requires extremely high temperatures, and vinegar and baking soda react at room temperature. The bubbles are carbon dioxide gas produced by the reaction, not steam from boiling.

The correct answer is that bubbles indicate a gas has been produced, which is one of the key signs of a chemical reaction occurring and a new substance being formed.