Representing a chemical change

A chemical change occurs when new substances are formed because bonds are broken and new ones are made. Chemical changes are generally difficult or impossible to reverse.

When wood burns, it produces ash, smoke, and gases — these are completely new substances that did not exist as wood before. You cannot collect the ash and gases and turn them back into wood.

The key feature of a chemical change is the formation of new substances that are difficult to reverse. The correct answer is: New substances are formed that cannot easily be turned back into wood.

The question asks us to identify the reason why burning wood is classified as a chemical change, not a physical change.

Changing shape (like tearing or cutting) is a feature of a physical change, not a chemical change. This option describes the wrong type of change.

A temperature change alone does not confirm a chemical reaction — dissolving some substances in water also changes temperature. Temperature change is only one possible sign, not the defining feature.

Becoming lighter describes a mass change observation, not the reason something is a chemical change. This doesn't explain the nature of the change.

The remaining option — new substances are formed that cannot easily be turned back into wood — correctly identifies irreversibility and new substance formation, which are the defining features of a chemical change.

A word equation follows the format: $\text{Reactant(s)}\to \text{Product(s)}$ The arrow points from reactants to products, showing the direction of the reaction.

Reactants are the starting materials — the substances that get used up. They are always placed on the left-hand side of the arrow.

The correct answer is: on the left-hand side of the arrow. The products go on the right-hand side.

The question asks about the position of reactants in a word equation. We need to recall the standard format for writing word equations.

The right-hand side of the arrow is where the products go — the new substances formed. Reactants are never written on the right.

Nothing is written above the arrow in a standard word equation. This is not part of the correct format.

The format of a word equation is always consistent — reactants on the left, products on the right. The position does not change depending on the reaction.

The only remaining option — on the left-hand side of the arrow — is correct. Reactants always go on the left.