A change in which no new substances are formed and the original substance can usually be recovered is called a ___________.

Introduction / Context:
This question tests your understanding of the basic difference between physical changes and chemical changes in matter. Distinguishing these two types of changes is one of the first conceptual steps in chemistry. A physical change alters the form or appearance of a substance without creating a new substance, while a chemical change produces one or more new substances with different properties.

Given Data / Assumptions:

• The change described does not produce any new substances.
• The original substance can usually be recovered by simple physical methods.
• Options include physical change, chemical change, rusting, galvanisation and combustion.
• We assume standard textbook definitions for these terms.

Concept / Approach:
A physical change is a change in the physical state, shape, size or appearance of a substance without change in its chemical composition. Examples include melting of ice, boiling of water, dissolving salt in water and breaking a piece of chalk. A chemical change involves a change in composition and formation of new substances, such as rusting of iron, burning of wood or souring of milk. Given that no new substances are formed in the change described, the correct classification is physical change.

Step-by-Step Solution:
Step 1: Note that the question clearly states that no new substances are formed. Step 2: Recall that in a physical change, the molecules of the substance remain the same, and only physical properties like shape, size, or state may change. Step 3: Rusting, galvanisation and combustion are all examples of chemical processes in which new substances are formed with different properties from the originals. Step 4: Chemical change is defined as a change where one or more new substances with different chemical composition and properties are produced. Step 5: Since the question specifically excludes the formation of new substances, chemical change and specific chemical processes like rusting, galvanisation and combustion cannot be the correct answers. Step 6: The only option that fits the description is physical change.

Verification / Alternative check:
Consider examples such as melting ice into water or boiling water into steam. In all these cases, only the state changes; the substance remains H2O. If you freeze the water again, it turns back into ice, showing that the original substance can be recovered. These are physical changes. In contrast, when iron rusts, the reddish brown substance formed is iron oxide, a different compound. You cannot easily get back pure iron from rust by simple physical means, which means rusting is a chemical change. This comparison confirms that the description in the question matches physical change.

Why Other Options Are Wrong:
Chemical change always involves the formation of new substances, which contradicts the condition given. Rusting is a specific example of chemical change involving oxidation of iron. Galvanisation is a chemical process in which a protective layer of zinc is applied to iron to prevent rusting, again involving chemical changes at the surface. Combustion is the burning of substances in oxygen and produces new substances such as carbon dioxide and water. None of these match a change where no new substances are formed.

Common Pitfalls:
Students sometimes confuse physical and chemical changes when the change in appearance is dramatic, such as dissolving or melting. It is important to focus on whether the chemical composition changes. If the process can be reversed by simple physical methods such as cooling, evaporation or filtration, it is likely a physical change. When energy changes, colour changes or gas evolution accompany the formation of new substances, the change is chemical. Keeping this distinction clear helps you avoid errors in classification questions.

Final Answer:
A change in which no new substances are formed is called a physical change.