A Level Chemistry OCR Practice Exam

What characterizes an anhydrous substance?

1. It contains water molecules

2. It contains at least one acid group

3. It contains no water molecules

4. It contains multiple halogens

The correct answer is: It contains no water molecules

An anhydrous substance is characterized by the absence of water molecules in its chemical structure. This definition is crucial in various contexts, such as when discussing salts, hydrates, and chemical reactions. For instance, many metal salts are found in hydrated forms where water molecules are incorporated into their crystalline structure. When these salts are heated or treated to remove the water, they transform into anhydrous forms, which can significantly change their properties, such as solubility and reactivity. Understanding that anhydrous specifically means "without water" helps in distinguishing it from other types of substances. For example, a substance that contains water molecules or is associated with an acid group does not fit the definition of an anhydrous substance. Similarly, the presence of multiple halogens does not relate to the concept of being anhydrous. This clear distinction is important in practical applications, such as preparing dry reagents for chemical reactions, where the presence of water could lead to inaccurate results or undesired side reactions.