A Level Chemistry OCR Practice Exam

What does the term 'anhydrous' refer to?

1. A substance that contains a low level of moisture

2. A compound with chemical water included within its structure

3. A substance that contains no water molecules

4. A hydrated compound at a high temperature

The correct answer is: A substance that contains no water molecules

The term 'anhydrous' specifically refers to a substance that contains no water molecules. In chemistry, this is used to describe compounds that have been dehydrated, meaning all associated water has been removed, either through physical means or during a chemical process. In various chemical contexts, anhydrous substances are important because the absence of moisture can affect the reactivity, stability, or properties of the material. For instance, anhydrous salts can behave differently than their hydrated counterparts in reactions, particularly in terms of solubility or ionization. The other options do not accurately represent the definition of anhydrous. A substance that contains low levels of moisture would still be considered hydrated, while a compound with chemical water included in its structure is explicitly hydrated. Lastly, the mention of a hydrated compound at a high temperature does not relate to the concept of being anhydrous, as such a substance would still contain water, albeit possibly in different phases. Thus, the definition clearly aligns with the understanding that an anhydrous substance is one that lacks any water molecules.