Final answer:
The subject of this question is Chemistry, and it involves checking for impurities in a foreign substance and evaluating its hardness level.
Step-by-step explanation:
In this question, the subject is Chemistry.
The question mentions the process of checking for impurities in a foreign substance and the hardness level. This process is commonly done in Chemistry to ensure the purity and quality of substances.
By examining the properties of substances, such as their hardness level or the presence of impurities, scientists can determine their suitability for various applications.