Chapter 2, Problem 1STP

Interpretation Introduction

Interpretation: Name of an element needs to be identified.

Concept Introduction: Practically all of the matter in environment consists of mixtures of different substance. For example, soil; it has many forms of components, air; it has mixture of many gases, etc.

Answer to Problem 1STP

Correct answer: Option D. Helium is an element.

Explanation of Solution

Reason for correct option: A substance which is made entirely from one type of atom, called element. Helium is a chemical element because its atomic number is $2$ and also, it has $2$ protons in its nucleus. So, the correct answer is option D.

Conclusion

Reasons for incorrect options:

Option A: Air is a mixture because it consists of gases such as hydrogen, nitrogen, oxygen, etc. So, this is an incorrect option.

Option B: Water is a compound because it consists of hydrogen and oxygen. So, this is an incorrect option.

Option C: Salt is a common term which is used for $NaCl,KCl$ or $NaHC{O}_3$ . So, this is an incorrect option.