To determine which pure substance can be classified as an element, let's examine each given substance:
1. [tex]\( H_2 \)[/tex] (Hydrogen gas):
- This consists of two hydrogen atoms bonded together.
- Hydrogen is a chemical element, denoted by the symbol H.
- Thus, [tex]\( H_2 \)[/tex] is an element.
2. [tex]\( NO_3 \)[/tex] (Nitrate):
- This consists of one nitrogen atom and three oxygen atoms.
- It is a compound, not an element, because it contains multiple types of atoms.
3. [tex]\( NaCl \)[/tex] (Sodium chloride, or table salt):
- This consists of one sodium atom and one chlorine atom bonded together.
- It is a compound because it also contains multiple types of atoms.
4. [tex]\( H_2SO_4 \)[/tex] (Sulfuric acid):
- This consists of two hydrogen atoms, one sulfur atom, and four oxygen atoms.
- It is also a compound, containing different types of atoms.
Based on this examination, the pure substance that can be classified as an element is [tex]\( H_2 \)[/tex].
Therefore, the correct answer is:
[tex]\[ 0 \][/tex] (corresponding to [tex]\( H_2 \)[/tex]).
