Which among the following is a pure substance?

Options

A. dilute sulphuric acid
B. concentrated sulphuric acid
C. aqueous NaCl
D. molten NaCl

Explanation

To determine which among the given options is a pure substance, we need to first understand what a pure substance is. A pure substance is defined as a material that has a consistent composition throughout and contains only one type of particle, whether it be atoms or molecules.

Let's analyze each option step by step:

1. **Option 1: Dilute Sulfuric Acid**
- Dilute sulfuric acid is a solution that contains sulfuric acid mixed with a significant amount of water. Since it contains more than one type of particle (water and sulfuric acid), it cannot be classified as a pure substance.

2. **Option 2: Concentrated Sulfuric Acid**
- Concentrated sulfuric acid is also a solution, but it has a higher concentration of sulfuric acid compared to water. However, it still contains some water, which means it is not composed of only one type of particle. Therefore, it is not a pure substance.

3. **Option 3: Aqueous NaCl**
- Aqueous NaCl refers to sodium chloride (table salt) dissolved in water. Since it contains both water and sodium chloride, it is a mixture and not a pure substance.

4. **Option 4: Molten NaCl**
- Molten NaCl is sodium chloride in its liquid form, achieved by heating it until it melts. In this state, NaCl consists solely of sodium and chloride ions, with no other substances present. Therefore, molten NaCl is a pure substance.

Based on the analysis above, the correct answer is **Option 4: Molten NaCl**.