Understanding Dilution of Concentrated Nitric Acid

Diluting concentrated acids, like nitric acid, is a common laboratory procedure. However, it requires careful attention to safety protocols because the process of dissolving concentrated acid in water is highly exothermic, meaning it releases a significant amount of heat.

Let's evaluate each statement given in the question regarding the dilution of concentrated nitric acid.

Analyzing the Statements on Nitric Acid Dilution

Statement 1: the concentration of \(\text{[H}_3\text{O}^+\text{]}\) ions/ volume increases

Concentration is generally defined as the amount of solute (in this case, nitric acid, which produces \(\text{H}_3\text{O}^+\) ions in water) per unit volume of the solution. When you dilute a solution, you add more solvent (water). This increases the total volume of the solution while the amount of solute remains constant.

Therefore, increasing the volume of the solution with the same amount of solute will result in a decrease in the concentration of the solute. The concentration of \(\text{H}_3\text{O}^+\) ions per unit volume actually decreases upon dilution.

Thus, statement 1 is incorrect.

Statement 2: water must be added slowly to concentrated acid

This describes an incorrect and potentially dangerous procedure for diluting concentrated acids. When water is added to concentrated acid, especially dense acids like sulfuric or nitric acid, the less dense water tends to sit on top of the acid.

The exothermic reaction occurs intensely at the interface between the two liquids. The heat generated can cause the water to boil rapidly and potentially splatter corrosive acid out of the container. This method is highly unsafe.

Thus, statement 2 is incorrect.

Statement 3: acid must be added slowly to water

This describes the correct and safe procedure for diluting concentrated acids. When concentrated acid is added slowly to a larger volume of water, the heat released during the dissolution process is absorbed and distributed by the larger thermal mass of the water.

Adding the acid slowly allows the heat to dissipate gradually, preventing the rapid temperature increase that could cause splashing or boiling. It is also recommended to stir the solution continuously while adding the acid and to add the acid down the side of the container to further control the process.

Thus, statement 3 is correct.

Based on the analysis, only statement 3 accurately describes a correct aspect of diluting concentrated nitric acid solution.

Revision Table: Key Points on Acid Dilution Safety

Additional Information on Handling Concentrated Acids

• Always wear appropriate Personal Protective Equipment (PPE), including safety goggles, acid-resistant gloves, and a lab coat.
• Perform dilution in a well-ventilated area or under a fume hood, especially with acids like nitric acid which can release irritating fumes.
• Use heat-resistant glassware (e.g., Pyrex or Kimax).
• Ensure there is sufficient water volume to absorb the heat generated.
• Have a plan in place for spills, including access to a neutralizer (like sodium bicarbonate) and safety showers/eyewash stations.