Why must one add water first and then add the acid rather than add the acid first and then water?

Water added to some acids creates so much heat that the solution actually boils and spatters. You don't want the spattering acid to erode your skin, your clothes, or the surroundings. Therefore, you pour the acid into the water. This way, the solution is the more dilute and working toward a more concentrated solution. The other way it is the most concentrated and working toward more dilute. Adding it the way it should be added still produces a great amount of heat but its more gradual and the solution never actually boils (nor spatters). By the way, this is the most important for concentrated sulfuric acid. Most other acids don't have the problem to that extent; but adding acid to water is a good rule to remember.

Alright thanks!

I meant, I was wondering that too, so thanks even though it isn't my question.

When working with acids, it is important to add water first and then add the acid, rather than the other way around. This is due to a chemical reaction called an exothermic reaction, which releases heat. Here's why this sequence is recommended:

1. Safety Precautions: Adding water before the acid helps minimize the potential hazards associated with the reaction. If you add acid to water, the reaction will occur slowly, allowing for better heat dissipation and less risk of splashing or boiling over.

2. Dilution: Adding water first allows for controlled dilution. When water is added to the acid, it disperses and becomes well-mixed, resulting in a more uniform solution. This gradual dilution helps manage the reaction's intensity and reduces the likelihood of a sudden release of heat or production of harmful vapors.

Now, let's discuss how to safely add water and acid:

1. Prepare the Container: Choose a suitable container that can resist the corrosive properties of the acid. Glass or high-density polyethylene (HDPE) containers are commonly used, but be sure to check the specific requirements for the type of acid you are handling.

2. Personal Protective Equipment: Prioritize safety by wearing appropriate personal protective equipment, including safety goggles, gloves, a lab coat, and possibly a face shield.

3. Slowly Add Water: Begin by filling the container with the desired amount of water. You can use a graduated cylinder or a pipette to measure the water accurately. Slowly pour the water into the container while stirring gently, if necessary.

4. Add Acid: Once the water is in the container, carefully add the acid. Aim for a slow, controlled pour to reduce the chance of splashing. Stir the mixture gently, if needed, to ensure thorough mixing.

5. Observe Safety Guidelines: Throughout the process, be cautious and aware of any changes. If you notice a sudden release of heat, an increase in temperature, or any unusual reaction, stop adding the acid, and seek appropriate guidance or assistance.

By following proper safety procedures and adding water first, you can help mitigate potential hazards while working with acids. Always consult specific guidelines or instructions provided with the particular acid you are handling, as different acids may have additional precautions or requirements.