When mixing acid and water, always:

When mixing acid and water, the correct practice is to pour acid into water. This method is important for safety reasons. When concentrated acid is added to water, it dissipates its heat into a larger volume of liquid, which helps to prevent splattering or violent reactions. This is critical because adding water to a concentrated acid can lead to an exothermic reaction that generates heat, potentially causing the acid to splash back or produce dangerous fumes.

In contrast, if water is poured into acid, the water can quickly turn into steam, and the rapid generation of steam can cause the mixture to foam or erupt violently. This is why storing acids in acid-resistant containers is also necessary but does not replace the correct procedure for mixing.

Mixing equal volumes of acid and water may seem practical but does not address the safety issues associated with adding water to acid. Therefore, the practice of always pouring acid into water ensures that the mixing is done safely and minimizes the risk of accidents.