Final answer:
To lower the pH in a swimming pool, muriatic acid (hydrochloric acid, HCl) can be added. This substance is diluted and then used to decrease the pH by increasing the concentration of hydronium ions in the water.
Step-by-step explanation:
To lower the pH in a swimming pool, muriatic acid may be added. Muriatic acid, also known as hydrochloric acid (HCl), is a common choice for adjusting pool pH levels. When the pool's pH is too high, indicating the water is too basic, the addition of muriatic acid increases the concentration of hydronium ions, thereby decreasing the pH. It is important to dilute the acid before adding it to the pool to ensure safety and to achieve the desired pH balance. On the contrary, if you need to increase the pH, substances like soda ash (sodium carbonate) are used because they can raise pH by decreasing the concentration of hydrogen ions in the water.