A substance that increases H and decreases pH when it dissociates in water is called an acid. Acids are substances that donate a hydrogen ion (H) when dissolved in water. When an acid dissociates, the H ion combines with a water molecule to form hydronium (H3O) ion, leading to an increase in H ions and a decrease in pH. Substances with a pH below 7 are considered acidic. Common examples of acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), and citric acid found in lemons or oranges. Acids play a crucial role in various chemical processes in everyday life and industrial applications.