Acids donate hydrogen ions to lower pH, while bases provide hydroxide ions to raise pH in a solution.
Acids are substances that provide hydrogen ions (H+) and lower pH, whereas bases provide hydroxide ions (OH-) and raise pH. The stronger the acid, the more readily it donates H+. For example, hydrochloric acid and lemon juice are very acidic and readily give up H+ when added to water. Conversely, bases are those substances that readily donate OH-. The OH- ions combine with H+ to produce water, which raises a substance's pH. Sodium hydroxide and many household cleaners are very alkaline and give up OH- rapidly when placed in water, thereby raising the pH.
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