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. ( source : https://courses.lumenlearning.com/wmopen-nmbiology1/chapter/the-ph-scale/ )
