Acidity and alkalinity refer to the levels of hydrogen ions in a solution. A solution is considered acidic if it has a high concentration of hydrogen ions, typically with a pH less than 7. Common examples of acidic substances include lemon juice and vinegar. These substances can taste sour and can react with certain metals. On the other hand, a solution is alkaline, or basic, if it has a low concentration of hydrogen ions, usually with a pH greater than 7. Examples of alkaline substances include baking soda and soapy water. Alkaline solutions often feel slippery and can neutralize acids.