Acids and bases are two types of substances with different properties. Acids are substances that taste sour and turn blue litmus paper red. They can be strong or weak. Some common acids are lemon juice (contains citric acid), vinegar (contains acetic acid), curd (contains lactic acid), and tamarind (contains tartaric acid). Bases are substances that taste bitter and feel soapy to touch. They turn red litmus paper blue. Bases can also be strong or weak. Some common bases are soap, baking soda, and lime water.