Acids are sour substances that turn litmus paper red, while bases are bitter substances that turn litmus paper blue. Examples of common acids include citric acid found in citrus fruits and hydrochloric acid, while bases include sodium hydroxide in soap and calcium hydroxide in limewater. Indicators like litmus paper, china rose, and turmeric are used to test whether a substance is acidic or basic by changing color in their presence. A neutralization reaction occurs when an acid and base combine, producing a salt and water along with the release of heat. Neutralization can be used to treat insect stings, indigestion, soil acidity, and neutralize acidic factory waste.