1. Sour taste: Acids have a sour taste. For example, lemon juice, vinegar, and hydrochloric acid all have a sour taste.

2. pH value: The pH value of a solution indicates its acidity or alkalinity. Acids have a pH value less than 7. The lower the pH value, the stronger the acid.

3. Reaction with metals: Acids react with metals to produce hydrogen gas. For example, when hydrochloric acid reacts with iron, it produces hydrogen gas and iron chloride.

4. Reaction with bases: Acids react with bases to produce salt and water. For example, when hydrochloric acid reacts with sodium hydroxide, it produces sodium chloride and water.

5. Litmus test: Litmus is a natural dye that changes color in the presence of acids and bases. Red litmus paper turns blue in the presence of an acid.

Alkalis:

1. Bitter taste: Alkalis have a bitter taste. For example, baking soda and lye both have a bitter taste.

2. pH value: Alkalis have a pH value greater than 7. The higher the pH value, the stronger the alkali.

3. Reaction with metals: Alkalis do not react with metals to produce hydrogen gas.

4. Reaction with acids: Alkalis react with acids to produce salt and water. For example, when sodium hydroxide reacts with hydrochloric acid, it produces sodium chloride and water.

5. Litmus test: Litmus is a natural dye that changes color in the presence of acids and bases. Blue litmus paper turns red in the presence of an alkali.