Acid is a type of chemical substance that tastes sour and can react strongly with other materials. Lemon juice, vinegar, and stomach acid are all acids you encounter in everyday life.

Scientists identify acids by how they behave: they turn blue litmus paper red, they can dissolve some metals, and they release hydrogen ions when dissolved in water. Strong acids like sulfuric acid or hydrochloric acid are dangerous and can burn skin or eat through materials, which is why chemistry teachers are so careful with them in labs. Weak acids like citric acid in oranges or acetic acid in vinegar are safe to eat and give foods their tangy flavor.

Your stomach produces hydrochloric acid (stomach acid) to help break down food. When too much acid builds up, you might feel heartburn. Rain becomes acid rain when pollution in the air mixes with water droplets, creating acids that can damage forests and buildings over time.

The opposite of an acid is a base or alkali. When acids and bases mix, they neutralize each other. Scientists measure how acidic something is using the pH scale, where low numbers mean more acidic and high numbers mean more basic.