Alkaline describes substances that are the chemical opposite of acids. On the pH scale, which measures how acidic or basic something is, alkaline substances score above 7. Pure water sits right at 7, perfectly neutral. Baking soda, soap, and bleach are all alkaline.

You can often identify alkaline substances by how they feel: slippery and a bit soapy to the touch. That's why washing your hands with soap (which is alkaline) makes them feel different from rinsing them with plain water. Alkaline substances can neutralize acids, which is why you might take an antacid (an alkaline medicine) for an upset stomach caused by too much stomach acid.

In everyday life, you'll hear about alkaline batteries (the common kind you put in flashlights and remote controls) and alkaline soil (which affects what plants can grow well). Some people even talk about “alkaline water,” though regular water works just fine for staying hydrated.

Scientists often use the word basic to mean the same thing as alkaline, so if you hear someone say a substance is basic, they mean it's on the alkaline side of the pH scale.