What is this material made of?

He is an expert in chemistry.

This lemon is sour.

Please add a little vinegar to this salad.

This lake is known as a dangerous body of water.

Please use the emergency stairs in case of a fire.

Due to the earthquake, the city declared a state of emergency.

The air in this area is greatly affected by pollutants emitted from factories.

The existence of dark matter has not yet been fully proven.

Antibiotics don't work for colds, so please don't take them unnecessarily.