Acids are most dangerous when concentrated. When an acid is concentrated, it has a higher concentration of hydrogen ions (H+ ions). These hydrogen ions are what give acids their corrosive properties. The more concentrated the acid, the more hydrogen ions it has and the more corrosive it is.

When an acid is diluted, the concentration of hydrogen ions is lower. This means that the acid is less corrosive and less dangerous. However, even diluted acids can still be dangerous, so it is important to always take precautions when handling them.