Definition
A chemical substance that is the opposite of an acid, having a pH greater than 7. Common alkalis include sodium hydroxide, potassium hydroxide, and ammonia. Alkalis react with acids to form salts and water, and many are corrosive to skin, metals, and aircraft materials such as aluminum.
Plain English
A type of chemical that is the opposite of an acid. Many alkalis are caustic and can damage skin or metal if not handled carefully.
Context Anchor
Seen in aircraft maintenance, battery servicing, corrosion control, and chemical safety discussions.
Derivation
From the Arabic 'al-qili,' meaning 'the ashes' — referring to the ashes of certain plants from which early alkaline substances like potash were extracted. Knowing this hints at alkalis being naturally occurring caustic substances long before modern chemistry named them.
Why Pilots Care
Nickel-cadmium aircraft batteries contain an alkaline electrolyte (potassium hydroxide) that is highly corrosive. Spills can damage aircraft structure and require different neutralizing procedures than acid spills from lead-acid batteries. Using the wrong neutralizer can make the damage worse.
Grounding Statement
If acid and a suitable alkali are brought together in the right way, they can reduce each other’s chemical effect.
Intuition Check
Do not read alkali as just “a strong cleaner.” An alkali is specifically a base, the chemical opposite of an acid, and strong alkalis can be hazardous too.
Example Sentence 1
The technician neutralized the alkali spill from the nickel-cadmium battery with a boric acid solution before cleaning the area.
Example Sentence 2
The maintenance manual warns against using unapproved alkali cleaners on aluminum skin without thorough rinsing.