Chemical

A chemical was a type of substance that could do several things. Chemicals could be represented by chemical formulas, such as NH4NO3. (TV: The Keys of Marinus)

Chemicals had several uses, including defoliation (TV: In the Forest of the Night). Humans sometimes stored chemicals in a chemical store. (TV: Warriors of the Deep)

Chemicals in biology

Life was a chemical process. (TV: The Mutants) The Fifth Doctor noted that a chemical reaction in a primeval swamp could create life on a planet, and theorised that the universe could have been created in a similar way. (TV: Terminus)

Mia Bennett proved that Earth plant life could grow on Martian soil with the help of chemicals. (TV: The Waters of Mars)

Human chemicals

The human body usually had a specific chemical composition, which could be corrupted. (TV: The Crimson Horror)

The 456 found that human children produced chemicals which felt "good". (TV: Children of Earth: Day Five)

The human brain contained chemical reactions, which could be changed by the Morpheus process. (TV: Sleep No More) A particular chemical in the brain promoted sleep, (TV: The Mark of the Rani) and as a result the process of rest provided chemical benefits. (TV: Sleep No More) Without these benefits, humans would become violent and aggressive. (TV: The Mark of the Rani)

The Twelfth Doctor once described Clara Oswald in an emotional state as "a mess of chemicals". (TV: Dark Water) Similarly, the Tenth Doctor noted that in an emotional situation such as walking down the aisle, chemicals such as adrenaline, acetylcholine, and endorphins were present in the human body. (TV: The Runaway Bride)

Dangers posed by chemicals

A sign warning of "Dangerous chemicals". (TV: School Reunion)

Chemicals could be dangerous to health or to the environment.

For instance, a chemical found in Raw Spectrox, described by Salateen as "similar to mustard nitrogen", was deadly to humans. (TV: The Caves of Androzani)

By 2007, a sign in a Deffry Vale High School classroom warned of "dangerous chemicals" within a cupboard. (TV: School Reunion)

Specific chemicals

Some types of chemicals included:

• Water (TV: The Power of the Daleks)
• Parrinium (TV: Death to the Daleks)
• Pentocyleinicmethyldrane (TV: The Sun Makers)
• Hexachromite (TV: Warriors of the Deep)
• Halides (TV: Frontios)
• Silica (TV: Frontios)
• Mustakozene-80 (TV: Timelash)
• Adrenaline (TV: The Runaway Bride)
• Acetylcholine (TV: The Runaway Bride)
• Endorphins (TV: The Runaway Bride)
• Mood drugs (TV: Gridlock)
• Ammonium nitrate (TV: Nikola Tesla's Night of Terror)
• Zinc (TV: Nikola Tesla's Night of Terror)