---
title: "Hypothetical types of biochemistry"
chunk: 4/7
source: "https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:51:22.917946+00:00"
instance: "kb-cron"
---

=== Ammonia ===
The ammonia molecule (NH3), like the water molecule, is abundant in the universe, being a compound of hydrogen (the simplest and most common element) with another very common element, nitrogen. The possible role of liquid ammonia as an alternative solvent for life is an idea dating back to 1954 at least, when J. B. S. Haldane raised the topic at a symposium about life's origin.
Many chemical reactions can occur in an ammonia solution, and liquid ammonia has chemical similarities with water. Ammonia dissolves most organic molecules at least as well as water does, and many elemental metals. Haldane indicated that various common water-related organic compounds have ammonia-related analogues; for instance, the ammonia-related amine group (−NH2) is analogous to the water-related hydroxyl group (−OH).
Ammonia, like water, can either accept or donate an H+ ion. When ammonia accepts an H+, it forms the ammonium cation (NH4+), analogous to hydronium (H3O+). When it donates an H+ ion, it forms the amide anion (NH2−), analogous to the hydroxide anion (OH−). Compared to water, however, ammonia is more inclined to accept an H+ ion, and less inclined to donate one; it is a stronger nucleophile. Ammonia added to water functions as an Arrhenius base: it increases the concentration of the anion hydroxide. Conversely, using a solvent system definition of acidity and basicity, water added to liquid ammonia functions as an acid, because it increases the concentration of the cation ammonium. The carbonyl group (C=O), which is much used in terrestrial biochemistry, would not be stable in ammonia solution, but the analogous imine group (C=NH) could be used instead.
However, ammonia has some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be a third, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. Gerald Feinberg and Robert Shapiro have questioned whether ammonia could hold prebiotic molecules together well enough to allow the emergence of a self-reproducing system. Ammonia is also flammable in oxygen and could not exist sustainably in an environment suitable for aerobic metabolism.

A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual in relation to life on Earth. Life on Earth usually exists between the melting point and boiling point of water, at a pressure designated as normal pressure, between 0 and 100 °C (273 and 373 K). When also held to normal pressure, ammonia's melting and boiling points are −78 °C (195 K) and −33 °C (240 K) respectively. Because chemical reactions generally proceed more slowly at lower temperatures, ammonia-based life existing in this set of conditions might metabolize more slowly and evolve more slowly than life on Earth. On the other hand, lower temperatures could also enable living systems to use chemical species that would be too unstable at Earth temperatures to be useful.
A set of conditions where ammonia is liquid at Earth-like temperatures would involve it being at a much higher pressure. For example, at 60 atm ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).
Ammonia and ammonia–water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn's largest moon Titan.