Reaction arrows (→, ⇌, ↑, ↓)
In a chemical equation the arrow → leads from the reactants on the left to the products on the right. The double harpoon ⇌ marks a reversible reaction at equilibrium. A small ↑ beside a product means a gas is given off, and ↓ means a solid precipitate forms.
Easily confused with
How to read it
OpenStax’s Chemistry 2e explains the convention: reactant formulas go on the left of the reaction arrow and product formulas on the right, so the equation reads as the reaction going from left to right. A reversible reaction gets a special double arrow, as in N₂O₄(g) ⇌ 2NO₂(g). The IUPAC Green Book gives each connecting sign its own meaning: = for a stoichiometric equation, → for a net forward reaction, ⇄ (two full arrows) for a reaction going in both directions, and ⇌ for equilibrium. For the vertical arrows, Abirami Lakshminarayanan’s article “Arrows in Chemistry” (Resonance, 2010) says ↑ marks the evolution of a gas and ↓ the formation of a precipitate, each written only on the product side, beside the product. A 2024 review of chemistry’s arrows in Química Nova, the Brazilian Chemical Society’s journal, gives the same two uses, with oxygen gas marked ↑ and a precipitate ↓.
Also searched as: →, ⇌, ↑, ↓, chemical equation arrows, double arrow in chemistry, down arrow in chemistry, equilibrium arrow, reaction arrows symbol, reversible reaction symbol, up arrow in chemistry.
The double-headed arrow ↔ is not a reaction arrow
The Green Book says the two-sided arrow ↔ should not be used for reactions, to avoid confusion with resonance structures: it links two drawings of the same molecule. Lakshminarayanan credits the resonance arrow to the German chemist Fritz Arndt.
Letters in brackets
States are also written as letters in brackets after a formula, upright and with no full stop: (g) for a gas or vapour, (l) for a liquid, (s) for a solid and (aq) for a solution in water, as in HCl(g), hydrogen chloride as a gas (Green Book).
Typing them
Unicode names ⇌ (U+21CC) RIGHTWARDS HARPOON OVER LEFTWARDS HARPOON and ⇄ (U+21C4) RIGHTWARDS ARROW OVER LEFTWARDS ARROW.
Read next
- Next in Physics and chemistry symbolsHalf-life (t½)t½ (also written t1/2 or T½) is the half-life: the time it takes for half of a radioactive substance to decay, or in chemistry for half of a reactant to be used up.
- The same shape in Blueprint symbolsStairsA staircase, seen from above. The arrow shows the direction of travel from this floor, labelled with the direction and the number of risers.
More physics and chemistry symbols
The same shape elsewhere
Browse symbols drawn with arrows or lines.
Sources
- Quantities, Units and Symbols in Physical Chemistry (the Green Book), IUPAC Green Book, 3rd edition, 2nd printing (2008), International Union of Pure and Applied Chemistry (IUPAC), Sec. 2.10.1 (iv) Equations for chemical reactions; (vi) States of aggregation
- Chemistry 2e, 13.1 Chemical Equilibria, OpenStax (Rice University), Chemical Equilibria
- Arrows in Chemistry, Abirami Lakshminarayanan, Resonance 15(1), January 2010, pp. 51–63, Indian Academy of Sciences, Sec. 1.2; 2.2 Equilibrium Arrows; 2.3 The Upward Arrow; 2.4 The Downward Arrow; 3.3 The Resonance Arrow
- Arrows: Range 2190–21FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Arrows, 21C4 and 21CC
- Uso de setas no contexto da aprendizagem em química e a importância do seu simbolismo, Tiago Gomes dos Santos, Valdiney Vieira da Silva, André Gustavo Ribeiro Mendonça, Iara Terra de Oliveira and Wander Gustavo Botero, Química Nova 47(4), 2024, doi:10.21577/0100-4042.20230128, Sociedade Brasileira de Química (Química Nova, via SciELO Brasil), Section on vertical arrows; Figure 10 (a), (b)
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 10, 2026. How we research
Added . Last updated .