In reversible reactions, the color change can occur depending on the direction of the reaction and
the conditions affecting it. These color changes are often due to the equilibrium position of the
reaction shifting between reactants and products when external conditions like temperature,
pressure, or concentration are changed. Here are a few common examples of reversible reactions
where color changes occur:
1. The Haber Process (Ammonia Synthesis)
● Reaction: N2(g)+3H2(g)⇌2NH3(g)N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)
● Color Change: In this process, nitrogen and hydrogen gases react to form ammonia gas. At
room temperature and normal pressure, ammonia is colorless. However, under certain
conditions, such as changing temperature or pressure, the equilibrium can shift, and the
mixture might appear slightly different due to the varying concentration of the ammonia.
● Example of Color Change:
○ At low temperature (cooling), more ammonia may form, and the gas mixture could
become slightly cloudy or change the way light passes through it.
2. Copper(II) Sulfate and Water (Hydration/Dehydration)
● Reaction: CuSO4⋅5H2O(s)⇌CuSO4(s)+5H2O(g)CuSO_4 \cdot 5H_2O(s) \rightleftharpoons
CuSO_4(s) + 5H_2O(g)
● Color Change: Copper(II) sulfate pentahydrate is blue, and when it is heated and loses its
water molecules, it turns from blue to white. This is a reversible reaction, as adding water
back to the white anhydrous copper(II) sulfate will restore its blue color.
● Example of Color Change:
○ Blue to White: On heating, it loses water and becomes white (anhydrous copper
sulfate).
○ White to Blue: Upon adding water back, it regains its blue color (hydrates again).
3. Iron(III) Thiocyanate Complex (FeSCN²⁺)
● Reaction: Fe3+(aq)+SCN−(aq)⇌FeSCN2+(aq)Fe^{3+}(aq) + SCN^-(aq) \rightleftharpoons
FeSCN^{2+}(aq)
● Color Change: In this reaction, the iron(III) ion (Fe³⁺) reacts with thiocyanate ions (SCN⁻) to
form the red FeSCN²⁺ complex. The color of the solution will change depending on the
concentration of reactants and products.
● Example of Color Change:
○ When the concentration of Fe³⁺ or SCN⁻ increases, the solution becomes more
intensely red.
○ When the equilibrium is shifted in the reverse direction (by adding excess of one
reactant or altering temperature), the red color fades or disappears as FeSCN²⁺
dissociates.
4. Chromium(III) Complex (Chromium Hydroxide)
● Reaction: Cr(OH)3(s)+3H+(aq)⇌[Cr(H2O)6]3+(aq)Cr(OH)_3(s) + 3H^+(aq) \rightleftharpoons
[Cr(H_2O)_6]^{3+}(aq)
● Color Change: Chromium(III) hydroxide is green, but when it reacts with an acid (proton), it
dissolves to form a red solution of the chromium ion complex.
● Example of Color Change:
○ Green to Red: Adding an acid to chromium hydroxide causes the color to change
from green to red due to the formation of the chromium ion complex.
5. Acid-Base Indicators (Litmus)
● Reaction: HIn⇌H++In−HIn \rightleftharpoons H^+ + In^-
● Color Change: Litmus is an indicator that changes color depending on the pH of the solution.
In an acidic solution, litmus turns red, while in a basic solution, it turns blue. This change
occurs because of the shift in equilibrium between the protonated (HIn) and deprotonated
(In⁻) forms.
● Example of Color Change:
○ Red in Acid: The solution is acidic, and the indicator shows red.
○ Blue in Base: The solution is basic, and the indicator turns blue.
6. Cobalt(II) Chloride Complex (Cobalt(II) Chloride Hydrate)
● Reaction: CoCl2⋅6H2O(s)⇌CoCl2(aq)+6H2OCoCl_2 \cdot 6H_2O(s) \rightleftharpoons
CoCl_2(aq) + 6H_2O
● Color Change: Cobalt chloride hydrate is blue, and when it loses water (dehydrates), it
becomes pink.
● Example of Color Change:
○ Blue to Pink: On heating, cobalt chloride loses water and changes from blue
(hydrated) to pink (anhydrous).
○ Pink to Blue: Adding water back restores the blue color.
7. Dichromate to Chromate (Chromium Compounds)
● Reaction: Cr2O72−(aq)+2H2O(l)⇌2CrO42−(aq)+2H+(aq)Cr_2O_7^{2-}(aq) + 2H_2O(l)
\rightleftharpoons 2CrO_4^{2-}(aq) + 2H^+(aq)
● Color Change: The color of a solution containing dichromate ions (Cr₂O₇²⁻) is orange, while
chromate ions (CrO₄²⁻) are yellow. This color change happens when the pH of the solution is
altered (adding an acid or base).
● Example of Color Change:
○ Orange to Yellow: In a basic solution, dichromate changes to chromate, and the
color shifts from orange to yellow.
○ Yellow to Orange: In an acidic solution, chromate converts to dichromate, and the
color shifts from yellow to orange.
