Why does chlorine act as a bleaching agent, and what is the role of moisture?

Chlorine's bleaching action is primarily due to its ability to form hypochlorous acid (HOCl) in the presence of moisture. When chlorine dissolves in water, it undergoes hydrolysis to produce HCl and HOCl. Hypochlorous acid is unstable and readily decomposes to yield nascent oxygen ($[O]$). This nascent oxygen is a powerful oxidizing agent that reacts with colored organic substances, oxidizing them into colorless compounds. Therefore, moisture is absolutely essential for chlorine to exhibit its bleaching properties; dry chlorine gas does not bleach.

Explain the difference in products when chlorine reacts with cold, dilute NaOH versus hot, concentrated NaOH.

The reaction of chlorine with sodium hydroxide is a disproportionation reaction, where chlorine is simultaneously oxidized and reduced. With cold, dilute NaOH, chlorine forms sodium chloride (NaCl) and sodium hypochlorite (NaOCl). Here, chlorine is in the -1 and +1 oxidation states, respectively. However, with hot, concentrated NaOH, chlorine disproportionates further, forming sodium chloride (NaCl) and sodium chlorate ($NaClO_3$). In this case, chlorine is in the -1 and +5 oxidation states. This difference highlights the temperature and concentration dependence of redox reactions involving halogens.

What is aqua regia, and why is it capable of dissolving noble metals like gold?

Aqua regia, meaning 'royal water,' is a highly corrosive mixture of concentrated nitric acid ($HNO_3$) and concentrated hydrochloric acid ($HCl$) in a 1:3 molar ratio. Neither acid alone can dissolve noble metals like gold or platinum. However, in aqua regia, nitric acid acts as a powerful oxidizing agent, oxidizing the metal, while hydrochloric acid provides chloride ions that react with the metal ions to form stable soluble complex anions (e.g., tetrachloroaurate(III) ion, $[AuCl_4]^-$). This removal of metal ions from solution shifts the equilibrium, allowing more metal to dissolve. The nascent chlorine and nitrosyl chloride formed also contribute to its strong oxidizing power.

Is HCl gas acidic? How does it differ from hydrochloric acid?

Anhydrous HCl gas is not acidic in nature. It is a covalent molecule and does not dissociate to produce $H^+$ ions in the absence of water. For example, dry HCl gas does not turn blue litmus paper red. However, when HCl gas dissolves in water, it ionizes completely to form hydronium ions ($H_3O^+$) and chloride ions ($Cl^-$), making the aqueous solution (hydrochloric acid) strongly acidic. The presence of water is crucial for HCl to exhibit its acidic properties, as water acts as a proton acceptor.

Describe the industrial preparation of chlorine using the Deacon's process.

The Deacon's process is an industrial method for producing chlorine by the catalytic oxidation of hydrogen chloride gas. In this process, a mixture of hydrogen chloride gas and atmospheric oxygen is passed over a catalyst, typically cuprous chloride ($CuCl_2$), at a high temperature of about 723 K. The reaction is $4HCl(g) + O_2(g) \xrightarrow{CuCl_2, 723 K} 2Cl_2(g) + 2H_2O(g)$. This method is significant because it utilizes HCl, which is often a byproduct of other industrial processes, making it an economically and environmentally viable route for chlorine production.

Why is chlorine used for water purification?

Chlorine is widely used for water purification due to its potent disinfectant properties. When chlorine is added to water, it reacts to form hypochlorous acid (HOCl) and hydrochloric acid (HCl). Hypochlorous acid is a powerful oxidizing agent that can penetrate the cell walls of bacteria, viruses, and other microorganisms, disrupting their cellular processes and effectively killing them. This action makes water safe for consumption by eliminating harmful pathogens. The residual chlorine also helps prevent recontamination in the distribution system.

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Chlorine and Hydrogen Chloride

Chemistry

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Version 1Updated 22 Mar 2026

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Chlorine (Cl), a member of Group 17 (halogens) in the periodic table, is a highly reactive non-metallic element existing as a diatomic molecule ( $Cl_2$ ) under standard conditions. It is characterized by its greenish-yellow color and pungent, suffocating odor. Its high electronegativity and electron affinity make it a potent oxidizing agent. Hydrogen chloride (HCl) is a colorless, pungent, and corr…

Quick Summary

Chlorine ( $Cl_2$ ) is a greenish-yellow, pungent gas, a highly reactive halogen from Group 17. Its electronic configuration ( $[Ne]3s^23p^5$ ) makes it a strong oxidizing agent, readily gaining an electron.

Industrially, it's produced by electrolysis of brine (chlor-alkali process) or Deacon's process (catalytic oxidation of HCl). In the lab, it's made from $MnO_2$ and HCl. Chlorine reacts with metals, non-metals, water (forming HCl and HOCl), and alkalis (disproportionation).

Its bleaching action is due to nascent oxygen from HOCl and requires moisture. It's vital for water disinfection, bleaching, and chemical synthesis. Hydrogen chloride (HCl) is a colorless, pungent gas, a covalent molecule.

When dissolved in water, it forms hydrochloric acid, a strong monoprotic acid. Lab preparation involves NaCl and concentrated $H_2SO_4$ . Industrially, it's from direct synthesis ( $H_2 + Cl_2$ ) or as a byproduct.

HCl gas is not acidic; its acidity manifests only in aqueous solution. It reacts with metals, bases, carbonates, and forms ammonium chloride with ammonia. Aqua regia (1:3 $HNO_3:HCl$ ) can dissolve noble metals.

HCl is crucial for steel pickling, chemical manufacturing, and as a lab reagent.

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Key Concepts

Bleaching Action of Chlorine

Chlorine's ability to bleach colored substances is a key property. This action is not direct but mediated by…

Disproportionation of Chlorine with Alkalis

Chlorine exhibits disproportionation reactions when it reacts with alkalis, meaning it is simultaneously…

Acidic Nature of HCl Gas vs. Hydrochloric Acid

It's a common misconception that HCl gas is an acid. Anhydrous hydrogen chloride gas is a covalent molecule…

Chlorine ($Cl_2$) — Greenish-yellow gas, pungent. Strong oxidizing agent.

Preparation — Lab:

MnO_2 + 4HCl \rightarrow MnCl_2 + Cl_2 + 2H_2O

. Ind: Deacon's (

4HCl + O_2 \xrightarrow{CuCl_2} 2Cl_2 + 2H_2O

), Electrolysis of brine (

2NaCl + 2H_2O \rightarrow 2NaOH + Cl_2 + H_2

Reactions

- With $H_2O$ : $Cl_2 + H_2O \rightleftharpoons HCl + HOCl$ . $HOCl \rightarrow HCl + [O]$ (bleaching). - With cold, dilute $NaOH$ : $Cl_2 + 2NaOH \rightarrow NaCl + NaOCl + H_2O$ . - With hot, conc. $NaOH$ : $3Cl_2 + 6NaOH \rightarrow 5NaCl + NaClO_3 + 3H_2O$ .

Bleaching — By oxidation (nascent oxygen), permanent, requires moisture.

Hydrogen Chloride (HCl) — Colorless gas, pungent. Highly soluble in water.

Preparation — Lab:

NaCl + H_2SO_4(conc) \xrightarrow{<200^circ C} NaHSO_4 + HCl

. Ind:

H_2 + Cl_2 \rightarrow 2HCl

Properties — Anhydrous gas is not acidic. Aqueous solution (hydrochloric acid) is a strong acid.

Reactions

- With metals: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$ . - With bases: $NaOH + HCl \rightarrow NaCl + H_2O$ . - With carbonates: $Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$ . - With $NH_3$ : $NH_3 + HCl \rightarrow NH_4Cl$ (white fumes).

Aqua Regia — 1:3

HNO_3:HCl

, dissolves noble metals.

Chlorine Loves Oxidation, Reacts Intensely, Needs Everything Moist for Bleaching. Hydrogen Chloride Loves Acid, Soluble In Water, Dry Gas Not Acidic.

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