| Criteria | Hydrogen Chloride (HCl) | Hydrochloric Acid (HCl in Water) | Remarks |
| Physical State | Colorless gas at room temperature | Aqueous solution (HCl dissolved in water) | HCl becomes hydrochloric acid only when dissolved in water. |
| Chemical Nature | Covalent compound | Strong acid (ionic character due to dissociation) | Dissociation in water changes the chemical behavior. |
| pH | Not applicable (gas) | Very low (typically pH 1–2 in concentrated form) | pH can only be measured in aqueous solutions. |
| Ionization | Does not ionize | Completely ionizes to H⁺ and Cl⁻ in water | Ionization in water gives it acidic properties. |
| Corrosiveness | Less corrosive as a gas but forms corrosive mist | Highly corrosive in solution | Hydrochloric acid is more hazardous in contact due to its aqueous nature. |
| Uses | Used in gas-phase reactions, semiconductor industry | Used in laboratories, digestion, pH control, and industry | Applications depend on physical state and reactivity. |
| Smell | Pungent, irritating odor | Same as gas but more intense in concentrated solutions | Smell due to release of HCl gas from solution. |
| Storage | In pressurized gas cylinders | In sealed containers, usually plastic or glass | Storage depends on form to prevent hazards. |
