Water gets ions primarily through a process called self-ionization. This process involves water molecules interacting with each other, which results in the formation of ions.
The Self-ionization of Water
The reference states that: "The reaction in which a water molecule donates one of its protons to a neighbouring water molecule, either in pure water or in an aqueous solution, is called the self-ionization of water."
This means that in pure water, some water molecules act as acids and donate a proton (H⁺), while other water molecules act as bases and accept the proton. This leads to the formation of two important ions:
- Hydronium ion (H₃O⁺): Formed when a water molecule accepts a proton.
- Hydroxide ion (OH⁻): Formed when a water molecule donates a proton.
This process can be represented by the following reversible chemical equation:
2H₂O ⇌ H₃O⁺ + OH⁻
Key Aspects of Water Ionization
Here are some important aspects to understand about how water gets ions:
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Extent of Ionization: Self-ionization happens to a very small extent in pure water. Only a tiny fraction of water molecules exists in ionized form at any given time. This is why pure water is considered to be a poor conductor of electricity.
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Dynamic Equilibrium: The self-ionization reaction is in a state of dynamic equilibrium. This means that water molecules are constantly ionizing and recombining back into water molecules at the same rate. Thus, the concentrations of hydronium and hydroxide ions remain very small and relatively constant in pure water.
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Temperature Dependence: The extent of self-ionization increases with temperature. Higher temperatures provide the energy to break hydrogen bonds and favor the separation of ions.
How External Substances Influence Ion Formation in Water
While self-ionization is the fundamental way water generates ions, the presence of other substances in water can greatly increase the concentration of ions.
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Acids and Bases: When acids or bases dissolve in water, they can dramatically increase the concentrations of hydronium (H₃O⁺) or hydroxide (OH⁻) ions, respectively.
- For example, hydrochloric acid (HCl) when dissolved in water, completely dissociates into H⁺ ions (which then form H₃O⁺) and Cl⁻ ions:
HCl + H₂O → H₃O⁺ + Cl⁻ - Similarly, sodium hydroxide (NaOH) dissociates in water to form Na⁺ ions and OH⁻ ions:
NaOH → Na⁺ + OH⁻
- For example, hydrochloric acid (HCl) when dissolved in water, completely dissociates into H⁺ ions (which then form H₃O⁺) and Cl⁻ ions:
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Salts: When ionic salts such as sodium chloride (NaCl) dissolve in water, they dissociate into their constituent ions (Na⁺ and Cl⁻), increasing the total concentration of ions.
- NaCl (s) → Na⁺ (aq) + Cl⁻ (aq)
Summary Table
| Mechanism | Description | Ions Produced |
|---|---|---|
| Self-ionization | Water molecules donate and accept protons | H₃O⁺ and OH⁻ |
| Acids in Water | Acids donate protons to water, increasing H₃O⁺ | H₃O⁺ and conjugate base |
| Bases in Water | Bases accept protons from water, increasing OH⁻ | OH⁻ and conjugate acid |
| Salts in Water | Salts dissociate into their constituent cations and anions. | Cations and Anions |
In conclusion, water gets ions through its inherent self-ionization process, which is enhanced significantly by the presence of acids, bases, and salts.
