Question

Write chemical reactions to show the amphoteric nature of water.

Short answer

Water can act as an acid with NH₃ and as a base with HCl, demonstrating its amphoteric nature.

Step-by-step solution

Understand Amphoteric Nature

Amphoteric substances can act as both acids and bases. Water is amphoteric because it can donate a proton (H+) in reactions where it acts as an acid, and it can accept a proton in reactions where it acts as a base.

Write the Reaction Where Water Acts as an Acid

When water acts as an acid, it donates an H+ ion to a base. One example of such a reaction is with ammonia (NH₃): \( \text{H}_2\text{O} + \text{NH}_3 \rightarrow \text{OH}^- + \text{NH}_4^+ \). Here, water donates a proton to ammonia, forming hydroxide ion (OH-) and ammonium ion (NH₄+).

Write the Reaction Where Water Acts as a Base

When water acts as a base, it accepts an H+ ion from an acid. An example of this is the reaction with hydrochloric acid (HCl): \( \text{H}_2\text{O} + \text{HCl} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^- \). Here, water accepts a proton to form the hydronium ion (H₃O+), and chloride ion (Cl-) is left in solution.

Key concepts

Proton Donation

In chemistry, when a molecule donates a proton, it means that it is releasing a hydrogen ion ( ext{H}^+). This process is fundamental to understanding how acids behave in chemical reactions. When water acts as an acid, it donates a proton to a base.
For instance, consider the reaction between water and ammonia ( ext{NH}_3):\[ \text{H}_2\text{O} + \text{NH}_3 \rightarrow \text{OH}^- + \text{NH}_4^+ \]In this reaction, water donates a proton to ammonia, thus forming a hydroxide ion ( ext{OH}^-) and an ammonium ion ( ext{NH}_4^+).
Understanding proton donation helps explain why water is considered amphoteric—it can behave like an acid, donating protons in certain reactions. This unique property is one reason why water is such an essential solvent in various chemical processes.

Proton Acceptance

Unlike proton donation, proton acceptance occurs when a molecule gains a hydrogen ion ( ext{H}^+). This is indicative of a base behavior. When water behaves as a base, it accepts a proton.
An example is the reaction with hydrochloric acid ( ext{HCl}):\[ \text{H}_2\text{O} + \text{HCl} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-\]In this case, water accepts a proton from ext{HCl}, forming the hydronium ion ( ext{H}_3 ext{O}^+) and the chloride ion ( ext{Cl}^-).
This acceptance of protons highlights water's ability to function as a base. Understanding this side of water’s versatility is critical for grasping how it participates in a myriad of chemical reactions, balancing between acidic and basic roles.

Hydronium Ion

The hydronium ion ( ext{H}_3 ext{O}^+) plays a central role in the chemistry of acids. It is formed when water accepts an extra proton.
This ion is crucial because it is essentially what you have when you dissolve an acid in water. For example, when ext{HCl} is dissolved:\[ \text{H}_2\text{O} + \text{HCl} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^- \]Understanding the presence of hydronium ions helps explain the acidic nature of a solution.
The concentration of ext{H}_3 ext{O}^+ in a solution determines its pH level. The greater the concentration, the more acidic the solution. Recognizing the formation and role of hydronium ions is critical for studying acid-base reactions.

Hydroxide Ion

The hydroxide ion ( ext{OH}^-) is the cornerstone of base chemistry. It forms when a molecule releases a hydroxide in the course of a chemical reaction. When water donates a proton, the hydroxide ion is produced. An example is its reaction with a base like ammonia:\[ \text{H}_2\text{O} + \text{NH}_3 \rightarrow \text{OH}^- + \text{NH}_4^+ \]Here, as water donates a proton ( ext{H}^+), it turns into a hydroxide ion.
The presence of hydroxide ions in a solution is indicative of its basicity. A higher concentration of ext{OH}^- turns the solution more basic, directly affecting the pH level and properties of the solution. By understanding the role of hydroxide ions, students can better grasp how different substances interact in solution, especially in alkaline reactions.