Question

What is the relationship between a hydroxide ion and a water molecule?

Short answer

A hydroxide ion is essentially a water molecule that has lost a proton (H⁺).

Step-by-step solution

Understanding Hydroxide Ion

A hydroxide ion is a negatively charged ion with the chemical formula OH⁻. It consists of one oxygen and one hydrogen atom and carries a negative charge because it has one more electron than the number of protons.

Understanding Water Molecule

A water molecule is a neutral molecule with the chemical formula H₂O. It is composed of two hydrogen atoms covalently bonded to one oxygen atom. Water molecules are polar, with partial positive charges on the hydrogen atoms and a partial negative charge on the oxygen atom.

Exploring Ionization of Water

Water molecules can undergo self-ionization, a process where two water molecules react to produce hydroxide ions (OH⁻) and hydronium ions (H₃O⁺). This is represented by the equation: 2H₂O ⇌ H₃O⁺ + OH⁻.

Relating Hydroxide Ion and Water Molecule

Since hydroxide ions are formed from water molecules during the ionization process, they directly share a connection. Specifically, a water molecule can lose a proton (H⁺) and form a hydroxide ion (OH⁻), demonstrating that the hydroxide ion is essentially a water molecule minus one proton.

Key concepts

Water Molecule

Water is a fascinating substance and its molecule has a simple, yet intriguing structure. It is composed of two hydrogen atoms bonded to one oxygen atom, and its chemical formula is written as H₂O.
The key aspect of a water molecule is its polarity. The oxygen atom is more electronegative, meaning it attracts electrons more strongly than the hydrogen atoms.
This creates a dipole moment, where the oxygen carries a slight negative charge, and the hydrogen atoms have slight positive charges.

• This polarity leads to strong hydrogen bonding between water molecules.
• It's why water has such high surface tension and boiling point compared to other small molecules.

Understanding the water molecule's structure helps explain its ability to participate in chemical reactions, such as the ionization process.

Ionization of Water

Ionization of water is a self-ionization process, meaning it occurs naturally among water molecules themselves.
In this process, pairs of water molecules react with each other to produce hydroxide ions (OH⁻) and hydronium ions (H₃O⁺).
The chemical equation for this reaction is: 2H₂O ⇌ H₃O⁺ + OH⁻
Although the concentration of these ions in pure water is very low, they play a significant role in determining water's pH.

• The presence of hydronium ions relates directly to the acidity of a solution.
• The hydroxide ions relate to the basicity.

This reaction is reversible, meaning hydronium and hydroxide ions can recombine to form water again.

Chemical Formula

The term 'chemical formula' refers to a written notation representing the composition of a compound.
For water, the chemical formula is H₂O, indicating it contains two hydrogen atoms and one oxygen atom.
On the other hand, the hydroxide ion is written as OH⁻, highlighting one oxygen atom bonded to one hydrogen atom, with an extra electron giving it a negative charge. Chemical formulas are important because:

• They show the proportions of substances involved in chemical reactions.
• They provide a quick way to understand the composition and behavior of molecules.

When examining ionization of water, recognizing these formulas helps visualize how water molecules transform into ions like OH⁻ and H₃O⁺.