D2.3.1 Solvation with water as the solvent Notes

Solvation with Water as the Solvent

1. When you add sugar to your tea, the sugar disappears, leaving a sweet solution.
2. This process, called solvation, is driven by the unique properties of water.

Water Is a Good Solvent Because of Its Polarity and Hydrogen Bonding

1. Polarity of Water

1. Water molecules have a bent shape, with oxygen at one end and hydrogen atoms at the other.
2. Oxygen is more electronegative than hydrogen, meaning it pulls shared electrons closer to itself.
3. This creates a partial negative charge ($\delta^-$) near the oxygen atom and a partial positive charge ($\delta^+$) near the hydrogen atoms.

2. Hydrogen Bonding

1. The partial charges in water molecules allow them to form hydrogen bonds:
2. The $\delta^+$ hydrogen of one water molecule is attracted to the $\delta^-$ oxygen of another.

Solvation Depends on The Nature of The Solute

1. These interactions depend on the nature of the solute: ionic or polar.

Solvation of Ionic Compounds

1. Ionic compounds, like NaCl, are made of positively charged cations (e.g., Na⁺) and negatively charged anions (e.g., Cl⁻).
2. When NaCl is added to water:
   1. Water molecules surround the ions.
   2. The partial negative charge of oxygen is attracted to Na⁺.
   3. The partial positive charge of hydrogen is attracted to Cl⁻.
3. These attractions overcome the ionic bonds in the crystal, causing it to dissociate into free ions.

Solvation of Polar Molecules

1. Polar molecules, like glucose, have regions with partial charges due to uneven electron distribution.
2. In glucose, hydroxyl groups (–OH) create these charges:
   1. The oxygen in –OH is partially negative.
   2. The hydrogen in –OH is partially positive.
3. Water molecules form hydrogen bonds with these groups, stabilizing the glucose in solution.

Why Do Some Substances Not Dissolve in Water?

1. Non-polar substances, like oils, lack charged or polar regions.
2. Without these, water molecules cannot form hydrogen bonds or strong interactions, so the substance remains undissolved.

Biological Importance of Solvation

1. Solvation is essential for life:
   1. Cytoplasm: A watery environment where ions and molecules are dissolved, enabling metabolic reactions.
   2. Transport: Blood plasma carries dissolved nutrients, gases, and waste products.
   3. Cellular Processes: Enzymes and substrates must be dissolved in water for reactions to occur.