Q.Differentiate between Soap and Synthetic Detergents.

Like Dissolves Like: Why It Holds

This is not a formula-driven concept — it's a qualitative principle rooted in thermodynamics and intermolecular forces. Let's build the reasoning step by step.

1. The Core Idea

Like Dissolves Like means:

A solute dissolves best in a solvent with similar intermolecular forces (polarity, hydrogen bonding, etc.).

• Polar solutes dissolve in polar solvents (e.g., salt in water).
• Nonpolar solutes dissolve in nonpolar solvents (e.g., oil in hexane).

2. Why? The Thermodynamic Reason

Dissolution is governed by Gibbs free energy change:

For dissolution to be spontaneous, $\Delta G_{\text{soln}} < 0$.

Step 1: Enthalpy ($\Delta H_{\text{soln}}$)

Dissolution involves three energy steps:

1. Separate solute particles (endothermic, $+\Delta H_1$)
2. Separate solvent molecules (endothermic, $+\Delta H_2$)
3. Mix solute and solvent (exothermic, $-\Delta H_3$)

The net enthalpy is:

• If solute and solvent are similar (both polar or both nonpolar), the new interactions (step 3) are almost as strong as the original ones (steps 1+2). So $\Delta H_{\text{soln}} \approx 0$ or slightly negative.
• If they are dissimilar (polar + nonpolar), the new interactions are weak — step 3 releases little energy, so $\Delta H_{\text{soln}}$ is large and positive.

Step 2: Entropy ($\Delta S_{\text{soln}}$)

Mixing always increases disorder ($\Delta S > 0$), which favours dissolution. But:

• For similar substances, mixing is easy — entropy gain is large.
• For dissimilar substances, the molecules repel each other (e.g., oil and water separate), so the entropy gain is small or even negative.

3. The Key Formula: "Like" Means Similar $\Delta H_{\text{mix}}$

There is no single formula for "Like Dissolves Like". Instead, the principle is captured by the Hildebrand solubility parameter ($\delta$):

Where:

• $\Delta H_{\text{vap}}$ = enthalpy of vaporisation
• $V_m$ = molar volume

Why this works:

• $\delta$ measures the cohesive energy density — how strongly molecules hold onto each other.
• Two substances with similar $\delta$ values have similar intermolecular forces → they mix with small $\Delta H_{\text{soln}}$ → dissolution is favourable.

Rule of thumb: If $|\delta_{\text{solute}} - \delta_{\text{solvent}}| < 2 \, \text{MPa}^{1/2}$, they are likely to dissolve.

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