Surface chemistry is one of the most fascinating and scoring parts of physical chemistry, especially in exams like JEE, NEET, and board-level assessments. Yet, many students find it hard to retain key trends like the rate of adsorption of gases or the efficiency of protective colloids. That’s where memory tricks and conceptual clarity help.
In this article, we’ll simplify the two essential topics:
- Adsorption of gases on solid surfaces
- Protective colloids and their role
Let’s dive into the logic, tricks, and real exam relevance.
Understanding Adsorption: The Surface Phenomenon
Adsorption is a surface phenomenon where gas molecules stick to the surface of solids or liquids. Unlike absorption, which is a bulk process, adsorption happens only on the surface.
There are two types of adsorption:
- Physisorption (physical forces, weak Van der Waals forces)
- Chemisorption (chemical bonds, stronger and often irreversible)
But which gas adsorbs more?
Order of Adsorption of Gases on Solids
Here's the commonly observed trend:
H₂O > SO₂ > Cl₂ > NH₃ > HCl > CO₂ > CH₄ > CO > O₂ > N₂ > H₂
To remember it, use the mnemonic from the image: "Pani soda cola aap se kaun mila, budh aur nahi dhuaan."
This helps you recall:
- Pani = H₂O
- Soda = SO₂
- Cola = Cl₂
- Aap = NH₃
- Se = HCl
- Kaun = CO₂
- Mila = CH₄
- Budh = CO
- Aur = O₂
- Nahi = N₂
- Dhuaan = H₂
Why This Order?
The extent of adsorption depends on:
- Polarity and solubility of the gas
- Molecular weight
- Critical temperature (Tc) of the gas Higher Tc = greater van der Waals forces = more adsorption
Hence, polar and heavier gases like SO₂ and H₂O are adsorbed more than lighter ones like H₂ or N₂.
Protective Colloids and Gold Number Concept
Colloids are mixtures where one substance is dispersed in another. In certain cases, we need to protect colloidal particles from coagulation (settling down).
That’s where protective colloids come in.
They prevent the aggregation of particles by forming a protective layer.
Examples of Protective Colloids:
- Gelatin
- Hemoglobin (Hb)
- Egg albumin
- Gum
- Starch
The effectiveness of a protective colloid is measured by its gold number. Lower the gold number, higher the protective power.
Order of Protective Power:
Gelatin > Hemoglobin > Egg Albumin > Gum > Starch
Mnemonic from the image: "Pila teen hai, anda albumin hai, gum me stay chain tha."
Break it down:
- Pila teen = Gelatin
- Anda = Egg
- Albumin = Albumin
- Gum = Gum
- Stay chain = Starch
Gelatin has the highest protection, starch the lowest.
Table: Summary of Surface Chemistry Trends
| Concept | Order (Most to Least) | Trick / Mnemonic |
|---|---|---|
| Adsorption of gases | H₂O > SO₂ > Cl₂ > NH₃ > HCl > CO₂ > CH₄ > CO > O₂ > N₂ > H₂ | Pani soda cola aap se kaun mila... |
| Protective power of colloids | Gelatin > Hemoglobin > Egg Albumin > Gum > Starch | Pila teen hai, anda albumin hai, gum me stay |
Why Is This Important in Exams?
- Direct MCQs from gas adsorption orders are common in NEET and JEE.
- Gold number-based questions are popular in CBSE and competitive chemistry tests.
- Mnemonics make revision faster during last-minute prep.
Real-Life Applications of Adsorption and Colloids
- Water Purification: Activated charcoal adsorbs impurities
- Catalysis: Surface adsorption is crucial in heterogeneous catalysis
- Medicine: Colloidal gold used in diagnostics
- Food and Pharma: Colloids like gelatin used in capsules, stabilizers
FAQs on Surface Chemistry Tricks
Q1. What is the difference between absorption and adsorption?
A1. Absorption is a bulk phenomenon; adsorption happens only on surfaces.Q2. Why does H₂O have the highest adsorption rate?
A2. Due to its high polarity and strong hydrogen bonding.Q3. What is the gold number in colloids?
A3. The minimum amount (in mg) of protective colloid required to prevent coagulation of gold sol in 10 mL of standard gold solution.Q4. Which protective colloid is the best?
A4. Gelatin, because it has the lowest gold number.
