# Colloids Charge and Coagulation

> Colloid charge and coagulation for JEE Chemistry with the Hardy-Schulze rule, NCERT coagulating values, gold number, zeta potential and electrophoresis.

- Canonical URL: https://prepelephant.com/topics/jee/chemistry/colloids-charge-coagulation
- Exam / course: JEE · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Colloids Charge and Coagulation", PrepElephant, https://prepelephant.com/topics/jee/chemistry/colloids-charge-coagulation

## Direct answer

Every lyophobic sol survives on a surface charge, acquired by preferential adsorption of one ion or by ionisation of surface groups — an AgI sol made with excess KI adsorbs iodide and goes negative. Coagulation demands neutralising that charge, and the Hardy-Schulze rule ranks the effort: flocculating power climbs steeply with counter-ion valence, so for negative sols Al3+ > Ba2+ > Na+ and for positive sols [Fe(CN)6]4− > SO4^2− > Cl−. The coagulating value — the minimum electrolyte concentration in millimoles per litre — is the inverse ranking, and NCERT's numbers for an As2S3 sol (NaCl 52, MgCl2 0.72, AlCl3 0.093) quantify the rule vividly.

## What you must remember

- **Hardy-Schulze orders:** negative sol (As2S3, AgI in iodide excess): Al3+ > Ba2+ > Na+; positive sol (Fe(OH)3): ferrocyanide ion > sulphate > chloride — the effective ion's charge decides, not the electrolyte's identity.
- **NCERT coagulating values for As2S3:** NaCl 52, MgCl2 0.72, AlCl3 0.093 mmol/L — aluminium needs roughly 560 times less sodium's dose, the numbers JEE recycles.
- **Charge origin:** preferential adsorption of the ion common to the particle's lattice, or ionisation of surface groups; the ion in excess during preparation donates the charge.
- **Double layer and zeta potential:** the slipping-plane potential; the sol destabilises as zeta potential collapses towards zero.
- **Electrophoresis:** charged particles migrate in an applied field — the experimental proof of sol charge; electro-osmosis is its stationary-phase mirror.
- **Gold number:** milligrams of protective lyophilic colloid that just fail to prevent coagulation of 10 mL of standard gold sol against 1 mL of 10% NaCl; smaller is stronger — gelatin 0.005-0.01 outranks gum arabic 0.15-0.25.
- **Other coagulation routes:** boiling, persistent dialysis, electrophoresis, and mixing oppositely charged sols — mutual coagulation, the working principle when alum clears muddy water.
- **Optical and mechanical identity:** the Tyndall effect scatters light only in colloids, and Brownian motion keeps particles suspended against gravity.

## Coagulation arithmetic and NCERT numbers

Start from the sol, then reason forward. Mixing silver nitrate into excess potassium iodide deposits AgI particles bathed in iodide; each adsorbs I−, so the sol is negative and the flocculating ion must be a cation. Compare candidates through the NCERT table: Na+ carries a coagulating value of 52 mmol/L, Mg2+ of 0.72, Al3+ of 0.093 — magnesium needs 72 times less than sodium, aluminium 560 times less. Each unit of valence buys an order of magnitude. Now flip to the positive side: the ferric hydroxide sol from ferric chloride hydrolysis adsorbs H+ and demands anions, with the tetravalent ferrocyanide ion the most efficient flocculant NCERT lists. Finally the protective case: adding gelatin before the electrolyte keeps a red gold sol red, because the lyophilic coat physically blocks ion approach — the same chemistry that keeps inks and milk stable, and the reason the gold number ranks protectors inversely to their numeric value.

## What JEE still asks of colloids

Surface chemistry, colloids included, was removed from the JEE Main syllabus in the 2023-24 rationalisation; the questions now live in JEE Advanced — whose revised syllabus retains colloids, electrophoresis, coagulation with the Hardy-Schulze rule and emulsions — and in NCERT-based school exams. Main-only aspirants can treat this page as concept reinforcement; Advanced candidates should hold it as scoring territory since the content rarely changes. The traps persist across both regimes. Flocculating power versus coagulating value: inverse rankings, so the ion with the smallest coagulating value is the most powerful flocculant. The gold number inversion: a smaller number means stronger protection, not weaker. And charge prediction: AgI from excess AgNO3 is positive, from excess KI negative — the side in excess supplies the adsorbed ion, a sentence that decides many one-mark questions.

## Frequently asked questions

### Which ion coagulates a negative sol most efficiently?

The counter-ion of highest charge — Al3+ over Ba2+ over Na+ — per the Hardy-Schulze rule, quantified by NCERT coagulating values of 0.093, 0.72 and 52 mmol/L for As2S3.

### What exactly is a coagulating value?

The minimum electrolyte concentration, in millimoles per litre, needed to precipitate a sol; it falls sharply as the counter-ion's charge rises.

### What does the gold number measure?

The protective power of a lyophilic colloid — the milligrams needed to just protect 10 mL of a standard gold sol; smaller values mean stronger protection, gelatin near 0.005-0.01 being the benchmark.

### Why is an AgI sol negative when prepared with excess KI?

The particles preferentially adsorb the lattice ion present in excess — iodide — so the dispersed phase carries negative charge stabilised by its double layer.

### What happens when two oppositely charged sols are mixed?

Their charges neutralise mutually and both coagulate — exploited when the positive aluminium hydroxide sol from alum clarifies negatively charged muddy water.