Maarg Manthan · Topic 6.7

Characteristics of Solutions

Science & Technology › Chemistry – Basic Concepts and Atomic Structure · Topic 6.7

A solution is a homogeneous mixture of two or more substances, in which the solute is dissolved in the solvent. In a true solution the particles are less than 1 nanometre across, so the solution is clear, does not scatter light and does not settle. Concentration is expressed as mass percentage, parts per million, molarity, which is moles of solute per litre of solution, or molality, which is moles of solute per kilogram of solvent. A solution may be unsaturated, saturated or supersaturated. Colligative properties depend on the number of dissolved particles and not on their nature, and include lowering of vapour pressure, rise in boiling point, fall in freezing point and osmotic pressure. Colloids and suspensions are mixtures with larger particles.

5 min read · UPSC Prelims · Science & Technology · Chemistry – Basic Concepts and Atomic Structure

At a Glance

True solution Particles under 1 nm; clear
Colloid 1 to 1000 nm; Tyndall effect
Suspension Over 1000 nm; settles
Molarity Moles per litre of solution
Colligative property Depends on particle number
Exam link UPSC Prelims, Science & Technology: Chemistry – Basic Concepts and Atomic Structure

Where Does It Fit?

This is the last topic of the macro on chemistry. It describes mixtures that dissolve, drawing on acids, salts and electrolytes, and prepares for chemistry in daily life. Previous topic: Electrochemistry and Its Applications. Next topic: Polymers and Biopolymers.

How Do Solutions, Colloids and Suspensions Differ?

Property	True solution, colloid, suspension
Particle size	Under 1 nm, 1 to 1000 nm, and above 1000 nm
Appearance	Transparent, translucent, opaque
Tyndall effect	No scattering of light in a true solution, scattering in a colloid, and scattering in a suspension
Settling	Does not settle, does not settle, and settles on standing
Separation	Cannot be filtered, cannot be filtered normally, and can be filtered
Examples	Salt in water, air, alloys; milk, blood, fog, smoke, jelly, ink; muddy water, chalk in water

How Is Concentration Measured and What Affects Solubility?

Mass percentage: grams of solute in 100 grams of solution.
Parts per million: used for very dilute solutions, such as pollutants in water or air.
Molarity: moles of solute per litre of solution, which changes slightly with temperature.
Molality: moles of solute per kilogram of solvent, which does not change with temperature.
Solubility: the maximum amount of solute that dissolves in a given amount of solvent at a given temperature. For most solids it rises with temperature, while for gases it falls.
Saturated solution: holds the maximum solute at that temperature. A supersaturated solution holds more than normal and crystallises when disturbed, as in rock candy.

What Are Colligative Properties and Gas Solubility Laws?

Property or law	Meaning and applications
Lowering of vapour pressure	Adding a non-volatile solute lowers the vapour pressure of the solvent, as stated by Raoult’s law
Elevation of boiling point	A solution boils at a higher temperature, so salt in cooking water raises its boiling point slightly
Depression of freezing point	A solution freezes at a lower temperature. Salt on icy roads melts ice, and antifreeze such as ethylene glycol protects car radiators
Osmotic pressure	The pressure needed to stop osmosis. Used in reverse osmosis for water purification and desalination, as in Cell Membranes and Cell-Cell Communication/Interactions
Henry’s law	The solubility of a gas in a liquid is proportional to its pressure above the liquid. Soda fizzes when the cap is opened. Scuba divers can get the bends, and climbers at altitude may suffer from low oxygen in blood
Van’t Hoff factor	Accounts for ionisation of solutes such as salts, which give more particles and a larger effect

What Else Should You Remember and Why Does It Matter?

Brownian motion: random movement of colloidal particles, observed by Robert Brown in 1827.
Types of colloids: sols such as paint and blood, gels such as jelly, emulsions such as milk and butter, foams such as whipped cream, and aerosols such as fog and smoke.
Coagulation: clumping of colloids, used in purifying water with alum and in the Cottrell smoke precipitator, and in the clotting of blood.
Solutions in the body: blood plasma and cell fluid are solutions and colloids, and intravenous saline is isotonic with blood, as in Osmoregulation, ADH and Formation of Urine; Thermoregulation.
Universal solvent: water dissolves many substances because of its polar molecules, and like dissolves like.
Alloys and gases: alloys are solid solutions and air is a gaseous solution, as in Elements, Compounds and Molecules.
Desalination: India is using reverse osmosis plants to produce potable water in coastal and island areas.
Environment link: dissolved oxygen in water, measured in parts per million, shows water quality, and warm water holds less of it.

As an analytical point, solutions explain how substances move in the body, how water is purified and why salt melts ice. Colligative properties depend only on how many particles are dissolved.

Why Does It Matter for the Exam?

Classification questions: whether a given mixture is a solution, colloid or suspension.
Property questions: Tyndall effect, colligative properties and their uses.
Applications: reverse osmosis, antifreeze and gas solubility.
Current affairs link: desalination plants and water quality standards.
Common errors: believing that milk is a true solution or that molarity is independent of temperature.

Why Is This Topic Important?

Facts Mixture types asked
Concept Colligative properties
Use Reverse osmosis

What Came Next?

The next macro covers chemistry in daily life and biomolecules. Next topic: Polymers and Biopolymers.

Key People and Terms

Solute Dissolved substance
Solvent Dissolving medium
Molarity Moles per litre of solution
Tyndall effect Scattering of light by particles
Colloid Mixture with intermediate-sized particles
Colligative Depending on particle number

Exam Corner

Points to Remember

True solution: particles under 1 nm.
Milk is a colloid.
Henry’s law for gases.
Salt lowers freezing point.
Brownian motion: Robert Brown, 1827.

Do Not Confuse With

Molarity and molality Per litre of solution versus per kg of solvent
Colloid and suspension Does not settle versus settles
Saturated and supersaturated Maximum versus beyond maximum

Memory Hook

Solution clear and small, colloid scatters light, suspension settles; salt lowers freezing, raises boiling, Henry’s law for the gas in the bottle.

Mains Angle

Questions link solutions with water treatment and daily life. These points add depth.

Types Solution, colloid, suspension
Properties Colligative
Use Desalination

A Question You May Face

An original practice question, not a past paper question.

“Distinguish between true solutions, colloids and suspensions, and explain the colligative properties of solutions with applications.”

How to Answer

Types Three mixtures.
Concentration Terms.
Colligative Four.
Use RO, antifreeze.

GS Relevance

Prelims: types of mixtures, concentration terms, colligative properties, colloids, uses. GS Paper 3: science and technology.

Frequently Asked Questions

What is the difference between a solution, a colloid and a suspension?

In a true solution the particles are smaller than 1 nanometre, so it is clear and does not scatter light or settle. In a colloid the particles are 1 to 1000 nanometres, so it scatters light and does not settle, and in a suspension the particles are larger, visible and settle on standing.

What is the Tyndall effect?

The Tyndall effect is the scattering of a beam of light by colloidal particles, which makes the path of the beam visible, as seen when sunlight passes through mist or smoke in a room. True solutions do not show it, so it is used to tell colloids from solutions.

What are colligative properties?

Colligative properties are properties of a solution that depend on the number of dissolved particles and not on their nature. They are lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure, and they explain the use of salt on icy roads and of antifreeze.

What does Henry's law state?

Henry's law states that at a given temperature the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid. It explains why a soft drink fizzes when opened and why divers breathing air at high pressure can suffer the bends when they rise too fast.

How is the concentration of a solution expressed?

Concentration can be expressed as mass percentage, parts per million for very dilute solutions, molarity as moles of solute per litre of solution, or molality as moles of solute per kilogram of solvent. Molality does not change with temperature, but molarity does because volume changes.

PYQ Practice — Statement Analysis

1 Milk is a colloid.

True

It shows the Tyndall effect.

2 A true solution shows the Tyndall effect.

False

Colloids do.

3 Salt lowers the freezing point of water.