Exploring Class 9 Chemistry: Properties and Separation of Mixtures

Fundamental Characteristics of Mixtures

A mixture is defined as a substance consisting of two or more components that are simply mixed together without being chemically combined. In a mixture, every constituent substance maintains its unique chemical properties. Most materials encountered in daily life are mixtures rather than pure substances; common examples include air, sea water, milk, soil, and steel. A key characteristic of mixtures is that they can be separated back into their individual substances using physical methods, and no new chemical substance is formed during the mixing process.

Mixtures are categorized into two primary types based on their composition. A homogeneous mixture possesses a uniform composition throughout, meaning the individual substances cannot be visually distinguished. For instance, a well-stirred sugar solution is equally sweet from the first sip to the last, indicating its uniformity. Other examples include salt solution, air, vinegar, and soda water. A homogeneous mixture is scientifically referred to as a solution. In contrast, a heterogeneous mixture lacks a uniform composition. Its different components remain visible and are distributed unevenly. For example, sand stirred into water remains visible and eventually settles at the bottom. Other examples include oil and water or a mixture of common salt and iron filings.

Particle Size Classification and Rationale for Separation

Mixtures are further classified into three distinct groups based on the size of their particles: solutions, colloids, and suspensions. The specific method chosen to separate the components of a mixture depends on physical differences between its substances, such as particle size, density, solubility, or boiling point. Crucially, no new substance is formed during physical separation; the process merely sorts out what was already present.

There are several reasons for separating mixtures. One reason is to obtain a substance in its pure form, such as extracting sugar from sugarcane juice, salt from sea water, or metals from ores. Another reason is to remove harmful or useless substances, such as the purification of drinking water. Finally, separation is used to collect useful substances from complex mixtures, such as obtaining petrol, diesel, and kerosene from crude oil.

The Laser Test Activity

To understand the interaction of light with different types of mixtures, the Laser Test can be performed using three beakers. Beaker A contains salt stirred in water, Beaker B contains chalk powder stirred in water, and Beaker C contains a few drops of milk in water. When a thin laser beam is shined through the beakers, the results differ. In Beaker A, which is a true solution, the path of light is not visible because the beam passes straight through the tiny particles. In beakers B and C, the larger particles scatter the light, making the path of the beam clearly visible. Safety note: one should never look directly into a laser beam.

Scientific Properties of Solutions

A solution is defined as a homogeneous mixture of two or more substances. It comprises two parts: the solute and the solvent. The solute is the substance that dissolves (usually present in a smaller amount), while the solvent is the substance in which the solute dissolves (usually present in a larger amount). Because water is capable of dissolving a wide variety of substances, it is known as the "universal solvent." The relationship is represented by the formula: $\text{Solute} + \text{Solvent} = \text{Solution}$

Solutions can exist in various states beyond solids dissolved in liquids. Examples of different solution types include:

Solid in liquid: Sugar or salt dissolved in water.
Gas in liquid: Soda water (carbon dioxide dissolved in water).
Liquid in liquid: Vinegar (acetic acid dissolved in water).
Gas in gas: Air (oxygen and other gases mixed in nitrogen).
Solid in solid: Alloys such as brass and bronze.

The properties of a solution include being a homogeneous mixture that is uniform throughout. The particles are extremely small, measuring less than $1\,nm$ ( $10^{-9}\,m$ ), which means they cannot be seen even with a microscope. Due to this small size, they do not scatter light, and the path of light is not visible through the solution. Furthermore, solutions are stable; solute particles do not settle when left standing and cannot be separated by filtration.

Concentration and Mathematical Formulas

The concentration of a solution is the amount of solute dissolved in a specific amount of solvent or solution. A solution with a minimal amount of solute is called dilute, while one with a large amount is called concentrated. Accurate concentration is vital in various applications, such as ORS drinks, pesticide sprays, or medicinal prescriptions. Concentration is typically expressed as a percentage using three primary formulas:

Mass by mass (used for solids and packaged foods): $\%\,m/m = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100$
Mass by volume (used in medicines and laboratories): $\%\,m/v = \frac{\text{mass of solute}}{\text{volume of solution}} \times 100$
Volume by volume (used for mixing two liquids): $\%\,v/v = \frac{\text{volume of solute}}{\text{volume of solution}} \times 100$

Specific examples include a saline drip used in hospitals, which is $0.9\%\,m/v$ sodium chloride ( $0.9\,g$ of salt per $100\,mL$ of solution), and white vinegar, which is labeled as $5\%\,v/v$ acetic acid. A memory trick for these formulas is "SOS," standing for "Solute Over Solution."

Solubility and Saturated Solutions

Solubility is defined as the maximum mass of a solute that can dissolve in $100\,g$ (or $100\,mL$ ) of a solvent at a specific temperature. A solution that cannot dissolve any more solute at a given temperature is called a saturated solution. Conversely, an unsaturated solution is one that can still dissolve more solute at that temperature. A supersaturated solution holds more solute than it normally can; it is created by dissolving extra solute in a hot solution and then cooling it carefully. If disturbed, the extra solute in a supersaturated solution will crystallize out.

Solubility varies significantly with environmental factors. For most solids dissolved in liquids, solubility increases as temperature rises—for example, hot water dissolves more sugar than cold water. For gases dissolved in liquids, solubility decreases as temperature rises, which is why warm soda loses its carbonation (goes flat) faster than cold soda. Higher pressure also increases the solubility of a gas in a liquid, which is the reason a soda bottle fizzes when opened.

The following table represents the solubility of various substances (in $g/100\,g$ water) at different temperatures:

Substance	$10^\circ\text{C}$	$20^\circ\text{C}$	$30^\circ\text{C}$	$40^\circ\text{C}$	$60^\circ\text{C}$	$80^\circ\text{C}$
Potassium nitrate	21	32	45	62	106	167
Sodium chloride	36	36	36.3	36.5	37	37
Potassium chloride	35	35	37.4	40	46	54
Ammonium chloride	24	37	41	41	55	66

Advanced Separation Methods

Separation techniques for homogeneous mixtures vary based on the physical properties of the components. Miscible liquids, which mix completely into a single layer (like water and alcohol), are separated by distillation. Immiscible liquids, which do not mix and form separate layers (like oil and water), are separated using a separating funnel based on density differences.

Filtration is used to separate an insoluble solid from a liquid; the solid remaining on the paper is the residue, and the clear liquid passing through is the filtrate. Evaporation involves heating a solution so the liquid vaporizes, leaving the dissolved solid behind; this is used to extract salt from sea water. Crystallization is a superior method to evaporation for obtaining pure crystals from a hot saturated solution as it cools. It prevents heat-sensitive substances, like sugar, from charring or decomposing. Natural examples of crystals include rock salt, candy sugar (mishri), snowflakes, and frost.

Distillation is employed when two miscible liquids have boiling points differing by at least $25^\circ\text{C}$ . The liquid with the lower boiling point evaporates first and is collected after passing through a condenser as the distillate. Fractional distillation is required when boiling points are closer than $25^\circ\text{C}$ . A fractionating column provides multiple surfaces for condensation and re-evaporation, allowing for more precise separation. It is used to refine crude oil into petrol, diesel, and kerosene, and to separate liquid air into nitrogen, oxygen, and argon. Memory trick: "25 or more, simple pours. Less than 25, go fractional."

Paper chromatography is a technique based on the different speeds at which dissolved substances travel through a material, influenced by their solubility. It is used to separate pigments in leaves, dyes in ink, and food colorings.

Heterogeneous Separation and Suspensions

A separating funnel is used for immiscible liquids; the denser liquid settles at the bottom and is drained through a stopcock first. Sublimation is the process where a solid turns directly into a vapour upon heating without becoming a liquid; the reverse is called deposition. This method can separate substances like camphor, ammonium chloride, naphthalene, and dry ice from non-subliming solids like sand.

Suspensions are heterogeneous mixtures with large solid particles (more than $1000\,nm$ ) that are visible to the naked eye and settle over time, making them unstable. Centrifugation is a method used when particles are too fine for filtration; the mixture is spun rapidly, and outward force pushes heavier particles to the bottom. This is famously used to separate cream from milk and blood into plasma and cells.

Coagulation involves adding a chemical called a coagulant (such as alum) to make fine suspended particles clump into larger, heavier lumps that settle. This is a critical step in water-treatment and sewage plants.

Alloys and Colloids

An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. Although they cannot be separated by physical methods, they are considered mixtures because their substances retain their properties and their composition can vary. Notable alloys include:

Brass: Copper + Zinc (used for being harder and brighter than pure copper).
Bronze: Copper + Tin (used for strength and corrosion resistance).
Stainless steel: Iron + Carbon, Chromium and Nickel (rust-resistant and very strong). Memory trick: "Brass has Zinc, Bronze has Tin."

A colloid is a mixture that appears homogeneous but is actually heterogeneous. Its particles ( $1\,nm$ to $1000\,nm$ ) are large enough to scatter light—a phenomenon called the Tyndall effect, named after John Tyndall. Colloidal particles do not settle because they are constantly struck by moving particles of the dispersion medium, resulting in a random zig-zag path. A colloid has two components: the dispersed phase (the particles) and the dispersion medium (the medium they are spread in). For example, in milk, fat droplets are the dispersed phase and water is the dispersion medium.

Types of colloids include:

Aerosol: Liquid or solid in gas (Fog, smoke, exhaust).
Foam: Gas in liquid (Shaving cream).
Emulsion: Liquid in liquid (Milk, face cream). An emulsifying agent helps keep them stable.
Sol: Solid in liquid (Mud, milk of magnesia).
Gel: Liquid in solid (Jelly, cheese, butter).
Solid sol: Solid in solid (Coloured gemstones, milky glass).

Comparative Analysis of Mixtures

Solutions, colloids, and suspensions are primarily differentiated by particle size. Solutions have particles less than $1\,nm$ , are transparent, stable, and do not show the Tyndall effect. Colloids have particles between $1\,nm$ and $1000\,nm$ , are translucent, stable, and show the Tyndall effect. Suspensions have particles larger than $1000\,nm$ , are opaque, unstable (settle down), and show the Tyndall effect until the particles settle.

To identify an unknown mixture: if it is transparent with no light beam visible, it is a solution. If it shows the Tyndall effect but does not settle or filter out, it is a colloid. If it is cloudy, visible, and settles or filters, it is a suspension. For a complex mixture like sand, salt, and naphthalene, the separation order is: 1. Sublime naphthalene, 2. Dissolve and filter out sand, 3. Evaporate the filtrate for salt.