What Is A Mixture?

A mixture is a substance formed by combining two or more different types of pure substances (elements or compounds) in any proportion. The components of a mixture are simply mixed together physically, without undergoing a chemical reaction to form a new substance. For example, dissolving salt in water creates a mixture. The salt (a pure substance) and water (a pure substance) are mixed, but they retain their individual chemical identities within the mixture.

Components of a mixture can often be separated by physical processes. For instance, salt can be recovered from saltwater by evaporation of the water.

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Types Of Mixtures

Mixtures are broadly classified based on the uniformity of their composition throughout the substance.

• Homogeneous Mixtures (Solutions): These mixtures have a uniform composition throughout. The components are evenly distributed at the particle level, and there are no physically distinct parts visible. Examples include sugar dissolved in water or salt dissolved in water. Even if the proportion of components varies (like adding more sugar), the mixture remains homogeneous, although its properties (like sweetness or colour intensity) will change. Solutions are homogeneous mixtures.
• Heterogeneous Mixtures: These mixtures have a non-uniform composition. They contain physically distinct parts, and the components are not evenly distributed. Examples include mixtures of sodium chloride and iron filings, salt and sulphur, or oil and water. You can often see the different components in a heterogeneous mixture.

An activity mixing copper sulphate powder with water shows that a small amount results in a uniformly coloured solution (homogeneous), while different amounts produce solutions of varying colour intensity, demonstrating variable composition within the homogeneous type. Mixing chalk powder or milk in water illustrates mixtures that are not homogeneous throughout.

What Is A Solution?

A solution is defined as a homogeneous mixture of two or more substances. Solutions are common in our daily lives, such as lemonade, soda water, and even air. While we often think of solutions as solids dissolved in liquids, solutions can exist in other states too:

• Solid solutions (e.g., alloys like brass, which is a homogeneous mixture of copper and zinc). Alloys are considered mixtures because they retain the properties of their constituent metals and have variable composition, even though the components cannot be separated by simple physical means.
• Gaseous solutions (e.g., air, which is primarily a homogeneous mixture of nitrogen and oxygen, along with small amounts of other gases).

The homogeneity of a solution exists at the particle level, meaning the particles of the components are uniformly distributed. For instance, every sip of lemonade tastes the same because the sugar, salt, and lemon juice particles are evenly spread throughout the water.

A solution has two main components:

• The Solvent: This is the component that dissolves the other component. It is typically present in the larger amount. In a sugar-water solution, water is the solvent.
• The Solute: This is the component that is dissolved in the solvent. It is usually present in the lesser quantity. In a sugar-water solution, sugar is the solute.

Examples of different types of solutions based on the state of solute and solvent:

• Solid in liquid: Sugar in water (Sugar=solute, Water=solvent)
• Solid in liquid: Tincture of iodine (Iodine=solute, Alcohol=solvent)
• Gas in liquid: Aerated drinks (Carbon dioxide=solute, Water=solvent)
• Gas in gas: Air (Nitrogen=solvent, Oxygen and other gases=solute)

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Properties Of A Solution

Solutions exhibit the following characteristics:

• Solutions are homogeneous mixtures.
• The particles in a solution are extremely small, with a diameter less than 1 nanometre ($< 10^{-9} \text{ m}$). This means they are invisible to the naked eye.
• Due to their tiny size, the particles in a solution do not scatter a beam of light passing through it. Therefore, the path of light is not visible within a solution.
• The solute particles cannot be separated from the solvent by simple physical methods like filtration.
• Solutions are stable; the solute particles do not settle down when the solution is left undisturbed.

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Concentration Of A Solution

The concentration of a solution refers to the amount of solute present in a given amount of solvent or solution. The relative proportion of solute and solvent can be varied, leading to different concentrations.

Based on the amount of solute dissolved, a solution can be described qualitatively as:

• Dilute: A solution with a relatively small amount of solute.
• Concentrated: A solution with a relatively large amount of solute. (These terms are comparative; one solution might be dilute compared to another but concentrated compared to a third).

At a specific temperature, a solution capable of dissolving no more solute is called a saturated solution. The maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature is known as the substance's solubility in that solvent at that temperature.

If a solution contains less solute than the saturation level at a given temperature, it is called an unsaturated solution.

Different substances generally have different solubilities in the same solvent at the same temperature. Also, solubility often changes with temperature (e.g., increases when heated, allowing more solute to dissolve, and might decrease upon cooling, causing excess solute to crystallise out).

Quantitatively, the concentration of a solution can be expressed in various ways. Three common methods are:

1. Mass by mass percentage: $$ \text{Mass percentage of solution} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100 $$
2. Mass by volume percentage: $$ \text{Mass by volume percentage of solution} = \frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100 $$
3. Volume by volume percentage: (Used for solutions of liquids in liquids) $$ \text{Volume by volume percentage of solution} = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100 $$

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What Is A Suspension?

A suspension is a heterogeneous mixture in which tiny solid particles are dispersed in a liquid medium, but they do not dissolve. Instead, the solid particles remain suspended throughout the bulk of the medium for some time. These suspended particles are large enough to be seen with the naked eye.

Examples include chalk powder mixed in water or muddy water.

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Properties Of A Suspension

Suspensions have distinct properties:

• They are heterogeneous mixtures.
• The particles are visible to the naked eye.
• The particles are large enough to scatter a beam of light passing through the suspension, making the path of light visible.
• Suspensions are unstable. When left undisturbed, the solid particles gradually settle down at the bottom due to gravity. Once the particles settle, the suspension breaks, and it no longer scatters light.
• The suspended particles can be separated from the liquid medium by filtration.

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What Is A Colloidal Solution?

A colloidal solution, or simply a colloid, is a mixture where the particle size is intermediate between that of a true solution and a suspension. Colloids appear homogeneous to the naked eye because the particles are not individually visible, but they are actually heterogeneous in nature at a microscopic level. Milk is a common example of a colloidal solution.

Although colloidal particles are too small to be seen directly, they are large enough to interact with light.

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Properties Of A Colloid

Colloids possess the following characteristics:

• They are heterogeneous mixtures, despite often appearing homogeneous.
• The size of colloidal particles is typically between 1 nm and 1000 nm ($10^{-9} \text{ m}$ to $10^{-6} \text{ m}$). They are too small to be seen with the naked eye.
• Colloidal particles are large enough to scatter a beam of visible light passing through the colloid. This phenomenon is called the Tyndall effect, named after the scientist John Tyndall. The Tyndall effect makes the path of the light beam visible.

The Tyndall effect can be observed in everyday situations, such as when a beam of sunlight enters a dusty room through a small opening, making the dust particles and the light path visible. Similarly, sunlight passing through the canopy of a dense forest shows the Tyndall effect due to the scattering of light by tiny mist droplets (a colloid of water in air).

• Colloids are generally quite stable. The particles do not settle down when the mixture is left undisturbed.
• Colloidal particles are too small to be separated by standard filtration. However, they can be separated by special techniques like centrifugation.

A colloidal solution has two parts:

• The Dispersed phase: These are the solute-like particles that are spread throughout the medium.
• The Dispersion medium: This is the medium in which the dispersed phase particles are suspended.

Colloids are classified based on the physical state (solid, liquid, or gas) of the dispersed phase and the dispersion medium. They are very common in daily life, as shown in the table below:

Dispersed phase	Dispersion Medium	Type	Example
Liquid	Gas	Aerosol	Fog, Clouds, Mist
Solid	Gas	Aerosol	Smoke, Automobile exhaust
Gas	Liquid	Foam	Shaving cream
Liquid	Liquid	Emulsion	Milk, Face cream
Solid	Liquid	Sol	Milk of magnesia, Mud
Gas	Solid	Foam	Foam rubber, Sponge, Pumice
Liquid	Solid	Gel	Jelly, Cheese, Butter
Solid	Solid	Solid Sol	Coloured gemstone, Milky glass

Physical And Chemical Changes

Matter can undergo changes, which can be classified as either physical or chemical.

Physical properties are characteristics that can be observed and measured without changing the chemical identity of the substance. These include properties like colour, hardness, rigidity, fluidity, density, melting point, and boiling point.

A physical change is a change that affects the physical properties of a substance but does not alter its chemical composition or nature. Changes of state (solid to liquid, liquid to gas, etc.) are classic examples of physical changes. When ice melts to water or water boils to form vapour, the substance changes form, but it is still chemically $\text{H}_2\text{O}$. The particles are the same chemically, even though their arrangement and energy differ, leading to different physical properties.

In contrast, chemical properties relate to a substance's ability to react with other substances or to change its composition. For example, the flammability of oil (its ability to burn) is a chemical property that distinguishes it from water (which is non-flammable and can extinguish fire). The odour of a substance can also be considered a chemical property as it relates to its chemical nature.

A chemical change (also called a chemical reaction) is a change that results in the formation of one or more new substances with different chemical properties and composition from the original substance(s). During a chemical change, the chemical identity of the substance is altered.

For example, burning is a chemical change because the substance reacts with oxygen in the air to produce new substances (e.g., ash, carbon dioxide, water vapour), and its original chemical composition is destroyed. Rusting of iron is another chemical change where iron reacts with oxygen and water to form iron oxide (rust), a new substance with different properties.

Sometimes, both physical and chemical changes can occur simultaneously, such as when a candle burns. The wax melts (physical change), while the wax also burns, reacting with oxygen to produce carbon dioxide and water (chemical change).

What Are The Types Of Pure Substances?

Pure substances can be categorised into two main types based on their chemical composition: elements and compounds.

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Elements

The concept of an element was first scientifically defined by Antoine Laurent Lavoisier, building on the work of Robert Boyle. An element is defined as a basic form of matter that cannot be broken down into simpler substances by ordinary chemical reactions (such as heating, electrolysis, or reaction with other chemicals).

There are currently over 100 known elements. Ninety-two of these occur naturally on Earth, while the rest are man-made.

Elements are commonly classified into three groups:

• Metals: These typically exhibit a characteristic metallic lustre (shine), are often silvery-grey or golden-yellow in colour, and are excellent conductors of heat and electricity. Metals are generally ductile (can be drawn into wires) and malleable (can be hammered into thin sheets). They are also sonorous (produce a ringing sound when struck). Examples include gold (Au), silver (Ag), copper (Cu), iron (Fe), sodium (Na), and potassium (K). Mercury (Hg) is a unique metal as it is liquid at room temperature, while gallium (Ga) and cesium (Cs) become liquid just above room temperature (around 303 K or $30^\circ\text{C}$). Most elements are solids at room temperature.
• Non-metals: These show a variety of colours and are generally poor conductors of heat and electricity. Unlike metals, they are typically not lustrous, sonorous, or malleable. Examples include hydrogen (H), oxygen (O), iodine (I), carbon (C - like coal or coke), bromine (Br), and chlorine (Cl). Bromine is a non-metal that is liquid at room temperature.
• Metalloids: These elements possess intermediate properties between those of metals and non-metals. Examples include boron (B), silicon (Si), and germanium (Ge).

Eleven elements exist in a gaseous state at room temperature.

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Compounds

A compound is a substance that is formed when two or more different elements chemically combine with each other in a fixed proportion by mass. This chemical combination creates a new substance with properties entirely different from those of its constituent elements.

Consider the difference between mixing iron filings and sulphur powder (a mixture) versus chemically reacting them by heating (forming a compound, iron sulphide).

• Mixing (Physical Change): If you simply mix iron filings and sulphur powder, you get a heterogeneous mixture. The individual particles of iron and sulphur are still present. You can still see distinct yellow sulphur particles and grey iron particles. You can separate the iron filings using a magnet because the iron still retains its magnetic property. The properties of the mixture are simply the average of the properties of its components. Adding a solvent like carbon disulphide dissolves the sulphur, separating it from the iron. Adding dilute acid would react with the iron (if not completely mixed) to produce hydrogen gas, characteristic of iron reacting with acid.
• Heating (Chemical Change): If you strongly heat the mixture of iron and sulphur, a chemical reaction occurs. A new substance, iron sulphide (FeS), is formed. This material is a compound. It has a uniform composition throughout (fixed proportion of iron and sulphur atoms chemically bonded) and a uniform texture and colour (often dark grey or black), unlike the original mixture. A magnet will not attract iron sulphide because the iron's magnetic property is lost in the chemical combination. When dilute acid is added to iron sulphide, it reacts to produce hydrogen sulphide gas, which smells like rotten eggs – a different gas than the hydrogen produced from iron reacting with acid. This shows that the chemical properties of the compound are completely different from its constituent elements.

The key differences between mixtures and compounds can be summarized in the following table:

Mixtures	Compounds
Elements or compounds are just physically mixed in any proportion, with no chemical reaction. No new chemical substance is formed.	Elements chemically react with each other in a fixed proportion to form a new compound. A new chemical substance is formed.
A mixture has a variable composition. The proportion of components can be changed.	A compound has a fixed composition. The elements are combined in a specific, unchanging ratio by mass.
A mixture shows the properties of its constituent elements or compounds.	The new substance (compound) has totally different properties compared to its constituent elements.
The constituents can generally be separated relatively easily by physical methods (e.g., filtration, evaporation, magnetism, decantation, etc.).	The constituents can only be separated by chemical or electrochemical reactions, as they are chemically bonded.

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