What Is A Solution?

A solution is defined as a homogeneous mixture of two or more substances.

Solutions are very common in our daily lives (e.g., lemonade, soda water). While typically considered as a solid dissolved in a liquid, solutions can also be formed by:

• Liquid in liquid
• Gas in liquid
• Gas in gas (like air)
• Solid in solid (like alloys)

An alloy is a solid solution, usually of metals, or a metal and a non-metal. Alloys show the properties of their constituent metals and can have variable compositions, even though they are not separable by simple physical methods. Brass, for instance, is a mixture of zinc and copper.

A solution has two main components:

• Solvent: This is the component present in the larger quantity that dissolves the other component. It determines the physical state of the solution.
• Solute: This is the component present in the smaller quantity that dissolves in the solvent.

Examples of Solutions:

• Sugar solution: Sugar (solute, solid) in Water (solvent, liquid).
• Tincture of Iodine: Iodine (solute, solid) in Alcohol (solvent, liquid).
• Aerated drinks (like soda water): Carbon dioxide (solute, gas) in Water (solvent, liquid).
• Air: Nitrogen (solvent, gas - $\approx 78\%$) and Oxygen (solute, gas - $\approx 21\%$), along with other gases in small amounts.

Properties of a Solution:

• Solutions are homogeneous mixtures.
• The particles in a solution are extremely small, less than 1 nanometer ($10^{-9}$ m) in diameter. They cannot be seen with the naked eye.
• Due to their tiny size, solution particles do not scatter a beam of light passing through them, so the path of light is not visible in a true solution.
• The solute particles do not settle down when the solution is left undisturbed, making solutions stable.
• The components of a solution cannot be separated by filtration.

Concentration Of A Solution

The concentration of a solution indicates the amount of solute present in a given quantity of the solution (or solvent).

Solutions can be described as dilute or concentrated. These are comparative terms; a dilute solution has less solute compared to a concentrated solution.

A solution in which no more solute can be dissolved at a specific temperature is called a saturated solution. The maximum amount of solute that can be dissolved in a given amount of solvent at a specific temperature to form a saturated solution is known as its solubility.

If the amount of solute in a solution is less than the solubility limit at that temperature, it is an unsaturated solution.

Solubility often increases with an increase in temperature, allowing more solute to dissolve.

Concentration can be expressed in various ways, commonly as a percentage:

• Mass by Mass Percentage: Calculated as: $$ \text{Mass percentage of solution} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100 $$ Where, Mass of solution = Mass of solute + Mass of solvent.
• Mass by Volume Percentage: Calculated as: $$ \text{Mass by volume percentage} = \frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100 $$
• Volume by Volume Percentage: Calculated for liquid-in-liquid solutions: $$ \text{Volume by volume percentage} = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100 $$

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What Is A Suspension?

A suspension is a heterogeneous mixture where solid particles are dispersed throughout a liquid medium but do not dissolve in it.

The particles in a suspension are large enough to be seen with the naked eye.

Properties of a Suspension:

• Suspensions are heterogeneous mixtures.
• The dispersed particles are visible to the naked eye.
• Suspension particles scatter a beam of light passing through, making the path of light visible (shows Tyndall effect).
• Suspensions are unstable; the solid particles tend to settle down over time when left undisturbed. This process is called sedimentation.
• The dispersed particles can be easily separated from the liquid by filtration.

Once the particles settle down, the suspension loses its ability to scatter light.

What Is A Colloidal Solution?

A colloid or colloidal solution is a type of heterogeneous mixture, although it may appear homogeneous due to the uniform distribution of particles.

The particle size in a colloid is intermediate, larger than particles in a true solution but smaller than those in a suspension (ranging approximately from 1 nm to 1000 nm).

These particles are too small to be seen individually with the naked eye.

Tyndall Effect: Colloidal particles are large enough to scatter a beam of light, making the path of light visible. This phenomenon is called the Tyndall effect. This effect can be seen when light passes through fog, mist, or smoke, or when a light beam enters a dusty room.

Properties of a Colloid:

• Colloids are heterogeneous mixtures.
• The particles are not visible to the naked eye.
• They scatter a beam of light (show Tyndall effect).
• Colloids are generally stable; the particles do not settle down when left undisturbed.
• The particles cannot be separated by simple filtration. However, they can be separated by a special technique called centrifugation.

A colloidal solution has two components:

• Dispersed Phase: This is the solute-like component or the particles that are dispersed in the medium.
• Dispersion Medium: This is the medium in which the dispersed phase particles are distributed.

Colloids are classified based on the physical states of the dispersed phase and the dispersion medium:

Separating The Components Of A Mixture

Most naturally occurring substances are mixtures, not pure substances. To obtain individual pure components from a mixture, various separation techniques are employed.

The method chosen depends on the nature and properties of the components in the mixture (e.g., state, particle size, boiling point, solubility, density, magnetic property, etc.).

Simple physical methods like handpicking, sieving, and filtration are effective for separating components of heterogeneous mixtures.

However, for many mixtures, especially homogeneous ones or those with very small particles, special techniques are required.

How Can We Obtain Coloured Component (Dye) From Blue/black Ink?

Ink is a mixture, typically of a dye (solute) dissolved in water (solvent).

To separate the dye from the water, the method of evaporation is used.

Process: Heat the ink indirectly by placing it in a watch glass over a beaker of boiling water. The water in the ink evaporates, leaving the solid dye behind on the watch glass.

This method is suitable for separating a volatile solvent from a non-volatile solute.

How Can We Separate Cream From Milk?

Milk is a heterogeneous mixture, actually a colloid (an emulsion of fat droplets in water).

Cream, which contains fat, can be separated from milk using centrifugation.

Process: Milk is spun rapidly in a centrifuging machine or a milk churner. This causes the denser components to move towards the bottom and the lighter components (like cream/fat) to stay near the top.

Principle of Centrifugation: This technique is based on the principle that denser particles are forced to the bottom and lighter particles stay at the top when spun rapidly.

Applications of Centrifugation:

• Separating blood and urine components in diagnostic laboratories.
• Separating butter from cream in dairies and homes.
• Squeezing water out of wet clothes in washing machines.

How Can We Separate A Mixture Of Two Immiscible Liquids?

Immiscible liquids are liquids that do not mix with each other (e.g., oil and water).

A mixture of immiscible liquids can be separated using a separating funnel.

Process: The mixture is poured into a separating funnel and allowed to stand undisturbed. The liquids separate into distinct layers based on their densities, with the denser liquid forming the lower layer. The stopcock at the bottom of the funnel is then opened to carefully drain out the lower layer. The stopcock is closed when the boundary between the two liquids reaches the stopcock.

Principle of Separating Funnel: This technique relies on the difference in densities of the immiscible liquids, causing them to form separate layers.

Applications of Separating Funnel:

• Separating mixtures of oil and water.
• In the extraction of iron, to remove the lighter slag (molten impurities) from the heavier molten iron.

How Can We Separate A Mixture Of Salt And Camphor?

Some substances have the property of sublimation, where they directly change from a solid to a gaseous state upon heating, without passing through the liquid state. Camphor is one such substance, while common salt (sodium chloride) does not sublime.

A mixture of a sublimable solid and a non-sublimable solid can be separated by sublimation.

Process: The mixture is heated, usually in a china dish covered with an inverted funnel. The sublimable component (camphor) turns into vapour and rises, then cools and solidifies (sublimes) on the cooler inner surface of the funnel or collecting device. The non-sublimable component (salt) remains in the china dish.

Examples of Sublimable Solids: Ammonium chloride, naphthalene, anthracene.

Is The Dye In Black Ink A Single Colour?

Black ink is usually not a single pure colour but a mixture of different coloured dyes.

These colours can be separated using a technique called chromatography, specifically paper chromatography in this case.

Process: A drop of ink is placed on a strip of filter paper (stationary phase). The lower edge of the paper is dipped into a solvent (mobile phase, usually water) in a jar, ensuring the ink spot is above the water level. As the solvent rises up the paper by capillary action, it dissolves the different dyes in the ink spot.

The dyes, being different substances, have different solubilities in the solvent and also different affinities for the filter paper. Dyes that are more soluble in the solvent and have less affinity for the paper travel faster and further up the paper. This separates the dyes into different coloured spots at different heights.

Principle of Chromatography: This technique is used to separate solutes that are dissolved in the same solvent. Separation is based on the differential movement of components between a stationary phase (filter paper) and a mobile phase (solvent).

The name "chromatography" comes from the Greek word "kroma" meaning colour, as it was initially used to separate colours.

Applications of Chromatography:

• Separating colours in dyes.
• Separating pigments from natural sources (like flower petals).
• Separating drugs from blood samples.

How Can We Separate A Mixture Of Two Miscible Liquids?

Miscible liquids are liquids that mix completely with each other (e.g., acetone and water).

A mixture of two miscible liquids can be separated by distillation, provided they have a significant difference in their boiling points (generally more than $25^\circ$C or 25 K).

Process: The mixture is heated in a distillation flask. The liquid with the lower boiling point vaporises first. The vapour is then cooled and condensed back into a liquid in a condenser, and collected in a separate container. The liquid with the higher boiling point remains in the distillation flask.

Principle of Distillation: Separation is based on the difference in the boiling points of the miscible liquids. This method works best for liquids that boil without decomposition.

For mixtures of miscible liquids with boiling points that are close to each other (difference less than $25^\circ$C or 25 K), fractional distillation is used.

The apparatus for fractional distillation includes a fractionating column placed between the distillation flask and the condenser. A fractionating column is often packed with glass beads or rings, which provide a large surface area. As the mixed vapours rise through the column, repeated vaporisation and condensation cycles occur. The vapour becomes richer in the more volatile component (lower boiling point) as it ascends, eventually condensing and separating more efficiently.

Applications of Fractional Distillation:

• Separating different gases from air.
• Separating different fractions from petroleum products (like petrol, kerosene, diesel).

How Can We Obtain Different Gases From Air ?

Air is a homogeneous mixture of gases, primarily nitrogen ($\approx 78\%$), oxygen ($\approx 21\%$), argon ($\approx 0.9\%$), and small amounts of other gases.

The components of air can be separated using fractional distillation of liquid air.

Process Steps:

1. Filtration: Air is filtered to remove dust particles.
2. Compression and Cooling: Air is compressed to a high pressure. Then, it is cooled using techniques like cooling by expansion, which liquefies the air. Carbon dioxide and water vapour are removed during cooling as they solidify at higher temperatures.
3. Fractional Distillation: The liquid air is slowly warmed up in a fractional distillation column. As the temperature increases, the different gases boil off and separate at different heights in the column based on their boiling points.

Boiling points of main components of air:

• Oxygen: -183$^\circ$C (90 K)
• Argon: -186$^\circ$C (87 K)
• Nitrogen: -196$^\circ$C (77 K)

Gases separate in increasing order of their boiling points as the liquid air warms up. Nitrogen (lowest boiling point) boils off first, followed by Argon, and then Oxygen (highest boiling point).

How Can We Obtain Pure Copper Sulphate From An Impure Sample?

Impurities often contaminate solid substances. Simple evaporation may not remove all impurities and can sometimes damage the desired solid (e.g., sugar charring).

Crystallisation is a process used to purify impure solid samples by forming pure crystals from a solution.

Process for Copper Sulphate Purification:

1. Dissolve the impure copper sulphate sample in a minimum amount of water to form a solution.
2. Filter the solution to remove any insoluble impurities.
3. Heat the filtrate (clear solution) gently to evaporate excess water until a saturated solution is obtained (indicated by a thin layer of crystals forming on the surface or a glass rod dipped in the solution and cooled).
4. Cover the saturated solution and leave it undisturbed at room temperature to cool slowly.
5. Over time, pure crystals of copper sulphate will form in the solution.
6. Separate the crystals from the remaining liquid by filtration.
7. Dry the crystals.

Principle of Crystallisation: As a saturated solution cools, the solubility of the solute decreases. The excess solute comes out of the solution in the form of pure crystals, leaving soluble impurities behind in the mother liquor.

Advantages of Crystallisation over Simple Evaporation:

• Prevents the decomposition or charring of some solids (like sugar) that may occur on heating to dryness.
• Removes soluble impurities that might remain dissolved in the solution even after initial filtration and would contaminate the solid upon simple evaporation.

Applications of Crystallisation:

• Purifying salt obtained from sea water.
• Separating crystals of alum (phitkari) from impure samples.

By selecting appropriate techniques based on the properties of the mixture's components, pure substances can be effectively obtained. Modern technology has led to the development of many advanced separation methods.

Drinking water supply systems in cities often involve a purification process that includes sedimentation, decantation, filtration, and chlorination.

Physical And Chemical Changes

Changes in matter can be classified as physical or chemical based on whether the chemical composition of the substance changes.

Physical properties are characteristics of a substance that can be observed or measured without changing its chemical identity. These include colour, hardness, rigidity, fluidity, density, melting point, boiling point, state (solid, liquid, gas), etc.

A physical change is a change that affects the physical properties of a substance but not its chemical composition. During a physical change, the substance remains chemically the same, even if its appearance changes. Interconversion of states of matter (melting, freezing, boiling, condensation, sublimation) are examples of physical changes. These changes are often reversible.

Chemical properties are characteristics that describe how a substance reacts or changes its chemical nature. Examples include inflammability (ability to burn), reactivity with acids, bases, or other substances.

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. The chemical composition of the original substance is altered. Chemical changes are generally irreversible.

For example, burning is a chemical change where a substance reacts with oxygen to produce new substances (like carbon dioxide and water), releasing heat and light.

When a candle burns, both types of changes occur: the wax melts (physical change), and the wax burns to produce carbon dioxide and water (chemical change).

What Are The Types Of Pure Substances?

Based on their chemical composition, pure substances are broadly classified into elements and compounds.

Elements

An element is a fundamental form of matter that cannot be broken down into simpler substances by ordinary chemical reactions (like heating, electrolysis, or chemical reactions with other substances).

Robert Boyle is credited as the first scientist to use the term 'element' (in 1661). Antoine Laurent Lavoisier established an experimentally useful definition, considering it a basic substance not decomposable by chemical means.

Elements are categorized into three main groups:

1. Metals:
   • Usually have a shiny appearance (lustre).
   • Are typically silvery-grey or golden-yellow.
   • Are good conductors of heat and electricity.
   • Are ductile (can be drawn into wires).
   • Are malleable (can be hammered into thin sheets).
   • Are sonorous (produce a ringing sound when struck).
   • Examples: Gold (Au), Silver (Ag), Copper (Cu), Iron (Fe), Sodium (Na), Potassium (K). Mercury (Hg) is the only metal that is a liquid at room temperature.
2. Non-metals:
   • Display a variety of colours.
   • Are poor conductors of heat and electricity.
   • Are generally not lustrous, sonorous, or malleable.
   • Examples: Hydrogen (H), Oxygen (O), Iodine (I), Carbon (C), Bromine (Br - liquid), Chlorine (Cl).
3. Metalloids:
   • Elements that exhibit properties intermediate between those of metals and non-metals.
   • Examples: Boron (B), Silicon (Si), Germanium (Ge).

More Facts about Elements:

• Over 100 elements are known currently.
• Most elements ($\approx 92$) occur naturally.
• The majority of elements are solid at room temperature.
• Eleven elements are gases at room temperature.
• Two elements (Mercury and Bromine) are liquids at room temperature.
• Gallium and Cesium become liquid slightly above room temperature ($303$ K or $30^\circ$C).

Compounds

A compound is a substance formed when two or more different elements are chemically combined in a fixed proportion by mass.

Unlike a mixture where components are just mixed and retain their properties, in a compound, the elements react to form a new substance with entirely different properties.

Activity illustrating the difference between a Mixture and a Compound (Iron and Sulphur):

Mixing iron filings and sulphur powder results in a mixture.

• The composition is variable.
• The components (iron and sulphur) retain their individual properties (e.g., iron is still attracted by a magnet).
• Adding carbon disulphide dissolves sulphur, leaving iron. Adding dilute acid to the mixture produces hydrogen gas (characteristic of iron reacting with acid).
• The components can be easily separated by physical means (like using a magnet).

• The composition is fixed (Fe and S are in a fixed mass ratio).
• The properties of the compound (Iron(II) sulphide) are entirely different from those of the constituent elements (iron and sulphur). Iron(II) sulphide is not attracted by a magnet.
• Adding carbon disulphide does not dissolve Iron(II) sulphide. Adding dilute acid to Iron(II) sulphide produces hydrogen sulphide gas (characteristic of sulfides reacting with acid), which has a rotten egg smell, distinct from hydrogen.
• The components (Iron and Sulphur) are chemically bonded and cannot be separated by simple physical methods. Separation requires chemical or electrochemical reactions.

Summary of Differences between Mixtures and Compounds:

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