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Exploring Mixtures and their SeparationClass 9 Science Notes

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Section 1 of 5· 1 of 5 complete

How Can We Classify Mixtures?

A mixture is a substance made by combining two or more different materials without a chemical reaction occurring between them. Mixtures can be broadly classified into two main types based on how uniformly their components are distributed.

Homogeneous Mixture: This is a mixture that has a uniform composition and appearance throughout. The individual components cannot be seen separately. A well-stirred mixture of sugar and water is a perfect example; every sip tastes equally sweet. Because the components are so thoroughly mixed, a homogeneous mixture is also called a solution.
- Examples include vinegar (acetic acid in water), salt water, and aerated drinks (carbon dioxide in water).
- A solution, once formed, remains homogeneous.
Heterogeneous Mixture: This is a mixture that does not have a uniform composition. The components are unevenly distributed, and you can often see the individual particles.
- Examples include sand stirred in water, oil and water, and iron filings mixed with sulfur.
- In a mixture of sand and water, the sand particles are easily visible and will eventually settle at the bottom.

Solutions

As we've seen, solutions are homogeneous mixtures. They consist of two main parts:

Solute: The substance that gets dissolved. In a sugar-water solution, sugar is the solute.
Solvent: The substance that does the dissolving. In a sugar-water solution, water is the solvent.

The amount of solute present in a solution is a key property that can be measured quantitatively.

Concentration of a solution

The proportion of solute and solvent in a solution is crucial in many real-world applications. For example, the Oral Rehydration Solution (ORS) used to treat dehydration must have specific amounts of salt and sugar dissolved in a fixed amount of water. Similarly, farmers must use the correct concentration of pesticides; too little may be ineffective, while too much can harm crops and the environment.

The concentration of the solution is a measure of the amount of solute that is dissolved in a given amount of solvent or solution. Understanding concentration is vital in fields like medicine, agriculture, and even cooking.

How do we express concentration?

There are several ways to express the concentration of a solution. Here, we will focus on three common methods based on percentage.

A. Mass by mass percentage (% m/m or % w/w)

This method expresses the mass of the solute present in 100 grams of the total solution. It is commonly used for solid solutes dissolved in liquid solvents or for mixtures of solids like spice blends.

The formula is:

\text{Mass by mass percentage} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100

Note

The mass of the solution is the sum of the mass of the solute and the mass of the solvent.

\text{Mass of solution} = \text{Mass of solute} + \text{Mass of solvent}

Example

Example If 10 g of salt is dissolved in 90 g of water, calculate the mass by mass percentage of the solution formed.

Given

Mass of salt (solute) = $10 \text{ g}$
Mass of water (solvent) = $90 \text{ g}$

To Find

Mass by mass percentage of the solution

Formula

\text{Mass of solution} = \text{Mass of solute} + \text{Mass of solvent}

\text{Mass by mass percentage} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100

Solution

First, calculate the total mass of the solution:

\text{Mass of solution} = 10 \text{ g} + 90 \text{ g} = 100 \text{ g}

Now, substitute the values into the percentage formula:

\text{Mass by mass percentage} = \frac{10 \text{ g}}{100 \text{ g}} \times 100 = 10\% \text{ m/m}

Final Answer The mass by mass percentage of the solution is $10\% \text{ m/m}$ .

B. Mass by volume percentage (% m/v or % w/v)

This method is useful when it is easier to measure the volume of a liquid solution than its mass, which is common in medicine and laboratories. It expresses the mass of the solute (in grams) present in 100 millilitres of the solution.

The formula is:

\text{Mass by volume percentage} = \frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100

Example

Example A saline drip used in hospitals is typically a

0.9\% \text{ m/v}

sodium chloride solution, meaning there is

0.9 \text{ g}

of salt in every

100 \text{ mL}

of solution.

Example

Example If 5 g of glucose is dissolved in water to make 100 mL of solution, calculate its concentration in mass by volume percentage.

Given

Mass of glucose (solute) = $5 \text{ g}$
Volume of solution = $100 \text{ mL}$

To Find

Mass by volume percentage of the solution

Formula

\text{Mass by volume percentage} = \frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100

Solution

Substitute the given values into the formula:

\text{Mass by volume percentage} = \frac{5 \text{ g}}{100 \text{ mL}} \times 100 = 5\% \text{ m/v}

Final Answer The concentration of the glucose solution is $5\% \text{ m/v}$ .

C. Volume by volume percentage (% v/v)

This method is used when mixing two miscible liquids, such as in perfumes, cosmetics, and vinegar. It expresses the volume of the solute (in millilitres) present in 100 millilitres of the solution.

The formula is:

\text{Volume by volume percentage} = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100

Example

Example If 1 mL of a liquid pesticide is mixed with a sufficient amount of water to form 100 mL of a pesticide spray, calculate its volume by volume percentage.

Given

Volume of pesticide (solute) = $1 \text{ mL}$
Total volume of solution = $100 \text{ mL}$

To Find

Volume by volume percentage of the solution

Formula

\text{Volume by volume percentage} = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100

Solution

Substitute the given values into the formula:

\text{Volume by volume percentage} = \frac{1 \text{ mL}}{100 \text{ mL}} \times 100 = 1\% \text{ v/v}

Final Answer The concentration of the pesticide spray is $1\% \text{ v/v}$ .

Solubility of substances

Solubility is defined as the maximum amount of a solute that can dissolve in a fixed quantity of a solvent (like 100 g or 100 mL) at a specific temperature. When a solvent cannot dissolve any more solute at that temperature, the solution is called a saturated solution.

Temperature is a critical factor because it affects solubility:

The solubility of most solid solutes in liquid solvents increases as the temperature increases.
The solubility of gases in liquids generally decreases as the temperature increases.

This property of changing solubility with temperature is the basis for some separation techniques. A solubility curve is a graph that shows how the solubility of a substance changes with temperature. By looking at the curve, we can determine how much solute will dissolve at any given temperature. For example, a steeply rising curve indicates that the substance's solubility increases significantly with temperature.

Methods of Separation of Homogeneous Mixtures

Obtaining pure substances from homogeneous mixtures requires techniques that exploit the different physical properties of the components.

Crystallization

Crystallization is a process used to obtain a pure solid substance in the form of crystals from a solution. A crystal is a solid in which the particles are arranged in a regular, repeating geometric pattern. You can see crystals in everyday life, such as rock salt, sugar, and snowflakes.

The principle behind crystallization is the difference in solubility of a substance at different temperatures. Here’s how it works:

A saturated solution of the substance is prepared at a high temperature.
The hot solution is filtered to remove any insoluble impurities.
The solution is then allowed to cool slowly. As it cools, the solubility of the solute decreases.
Since the solution can no longer hold all the dissolved solute, the excess solute separates from the solution and forms pure crystals.

Crystallization is an excellent method for purifying solids. The rate of cooling affects the size of the crystals: slow cooling produces larger, well-formed crystals, while rapid cooling results in smaller crystals.

Warning

Safety First Some chemicals used for crystallization, like copper sulfate, are toxic. Experiments involving chemicals like sulfuric acid should be handled with extreme care and only under adult supervision.

Distillation

Distillation is a technique used to separate components of a liquid mixture based on differences in their boiling points. It is particularly effective for:

Separating two miscible liquids with significantly different boiling points (a difference of at least $25^\circ\text{C}$ ).
Separating a liquid solvent from a dissolved non-volatile solid (like separating pure water from salt water).

The process involves:

Heating the liquid mixture in a distillation flask. The liquid with the lower boiling point evaporates first, turning into vapor.
Directing this vapor into a condenser, which is a cool tube.
The vapor cools down in the condenser and turns back into a pure liquid (condenses).
This pure liquid, called the distillate, is collected in a separate container.

Example

A mixture of acetone (boiling point

\approx 56^\circ\text{C}

) and water (boiling point

100^\circ\text{C}

) can be separated by distillation. When heated, the acetone will vaporize first, and can be condensed and collected, leaving the water behind.

A more advanced technique called fractional distillation is used when the boiling points of the liquids are very close (less than $25^\circ\text{C}$ apart). This process is used on an industrial scale in petroleum refineries to separate crude oil into useful products like petrol, diesel, kerosene, and LPG (Liquefied Petroleum Gas).

Paper Chromatography

The word chromatography comes from the Greek words chroma (meaning 'colour') and graphein (meaning 'to write'). It is a powerful technique used to separate the different components of a mixture.

Paper chromatography works on the principle that different substances travel at different speeds through a stationary material (the paper) as a solvent moves over it.

A spot of the mixture (like black ink) is placed on a strip of special chromatographic paper.
The bottom edge of the paper is dipped into a solvent (like water or alcohol), ensuring the spot is above the solvent level.
As the solvent travels up the paper by capillary action, it dissolves the mixture and carries it up the paper.
Components that are more soluble in the solvent and interact less with the paper travel faster and farther up the paper.
This differential movement separates the mixture into distinct spots or bands of its individual components.

This method is widely used to separate pigments in inks, dyes, and even natural sources like flower petals and spinach leaves.

How Can We Separate the Components of Heterogeneous Mixtures?

Separating heterogeneous mixtures often involves simpler physical methods, as the components are not uniformly mixed.

Separation of two immiscible liquids

Immiscible liquids are liquids that do not mix to form a single layer, such as oil and water. They can be separated based on their different densities using a piece of lab equipment called a separating funnel.

The process is straightforward:

The mixture of immiscible liquids is poured into the separating funnel and left to stand.
The liquids will separate into distinct layers, with the denser liquid settling at the bottom and the less dense liquid floating on top.
The stopcock at the bottom of the funnel is opened carefully to drain out the bottom layer into a container.
The stopcock is closed just as the interface between the two liquids reaches the bottom, and the top layer can then be collected in a separate container.

Sublimation

Some solid substances have a unique property: when heated, they can turn directly from a solid into a gas without melting into a liquid. This process is called sublimation. The reverse process, where a gas turns directly into a solid, is called deposition.

Sublimation can be used to separate a mixture if one of its components can sublime while the others cannot.

Example: A mixture of sand and camphor.
1. When the mixture is gently heated, the camphor sublimes (turns into vapor).
2. The camphor vapor can be collected on a cool surface (like the inside of an inverted funnel), where it undergoes deposition and turns back into solid, pure camphor.
3. The non-sublimable sand is left behind.

Other common sublimable substances include naphthalene (mothballs) and solid carbon dioxide (dry ice).

Note

Alloys are special types of mixtures, usually homogeneous mixtures of two or more metals (like brass, a mix of copper and zinc) or a metal and a non-metal (like stainless steel). Because their components are so tightly intermixed at the atomic level, alloys cannot be separated by simple physical methods like the ones described above.

Suspensions

A suspension is a heterogeneous mixture in which solid particles are dispersed in a liquid but are large enough to be seen with the naked eye and eventually settle out if left undisturbed. Muddy water is a classic example.

While simple filtration can remove large particles from a suspension, very fine particles may pass through the filter paper. For these, more advanced techniques are needed.

A. Centrifugation

Centrifugation is a process that uses high-speed rotation to separate components of a mixture based on their density.

The mixture is placed in tubes in a machine called a centrifuge, which spins them at very high speeds.
The spinning creates a strong centrifugal force, which is an apparent outward force that pushes the denser particles to the bottom of the tube.
The lighter liquid, called the supernatant, remains on top and can be carefully poured off.

Centrifugation is widely used in medical labs to separate blood cells from plasma and in dairies to separate cream from milk.

B. Coagulation

Sometimes, suspended particles are so fine that they won't settle on their own and are too small for centrifugation to be effective. In such cases, coagulation can be used.

A chemical called a coagulant (like alum, known as fitkari) is added to the mixture.
The coagulant causes the fine, suspended particles to clump together to form larger, heavier masses.
This process of clumping is called coagulation.
These larger clumps then become heavy enough to settle to the bottom through sedimentation, and the clear liquid can be separated.

A common example of coagulation in daily life is the making of cheese (paneer), where an acid like lemon juice is added to milk, causing the milk proteins to coagulate and form solid cheese.

Colloids

What about mixtures like milk or blood? They appear homogeneous, the particles don't settle, but they aren't true solutions. These are called colloids.

A colloid is a type of mixture where one substance of microscopically dispersed insoluble particles is suspended throughout another substance. The particle size in a colloid is intermediate between that of a solution and a suspension.

Solution particles: < 1 nanometer (nm)
Colloidal particles: 1 - 1000 nm
Suspension particles: > 1000 nm

Even though colloids appear uniform, they are technically heterogeneous mixtures because they consist of distinct particles. However, these particles are small enough that they remain suspended indefinitely and do not settle due to gravity. Examples include milk, blood, fog, tomato sauce, and ice cream.

Tyndall Effect

A key property that distinguishes colloids from true solutions is their ability to scatter light. The Tyndall effect is the scattering of a light beam by particles in a colloid or a fine suspension. This scattering makes the path of the light beam visible.

When a beam of light passes through a true solution, its path is invisible because the solute particles are too small to scatter the light.
When the same beam passes through a colloid (like milk in water) or a suspension (like chalk powder in water), the particles are large enough to scatter the light in all directions, making the beam's path visible from the side.

You can observe the Tyndall effect in everyday life, such as when a sunbeam enters a dusty room or when car headlights cut through fog.

In a colloid, the components are referred to as:

Dispersed phase: The solute-like particles that are scattered throughout the medium.
Dispersion medium: The solvent-like substance in which the particles are dispersed.

An emulsion is a special type of colloid where both the dispersed phase and the dispersion medium are liquids (e.g., oil and water). Emulsions are often stabilized by emulsifying agents. Milk is an example of an oil-in-water emulsion, where fat globules (oil) are dispersed in water, stabilized by proteins.

Comparison of Solutions, Colloids, and Suspensions

Property	Solution	Colloid	Suspension
Nature	Homogeneous	Appears homogeneous, but is heterogeneous	Heterogeneous
Particle Size	< 1 nm	1 - 1000 nm	> 1000 nm
Visibility	Particles are not visible	Particles are not visible to the naked eye	Particles are visible to the naked eye
Settling	Particles do not settle	Particles do not settle	Particles settle down when left undisturbed
Filtration	Cannot be separated by filtration	Cannot be separated by filtration	Can be separated by filtration
Tyndall Effect	Does not show Tyndall effect	Shows Tyndall effect	Shows Tyndall effect

Separation techniques are not just for the lab; they are fundamental to nature and modern life. Our kidneys, for instance, act as sophisticated filters to clean our blood. Technologically, we face huge separation challenges like removing plastics from oceans and treating sewage water using processes like sedimentation, coagulation, and filtration. By sorting waste at home and developing ways to recycle materials from old electronics, we use the principles of separation to help create a cleaner and more sustainable world.