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Mixtures, Pure Substances, and Separation

Is Matter Around Us Pure? (Mixtures)

When we look around us, we see a variety of substances. Some of these appear to be pure, while others are clearly mixtures. In science, the term 'pure' is different from common usage. Scientifically, a pure substance consists of a single type of particle, which could be atoms or molecules. A mixture, on the other hand, contains more than one type of pure substance mixed together.

Most of the matter around us exists as mixtures of two or more pure components, for example, air (a mixture of gases), soil (a mixture of minerals, organic matter, etc.), milk, etc. Mixtures can be homogeneous or heterogeneous.

A mixture is formed when two or more substances (elements or compounds) are mixed together physically in any proportion, and the components retain their individual properties. Mixtures can be classified into two main types based on the uniformity of their composition:

What Is A Solution?

A solution is a homogeneous mixture of two or more substances. You might have come across various types of solutions in daily life, such as lemonade (nimbu paani), soda water, etc. Lemonade is a mixture of lemon juice, sugar, and water. All these substances are uniformly distributed, meaning the lemonade tastes the same throughout. This uniformity is characteristic of a solution.

A solution has a solvent and a solute as its components.

Solvent: The component of the solution that dissolves the other component in it (usually the component present in larger quantity).

Solute: The component of the solution that is dissolved in the solvent (usually the component present in lesser quantity).

For example, in a sugar solution, sugar is the solute and water is the solvent. Solutions are not just liquids. They can be solid solutions (alloys), gaseous solutions (air), etc.

Properties of a solution:

  1. A solution is a homogeneous mixture.
  2. The particles of a solution are smaller than 1 nm ($10^{-9}$ metre) in diameter. So, they cannot be seen by the naked eye.
  3. Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
  4. The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, i.e., a solution is stable.

Concentration Of A Solution

The concentration of a solution is the amount of solute present in a given amount of solvent or solution. A solution can be dilute, concentrated, or saturated.

The concentration of a solution can be expressed in various ways. Common methods include:

1. Mass by mass percentage of a solution:

This is defined as the mass of solute present in 100 grams of the solution.

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

Note that Mass of solution = Mass of solute + Mass of solvent.

Example 1. A solution is prepared by dissolving 20 grams of sugar in 180 grams of water. Calculate the mass percentage of the sugar solution.

Answer:

Mass of solute (sugar) = 20 g

Mass of solvent (water) = 180 g

Mass of solution = Mass of solute + Mass of solvent = 20 g + 180 g = 200 g

Mass percentage of solution = $\frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100 = \frac{20 \text{ g}}{200 \text{ g}} \times 100 = 10\%$

2. Mass by volume percentage of a solution:

This is defined as the mass of solute present in 100 ml of the solution.

$ \text{Mass by volume } \% = \frac{\text{Mass of solute (g)}}{\text{Volume of solution (ml)}} \times 100 $

Example 2. 15 grams of salt is dissolved in water to make 250 ml of solution. Calculate the mass by volume percentage concentration.

Answer:

Mass of solute (salt) = 15 g

Volume of solution = 250 ml

Mass by volume percentage = $\frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100 = \frac{15 \text{ g}}{250 \text{ ml}} \times 100 = 6\%$

3. Volume by volume percentage of a solution:

This is defined as the volume of solute present in 100 ml of the solution (used when both solute and solvent are liquids).

$ \text{Volume by volume } \% = \frac{\text{Volume of solute (ml)}}{\text{Volume of solution (ml)}} \times 100 $

Example 3. A solution is made by mixing 30 ml of alcohol with 70 ml of water. Calculate the volume percentage of alcohol in the solution.

Answer:

Volume of solute (alcohol) = 30 ml

Volume of solvent (water) = 70 ml

Volume of solution = Volume of solute + Volume of solvent = 30 ml + 70 ml = 100 ml

Volume by volume percentage of alcohol = $\frac{\text{Volume of alcohol}}{\text{Volume of solution}} \times 100 = \frac{30 \text{ ml}}{100 \text{ ml}} \times 100 = 30\%$

What Is A Suspension?

A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. These particles are large enough to be seen with the naked eye. An example is mixing sand in water. The sand particles do not dissolve but stay suspended in the water, and if left undisturbed, they will settle down at the bottom.

Properties of a suspension:

  1. A suspension is a heterogeneous mixture.
  2. The particles of a suspension can be seen by the naked eye.
  3. The particles of a suspension scatter a beam of light passing through it and make its path visible. This effect is known as the Tyndall effect.
  4. The solute particles settle down when a suspension is left undisturbed, i.e., a suspension is unstable.
  5. The particles can be separated from the mixture by filtration.

What Is A Colloidal Solution?

A colloidal solution (or colloid) is a heterogeneous mixture, although it appears homogeneous to the naked eye. The particles of a colloid are uniformly spread throughout the solution. Compared to true solutions, the particles in a colloid are larger, but they are still small enough that they cannot be seen individually with the naked eye and do not settle down when left undisturbed.

Examples of colloids include milk, blood, ink, fog, smoke, jelly, etc.

A colloidal solution is also a mixture of two components: the dispersed phase and the dispersion medium.

Dispersed phase: The solute-like component or the dispersed particles in a colloid form the dispersed phase.

Dispersion medium: The component in which the dispersed phase is suspended is known as the dispersion medium.

Colloids are classified according to the state (solid, liquid, or gas) of the dispersed phase and the dispersion medium. A table illustrating some common types of colloids is provided 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 (Solid Foam) Foam, Rubber, Sponge, Pumice
Liquid Solid Gel Jelly, Cheese, Butter
Solid Solid Solid Sol Coloured gemstone, Milky glass

Properties of a colloidal solution:

  1. A colloid is a heterogeneous mixture.
  2. The size of particles of a colloid is too small to be individually seen by the naked eye.
  3. Colloids are big enough to scatter a beam of light passing through it and make its path visible (Tyndall effect).
  4. They do not settle down when left undisturbed, i.e., a colloid is quite stable.
  5. The particles cannot be separated from the mixture by the process of filtration. However, special techniques of separation known as centrifugation can be used to separate the colloidal particles.

Tyndall effect: The scattering of light by colloidal particles is called the Tyndall effect. This effect is seen when a beam of light passes through a colloidal solution. The colloidal particles are large enough to scatter light, making the path of the light beam visible. This effect is observed in everyday life, for example, when a beam of sunlight enters a dusty room through a small hole, the path of the light becomes visible due to the scattering of light by tiny dust particles suspended in the air (an aerosol colloid). It is also seen in a forest when light passes through the canopy; the mist containing tiny water droplets (aerosol) scatters the light.

Illustration of Tyndall effect in a colloid

Brownian motion: Colloidal particles are constantly moving in a random, zig-zag path. This random movement is called Brownian motion. It is caused by the collision of the colloidal particles with the fast-moving molecules of the dispersion medium. This motion provides stability to colloidal solutions by preventing the particles from settling down.


Separating The Components Of A Mixture

Most natural substances are not chemically pure. Different methods of separation are used to get individual components from a mixture. The choice of separation method depends on the properties of the constituents of the mixture. Simple physical methods like handpicking, sieving, and filtration are used to separate heterogeneous mixtures. Special techniques are used for separating the components of mixtures.

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

Ink is a mixture of dye (coloured component) and water. The dye is a solute dissolved in the water (solvent). We can separate the volatile solvent (water) from the non-volatile solute (dye) using the method of evaporation.

Process:

  1. Take a beaker filled with water.
  2. Place a watch glass on the mouth of the beaker.
  3. Put a few drops of ink on the watch glass.
  4. Now, start heating the beaker. Do not heat the ink directly on the flame.
  5. You will see that the water in the beaker heats up, which in turn heats the ink on the watch glass.
  6. As the water evaporates from the watch glass, the solid dye is left behind.

Conclusion: This activity shows that ink is a mixture, and we can separate the coloured dye from the water by evaporation. Water evaporates while the solid dye is left behind on the watch glass.

How Can We Separate Cream From Milk?

Milk is actually a heterogeneous mixture of fat particles dispersed in water. We can separate cream (which is richer in fat) from milk using the method of centrifugation.

Principle: Centrifugation is a method used to separate small solid particles from a liquid by spinning the mixture at high speed. The denser particles are forced to the bottom, and the lighter particles remain at the top.

Process:

  1. Take some full-cream milk in a test tube.
  2. Centrifuge it by using a centrifuging machine for two minutes. (If a centrifuging machine is not available in the school, you can do this activity by using a hand churner, commonly used in Indian households to separate cream from milk/curd).
  3. Observe the test tube after centrifugation.

Observation: The cream separates from the milk and collects on top.

Explanation: When milk is centrifuged rapidly, the denser components (cream/fat) are forced to the bottom, and the lighter components (skimmed milk) remain at the top.

Applications of Centrifugation:

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 two immiscible liquids can be separated using a separating funnel.

Principle: Immiscible liquids separate into layers depending on their densities. The denser liquid settles at the bottom, and the lighter liquid floats on top.

Process:

  1. Pour the mixture of kerosene oil and water into a separating funnel.
  2. Let it stand undisturbed for some time so that separate layers of oil and water are formed. (Oil is less dense than water, so it forms the upper layer, and water forms the lower layer).
  3. Open the stop-cock of the separating funnel and pour out the lower layer of water carefully into a beaker.
  4. Close the stop-cock of the separating funnel as the oil reaches the stop-cock.
Separating funnel setup to separate immiscible liquids

Applications of Separating Funnel:

How Can We Separate A Mixture Of Salt And Camphor?

Some solid substances change directly from solid to gas phase on heating, a process called sublimation. This property can be used to separate a mixture containing a sublimable component (like camphor, naphthalene, anthracene, iodine) from a non-sublimable component (like salt).

Process:

  1. Take a mixture of salt and camphor in a china dish.
  2. Place an inverted funnel over the china dish.
  3. Put a cotton plug on the stem of the funnel.
  4. Now, heat the china dish gently.
  5. The camphor will sublime and convert directly into vapour.
  6. The vapours of camphor will rise up and get condensed on the cooler inner walls of the funnel, forming solid camphor.
  7. The salt will be left behind in the china dish.
Sublimation setup to separate camphor and salt

Conclusion: Camphor, being a sublimable substance, separated from the salt by converting directly into a gas upon heating and then back to solid upon cooling. Salt remained in the solid state.

Is The Dye In Black Ink A Single Colour?

Black ink is usually a mixture of several colours. We can separate the different coloured components of the dye using chromatography.

Principle: Chromatography is a technique used for separation of those solutes that dissolve in the same solvent. Paper chromatography separates components of a liquid mixture by distributing them between a stationary phase (the chromatography paper, which holds water molecules) and a mobile phase (the solvent, usually water, which moves up the paper). The components of the mixture travel at different speeds depending on their solubility in the mobile phase and their adsorption to the stationary phase.

Process (Paper Chromatography):

  1. Take a thin strip of filter paper.
  2. Draw a line using a pencil approximately 3 cm above the lower edge.
  3. Put a small drop of ink at the centre of the pencil line.
  4. Let it dry.
  5. Take a glass beaker/jar filled with water.
  6. Carefully hang the filter paper strip in the jar such that the drop of ink on the paper is just above the water level.
  7. Leave it undisturbed.
  8. As water rises up on the filter paper, it carries the ink drop with it. The different colours in the ink, having different solubilities and affinities for the paper, separate as they move up.
Paper chromatography setup for separating colours in ink

Observation: Different coloured spots appear at different heights on the paper strip, indicating that the ink was a mixture of different colours.

Applications of Chromatography:

How Can We Separate A Mixture Of Two Miscible Liquids?

Miscible liquids are liquids that mix completely (e.g., water and alcohol). If the boiling points of the miscible liquids are significantly different (difference of at least 25°C), we can separate them using distillation.

Principle: Distillation is used for the separation of components of a mixture of miscible liquids that boil without decomposition and have sufficient difference in their boiling points.

Process (for separating acetone and water):

  1. Take the mixture of acetone and water in a distillation flask.
  2. Fit the flask with a thermometer and a condenser.
  3. Arrange the apparatus as shown in the diagram.
  4. Heat the mixture slowly, keeping a close watch at the thermometer.
  5. Acetone (boiling point approx. 56°C) vaporises first and is condensed by the condenser, where the vapours are cooled by circulating water, turning them back into liquid.
  6. The liquid acetone is collected in the receiver flask.
  7. Water (boiling point 100°C) is left behind in the distillation flask.
Distillation apparatus for separating miscible liquids with different boiling points

Conclusion: Acetone, having a lower boiling point, evaporated, condensed, and was collected separately from water.

Fractional Distillation: If the difference in boiling points of the miscible liquids is less than 25°C, fractional distillation is used. A fractionating column is fitted between the distillation flask and the condenser. The fractionating column is packed with glass beads or rings, which provide a large surface area for repeated condensation and vaporisation. This process allows for a better separation of liquids with close boiling points.

Fractional distillation apparatus

Applications of Fractional Distillation:

How Can We Obtain Different Gases From Air ?

Air is a homogeneous mixture of different gases like nitrogen, oxygen, argon, carbon dioxide, etc. The components of air can be separated by fractional distillation of liquid air.

Process:

  1. Air is compressed to a high pressure and then cooled to a very low temperature using methods like repeated compression and expansion. This process liquefies the air.
  2. The liquid air is then slowly warmed up in a fractional distillation column.
  3. As the liquid air warms up, the different gases evaporate at different temperatures according to their boiling points.
  4. Gases with lower boiling points will vaporise earlier and rise higher in the column before being condensed and collected.

The approximate boiling points of the major components of air are:

Since their boiling points are relatively close, fractional distillation is necessary for effective separation. Nitrogen boils off first, followed by Argon, and then Oxygen is left behind or collected last.

Flow diagram for separation of components of air

Conclusion: Different gases from air are separated based on their different boiling points through fractional distillation of liquid air.

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

Impure solid substances can be purified using the method of crystallisation.

Principle: Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. It is preferred over evaporation for obtaining pure solids because:

Process:

  1. Take some impure copper sulphate sample in a china dish.
  2. Dissolve it in minimum amount of water.
  3. Filter the solution to remove insoluble impurities.
  4. Heat the copper sulphate solution gently to evaporate the excess water, bringing it to a saturated state. You can test for saturation by dipping a glass rod in the solution and if crystals form on the rod when cooled, the solution is saturated.
  5. Cover the solution with a filter paper and leave it undisturbed at room temperature to cool slowly.
  6. After some time, pure crystals of copper sulphate will separate out.
  7. Filter the crystals from the liquid (mother liquor) and dry them.
Crystallisation process

Applications of Crystallisation:


Physical And Chemical Changes

Matter undergoes various changes. These changes can be broadly classified into two types: physical changes and chemical changes.

Physical Change: A physical change is a temporary change in which only the physical properties of a substance, such as its state, shape, size, or appearance, change. The chemical composition of the substance remains the same. No new substance is formed.

Chemical Change: A chemical change (also called a chemical reaction) is a permanent change in which the chemical composition of a substance is altered, resulting in the formation of one or more new substances with different properties. The original substance loses its identity.

Key Differences between Physical and Chemical Changes:

Feature Physical Change Chemical Change
Nature Temporary change Permanent change
Composition Chemical composition remains the same Chemical composition changes
New substance formed No new substance is formed New substance(s) with new properties are formed
Reversibility Often reversible Generally irreversible
Energy change Relatively small energy change Significant energy change (heat, light, sound may be produced or absorbed)
Mass Mass of substance remains the same Mass of reactants equals mass of products (Law of Conservation of Mass applies)

What Are The Types Of Pure Substances?

Based on their chemical composition, pure substances can be classified into elements and compounds. A pure substance consists of a single type of particle (atom or molecule).

Elements

An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions. Elements are made up of only one kind of atom.

The word element was first used by Robert Boyle in 1661. Antoine Laurent Lavoisier, a French chemist, was the first to establish an experimentally useful definition of an element.

Elements can be broadly classified into three groups based on their properties:

  1. Metals: They usually have a shiny appearance (lustre), are malleable (can be hammered into thin sheets), are ductile (can be drawn into wires), are good conductors of heat and electricity, and are sonorous (produce a ringing sound when hit). Examples: Iron (Fe), Copper (Cu), Gold (Au), Silver (Ag), Sodium (Na). Most metals are solids at room temperature, except for Mercury (Hg), which is a liquid.
  2. Non-metals: They generally do not have lustre, are not malleable or ductile, are poor conductors of heat and electricity (except Graphite, an allotrope of Carbon), and are not sonorous. Examples: Oxygen (O), Hydrogen (H), Nitrogen (N), Sulphur (S), Carbon (C), Iodine (I), Bromine (Br), Chlorine (Cl). Non-metals exist as solids, liquids, or gases at room temperature (e.g., Carbon, Sulphur are solids; Bromine is a liquid; Oxygen, Hydrogen are gases).
  3. Metalloids: These are elements that show properties intermediate between those of metals and non-metals. Examples: Boron (B), Silicon (Si), Germanium (Ge).

As of now (with ongoing discoveries), there are over 118 known elements. Around 94 are naturally occurring, while others have been synthesised in laboratories. A vast number of substances we see around us are formed from the combination of these elements.

Compounds

A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion. For example, water (H$_2$O) is a compound formed by Hydrogen and Oxygen chemically combined in a fixed ratio of 2:1 atoms or 1:8 by mass. Carbon dioxide (CO$_2$) is another example, formed from Carbon and Oxygen.

Properties of a compound:

  1. A compound is formed by the chemical combination of elements.
  2. The elements are combined in a fixed ratio by mass.
  3. A compound is a homogeneous substance.
  4. The properties of a compound are entirely different from the properties of its constituent elements. For example, Hydrogen is a gas that burns, Oxygen is a gas that supports burning, but Water (formed from Hydrogen and Oxygen) is a liquid that extinguishes fire.
  5. The constituents of a compound cannot be separated by simple physical methods. They can only be separated by chemical or electrochemical reactions.
  6. A compound has a fixed melting point and boiling point.
  7. Energy (in the form of heat or light) is usually absorbed or released during the formation of a compound.

Differences between Mixtures and Compounds:

Mixture Compound
Elements or compounds are mixed together physically in any proportion. Elements are chemically combined in a fixed proportion by mass.
Composition is variable. Composition is fixed.
A mixture shows the properties of its constituent components. The properties of a compound are entirely different from those of its constituent elements.
The constituents can be separated by simple physical methods. The constituents can only be separated by chemical or electrochemical reactions.
Energy is usually not absorbed or released during the formation of a mixture. Energy is usually absorbed or released during the formation of a compound.
Does not have a fixed melting or boiling point (unless it's a pure component). Has a fixed melting and boiling point.