MIXTURES CHEMISTRY STANDARD : 8

1.MIXTURES •

Matter can be broken down into two categories: pure substances and mixtures. Pure substances are further broken down into elements and compounds. Mixtures are physically combined structures that can be separated into their original components.

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A chemical substance is composed of one type of atom or molecule.

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A mixture is composed of different types of atoms or molecules that are not chemically bonded.

Therefore, when two or more substances mix with each other without participating in a chemical change, the resulting substance is called a Mixture. Here some of the examples of mixtures are shown below, which include air, milk, soil, brass. Air is a mixture of many gases and dust, milk is a mixture of carbohydrate, protein, fats and water, soil is a mixture of many things and brass is an alloy which is a mixture of copper and zinc.

Types of Mixtures : HOMOGENEOUS AND

HETEROGENEOUS MIXTURES

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If the composition of the constituents is the same throughout the mixture, it is known as a homogeneous mixture.

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All the solutions are homogenous mixtures.

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Eg: sugar solution, salt solution, mixture of alcohol and water, ornamental gold.

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The components of such mixtures cannot be seen separately with the naked eye.

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If the components in a mixture are not distributed uniformly, it is known it is known as heterogeneous mixture.

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Eg: a mixture of salt and sand, muddy water, a mixture of water and kerosene.

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Here the components can be seen separately with the naked eye.

Let’s do an experiment. 

Take equal amounts of water in three separate beakers. Add sugar particles in the first, chalk powder in the second and milk in the third beaker. Stir them well.

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Keep the beakers undisturbed for some time. Stir it again and pass an intense beam of light through the sides of the three beakers. https://www.youtube.com/watch?v=XEAiLm2zuvc&t=2 32s

1.1

SOLUTION

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When one substance dissolves into another, a solution is formed. A solution is a homogeneous mixture consisting of a solute dissolved into a solvent . The solute is the substance that is being dissolved, while the solvent is the dissolving medium. Solutions can be formed with many different types and forms of solutes and solvents.

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The particles of solution are very small. The size of the particle is usually less than 1nm.

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They cannot scatter any light.

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Hence we are unable to see the path of light inside homogenous mixture

https://www.youtube.com/watch?v=G51be WWhCv0 Normally, the constituent present in smaller amount is considered as the solute and the constituent present in larger amount is considered the

solvent. In aqueous solutions, water is always the solvent irrespective of its amount. Concentration of solution : Take equal amounts of water in three glass tumblers. Add one One tablespoon of glucose, to the second beaker add two tablespoon of glucose(you can use tang powder also), and to the third one add four tablespoons of glucose and stir. Observe the difference in the colour of the solutions in the two tumblers. Concentration of a solution refers to the amount of solute dissolved in a fixed amount of solvent. A solution containing a small amount of solute is known as a dilute solution and that which contains large amount of solute is known as a concentrated solution.

1.1a Expressing concentration of a solution https://www.youtube.com/watch?v=leqokXN_ntk&t=122s Concentration of solutions can be expressed in different ways. Mass percentage: The amount of solute that gets dissolved in a solution can be expressed in percentage using this method. This denotes the amount of solute present in hundred grams of the solution. Mass percentage = Parts per million (ppm) This denotes the number of parts by mass of a solute present in ten lakh (one million) parts by mass of a solution. 'ppm' is used to express the presence of very small amounts of a solute. For example, the allowed limit of chlorine in drinking water is 4 ppm. Volume percentage, molarity, molality and normality are some other scales used to express the concentration of solutions.

Do different materials dissolve to the same extent in a given solvent? Take 50 mL each of water in two beakers. Take 100 g each of powdered common salt (sodium chloride) and ammonium chloride. Stir and dissolve the common salt gradually in the water in one of the beakers. Continue doing this till no more salt dissolves. Such a solution, in which maximum amount of solute has been dissolved, is known as a saturated solution. Amount of solute at a given temperature is known as a saturated solution. The solution before reaching saturation is known as an unsaturated solution. Unsaturated solutions can dissolve more amounts of solute. Repeat the above activity by adding sugar gradually to the second beaker. Compare the amount of ammonium chloride with that of common salt. Which is greater? The amount of a solute in grams required to convert 100 g of a solvent into a saturated solution at a given temperature is known as the solubility of the solute in that solvent.

https://www.youtube.com/watch?v=95ChO2oEJPA

Supersaturated solution and crystallization 

Add a little more of common salt to the saturated salt solution (or sugar or potassium permanganate) and heat it. What happens?

Such a solution, which contains more amount of solute than that is required to saturate it, is known as a supersaturated solution. Crystallization refers to the formation of solid crystals from a homogeneous solution. It is essentially a solid-liquid separation technique and a very important one at that.

In order for crystallization to take place a solution must be "supersaturated". Supersaturation refers to a state in which the liquid (solvent) contains more dissolved solids (solute) than can ordinarily be accommodated at that temperature.

https://www.youtube.com/watch?v=QdwKhbtzsug

1.2 suspension 

Size of the particles in a suspension is usually larger than 1000nm.

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The particles are insoluble in water and are big enough to scatter the light passing through them.  This effect of scattering of light is called Tyndall effect. 

However, these particles eventually settle down. After sometime, the Tyndall effect may not be observed in suspension.

1.3 colloids 

The particle size is between 1-1000nm.  Particles does not settle down.  Path of the light is visible through the mixture. Mixture appears to be homogenous, 

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But on observing under highly powered microscope, we came across the conclusion that mixture appears to be homogenous but it is actually a heterogeneous mixture.

In a colloid particles never settle down. Hence the Tyndall effect is always observed in colloids.

Let us assess