NCERT Class 9 Science Exploration Chapter 05 Exploring Mixtures and their Separation PDF Download

Chapter 5: Exploring Mixtures and their Separation

Have you ever wondered how sweet, white crystals of sugar are obtained from tall, green sugarcane plants? Or how do doctors detect diseases like malaria using just a few drops of blood? Many such everyday activities are made possible by techniques based on the fascinating science of separating mixtures.

You have learnt about mixtures and some simple methods to separate them. In this chapter, you will explore mixtures in greater depth, including their properties, behaviour and the various techniques used to separate them. From industrial processes like sugar production to life-saving medical tests, the separation of mixtures plays a crucial role in our daily lives.

Think It Over

• Why do suspended particles settle in muddy water over time but not in milk?
• How is evaporation different from boiling?
• Why do you see bright rays of sunlight when it passes through small gaps between the leaves of a dense tree?

5.1 How Can We Classify Mixtures?

You have learnt that a mixture of sugar and water has a uniform composition throughout. A well-stirred mixture of sugar and water is equally sweet in the first and the last sip. Such a mixture is called a homogeneous mixture (Fig. 5.1) or a solution. Other examples of homogeneous mixtures are vinegar (acetic acid in water), aerated drinks like soda (carbon dioxide in water), etc. A solution always remains homogeneous.

On the other hand, a stirred mixture of sand and water is not uniform. The sand particles are easily visible in the water and settle with time. Such a mixture is called a heterogeneous mixture (Fig. 5.2). Is the mixture of oil and water homogeneous or heterogeneous? Can you think of some other heterogeneous mixtures?

[Figure 5.1: Homogeneous mixture, See in your textbook]

[Figure 5.2: Heterogeneous mixture, See in your textbook]

Activity 5.1: Let us experiment - Group activity

Divide the class into three groups - A, B and C. Each group prepares a mixture in a colourless, transparent glass tumbler or a 100 mL beaker by following these steps:

1. Group A: Add one spatula of common salt to 50 mL of water in a beaker and stir it well. Label it A.
2. Group B: Add one spatula of chalk powder to 50 mL of water in a beaker and stir it well. Label it B.
3. Group C: Add a few drops of milk to 50 mL of water in a beaker and stir it well. Label it C.
4. Are the particles visible in each mixture? Record your observations.
5. Direct the light from a laser pointer through the beakers containing the mixtures (Fig. 5.3) and observe it from the side of the beaker in a direction perpendicular to the laser beam. Record your observations.

Safety first: Do not look directly into the laser beam. It can cause irreversible eye damage.

6. Predict what you would observe in each of the beakers if you leave them undisturbed for a few minutes.
7. Set up a filtration apparatus and filter each mixture separately. Is there any residue left on the filter paper?
8. Based on your observations, do you think these are the same types of mixtures or are they different?

[Figure 5.3: Passing laser light through various mixtures (a) salt and water, (b) chalk powder and water, and (c) milk and water, See in your textbook]

These are indeed different types of mixtures. How? Let us explore further!

Teacher's Note

When you pass laser light through a homogeneous mixture like salt solution, the light passes straight through without scattering. In heterogeneous mixtures like chalk powder and water, the particles scatter the light, and you see a bright beam. This is the key difference to remember: homogeneous means uniform throughout and appears clear, while heterogeneous has visible particles or phases you can tell apart.

5.2 Solutions

You have learnt that solutions are homogeneous mixtures. You have also learnt that solutions are prepared when a solute (the substance that gets dissolved) is mixed with a solvent (the substance that dissolves the solute). In the mixture of sugar and water, sugar is the solute, and water is the solvent. In what proportion are a solute and a solvent present in a solution? Can these be expressed quantitatively?

5.2.1 Concentration of a solution

You have learnt how Oral Rehydration Solution (ORS) is prepared. We add specified amounts of salt and sugar to a fixed amount of water to prepare ORS. If we change the amount of salt or sugar added to the same volume of water, or change the volume of water for the specified amounts of salt and sugar, we will get a solution of salt and sugar in water, but that will not be ORS. In other words, we cannot freely add any amount of salt and sugar to a fixed amount of water to make ORS.

Let us take another example. When farmers spray pesticides on their crops, they must mix the right amount of pesticide with a fixed amount of water to prepare a solution. If they do not do so, what is likely to happen? Too little pesticide may not protect crops, while too much can damage crops, soil and the environment.

Can you think of other such examples?

The right proportion is always essential when preparing a solution.

The amount of solute dissolved in a given amount of solvent or solution is termed as the concentration of the solution.

Understanding concentration is essential not only in science laboratories but also in everyday life, whether in medicine, agriculture, food, cosmetics, or even while making a simple cup of tea!

Teacher's Note

Concentration answers the question "How much solute is in this solution?" If you make tea with one spoon of tea leaves in a cup, that is different from one spoon in a bucket of water - the concentration is different. When you see labels on medicines, cordials, or cleaning products saying "dilute 1 part in 10 parts water", they are telling you the concentration. Always check this on the label before using, because getting it wrong can make the product ineffective or even harmful.

5.2.2 How do we express concentration?

Let us now explore the different ways to express the concentration of a solution, and understand where and why each method is more suitable.

Collect some commercial packaged products (Fig. 5.4) and note down the information given for each item.

Key Points

• A homogeneous mixture has uniform composition throughout, while a heterogeneous mixture has visible different parts or phases that can be easily identified.
• A solution is a homogeneous mixture made of a solute (the dissolved substance) and a solvent (the substance doing the dissolving).
• The concentration of a solution tells you how much solute is dissolved in a given amount of solvent or total solution, and getting the right concentration is important in medicine, farming, and many everyday applications.

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