NCERT Class 8 Science Curiosity Chapter 09 The Amazing World of Solutes Solvents and Solutions PDF Download

Chapter 9: The Amazing World of Solutes, Solvents, and Solutions

You must have taken an Oral Rehydration Solution (ORS) at some time in your life. ORS is used to treat dehydration by keeping your body hydrated. You have learnt to prepare ORS at home in Curiosity, Grade 6. You may have wondered why every sip of your homemade ORS tastes the same, no matter how much you drink. Why does it not taste salty in one sip and sweet in another? This is because when you add sugar and salt to water, they form a mixture in which the components are evenly distributed throughout.

Can you predict whether this mixture is uniform or not (Fig. 9.1)? What happens when chalk powder is mixed with water - does it form a uniform mixture?

When salt and sugar are mixed with water, a uniform mixture is formed, whereas when chalk powder or sand, or sawdust is mixed with water, the components are not evenly distributed. Such mixtures are known as non-uniform mixtures (Fig. 9.2a and 9.2b).

Let us explore the science of mixing things together.

[Figure 9.1: Mixture of sugar, salt, and water, See in your textbook]

[Figure 9.2: Mixture of (a) Sand and water; (b) Sawdust and water, See in your textbook]

9.1 What Are Solute, Solvent, and Solution?

A uniform mixture, such as that of salt or sugar, and water, is called a solution. Whenever a solid is mixed with a liquid to form a solution, the solid component is called the solute, and the liquid component is called the solvent. The solute dissolves in the solvent to form a solution (Fig. 9.3).

Solute + Solvent = Solution

When a solution is formed by mixing two liquids, it is not always clear which substance is dissolving the other. In such cases, the substance present in smaller amount is called the solute, while the one in larger amount is called the solvent.

[Figure 9.3: Magnified schematic picture of a solute evenly distributed in a solvent, See in your textbook]

We know air is a mixture. Would a mixture of gases also be considered a solution?

Ever heard of ...

The Chashni (sugar syrup) of the Indian sweet Gulab jamun is made of a large amount of sugar (solid) dissolved in a small amount of water (liquid). However, the water is still considered as the solvent and sugar as the solute (Fig. 9.4)!

[Figure 9.4: Gulab jamuns dipped in sugar syrup, See in your textbook]

Teacher's Note

Remember: in a solution with two liquids, the solute is always the substance present in smaller amount, and the solvent is the one in larger amount - even if the solute appears to be the main ingredient. In the Gulab jamun syrup, there is much more sugar than water, but water is still the solvent because it is the liquid that does the dissolving.

9.2 How Much Solute Can a Fixed Amount of Solvent Dissolve?

Activity 9.1: Let us investigate

• Take a clean glass tumbler and fill it half with water.
• Add one spoon of salt into it and stir well till it dissolves completely (Fig. 9.5).
• Gradually add a spoonful of salt into the glass tumbler and stir. Observe how many spoons of salt you can add before it stops dissolving completely.
• Record your observations in Table 9.1.

Some discussion points

• How many spoons of salt were you able to dissolve before some of it remained undissolved?
• What does this indicate about the capacity of water to dissolve salt?

What will happen if we keep on adding more salt in a given amount of water?

Teacher's Note

The point at which salt stops dissolving shows you the solubility limit of salt in that amount of water at room temperature. Different solutes have different solubility limits - some dissolve more easily than others. When you add more salt than water can dissolve, the extra salt just sits at the bottom undissolved; you can tell because you will see solid particles that won't mix in no matter how much you stir.

Key Points

• A solution is a uniform mixture where one substance (the solute) is evenly distributed in another (the solvent). Salt and sugar form solutions with water, but sand and chalk do not.
• In a solution where a solid mixes with a liquid, the solid is the solute and the liquid is the solvent. If two liquids mix, the one present in smaller amount is the solute.
• A fixed amount of solvent can only dissolve a limited amount of solute. Once this limit is reached, extra solute will not dissolve and remains as solid particles at the bottom.

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