Class 8 Science NCERT Notes – Chapter 9: The Amazing World of Solutes, Solvents and Solutions (PDF, MindMap, Q&A, Quizzes)

Student Study Notes: Science Curiosity Chapter 9: The Amazing World of Solutes, Solvents and Solutions.

Part 1: Core Concepts Review

1.1 Mixtures and Solutions

A mixture is formed when two or more substances are combined. Mixtures can be classified into two main types based on the distribution of their components:

• Uniform Mixture: A mixture where the components are evenly distributed throughout. These mixtures are also known as solutions. When sugar and salt are dissolved in water, they form a uniform mixture where every sip tastes the same. The individual components are not visible separately.

• Non-uniform Mixture: A mixture where the components are not evenly distributed. The components can often be seen with the naked eye or a magnifying device. Examples include mixing chalk powder, sand, or sawdust with water.

1.2 Components of a Solution

A solution is composed of a solute and a solvent.

• Solute: The substance that dissolves.
  • In a solid-liquid solution (e.g., salt in water), the solid is the solute.
  • In a liquid-liquid solution, the substance present in the smaller amount is considered the solute.
  • In a gaseous solution like air, gases other than nitrogen (e.g., oxygen, argon, carbon dioxide) are the solutes.

• Solvent: The substance in which the solute dissolves.
  • In a solid-liquid solution, the liquid is the solvent.
  • In a liquid-liquid solution, the substance present in the larger amount is the solvent.
  • In the air, nitrogen is the solvent because it is present in the largest amount.

• Solution: The uniform mixture formed when a solute dissolves in a solvent. The fundamental relationship is: Solute + Solvent → Solution.

An interesting exception is chashni (sugar syrup), where a large amount of solid sugar (solute) is dissolved in a small amount of liquid water, but the water is still considered the solvent.

1.3 Concentration, Saturation, and Solubility

• Concentration: This term describes the amount of solute present in a fixed quantity of a solution or solvent.
  • Dilute Solution: Contains a relatively small amount of solute.
  • Concentrated Solution: Contains a relatively large amount of solute.
  • “Dilute” and “concentrated” are relative terms used to compare solutions.

• Saturation: This describes the capacity of a solvent to dissolve a solute at a specific temperature.
  • Unsaturated Solution: A solution in which more solute can be dissolved at a given temperature.
  • Saturated Solution: A solution that has reached its limit and cannot dissolve any more solute at a particular temperature. If more solute is added, it will settle at the bottom.

• Solubility: This is a specific measure defined as the maximum amount of a solute that can dissolve in a fixed quantity of a solvent to form a saturated solution at a particular temperature.

1.4 Factors Affecting Solubility

Temperature is a key factor that influences the solubility of substances.

• Solids in Liquids: For most solid substances, solubility increases as the temperature of the solvent increases. For example, water at 70°C can dissolve more baking soda than water at 50°C or 20°C. A saturated solution at a lower temperature can become unsaturated if the temperature is raised.

• Gases in Liquids: The solubility of gases generally decreases as the temperature of the liquid increases. Cold water can hold more dissolved oxygen than warm water. This is crucial for sustaining aquatic life, which depends on dissolved oxygen.

1.5 Density

Density is a fundamental property of matter that describes the “heaviness” of an object relative to the space it occupies.

• Definition: Density is defined as the mass present in a unit volume of a substance.

• Formula: Density = Mass / Volume

• Units: The SI unit for density is kilograms per cubic meter (kg/m³). Other common units include grams per milliliter (g/mL) or grams per cubic centimeter (g/cm³).

• Relative Density: A unitless number that compares the density of a substance to the density of water at the same temperature. It is calculated as: (Density of substance) / (Density of water).

1.6 Measurement of Mass and Volume

To calculate density, one must first measure the mass and volume of a substance.

Measuring Mass

• Mass: The quantity of matter present in an object. Its units are grams (g) and kilograms (kg).

• Balance: The instrument used to measure mass. A digital weighing balance is a common type.

• Procedure (Digital Balance):
  1. Turn on the balance and ensure it reads zero.
  2. Place a container (e.g., watch glass) on the pan.
  3. Press the “tare” or “reset” button to zero the balance again.
  4. Place the object in the container to get its mass directly.

• Mass vs. Weight: While often used interchangeably, mass is the amount of matter, whereas weight is the force of gravity on an object. Most balances measure weight but are calibrated to display mass in grams or kilograms.

Measuring Volume

• Volume: The amount of space occupied by an object or substance. SI unit is the cubic meter (m³). For liquids, liters (L) and milliliters (mL) are common (1 mL = 1 cm³).

• Measuring Cylinder: A narrow, tall, transparent container with markings used to measure the volume of liquids.

• Procedure (Liquids):
  1. Place the cylinder on a flat surface.
  2. Pour the liquid to the desired level.
  3. Read the volume by aligning your eye with the meniscus (the curved surface of the liquid). For water and other colorless liquids, read the bottom of the meniscus. For colored liquids, read the top of the meniscus.

• Volume of Regular Solids: Calculated using a formula. For a cuboid: Volume = length × width × height.

• Volume of Irregular Solids: Measured using the water displacement method:
  1. Record an initial volume of water in a measuring cylinder.
  2. Submerge the object completely in the water.
  3. Record the final volume.
  4. The volume of the object is the final volume minus the initial volume.

1.7 Factors Affecting Density

• Temperature: For most substances (solids, liquids, and gases), density decreases when temperature increases. As a substance is heated, its particles spread out, increasing its volume while its mass remains constant. This principle explains why hot air is less dense and rises, which is how hot air balloons work.
  • Special Case of Water: Water is densest at 4°C. When it freezes into ice at 0°C, it expands, taking up more space. This makes ice less dense than liquid water, causing it to float.

• Pressure: The effect of pressure on density varies by the state of matter.
  • Gases: Increasing pressure pushes gas particles closer, decreasing volume and increasing density.
  • Liquids and Solids: They are nearly incompressible, so pressure has a negligible effect on their density.

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Part 2: Knowledge Check Quiz

Answer the following questions in 2-3 complete sentences based on the information provided in the study guide.

1. What is the primary difference between a solute and a solvent when forming a solution from two liquids?

2. Explain the concept of a saturated solution and what happens if more solute is added to it at the same temperature.

3. How does an increase in water temperature affect the amount of oxygen available for fish, and why?

4. Define density and state the mathematical formula used to calculate it.

5. Describe the water displacement method for determining the volume of an irregularly shaped object like a stone.

6. Explain why air is classified as a solution and identify its main solvent and some of its solutes.

7. What is the difference between a uniform mixture and a non-uniform mixture? Provide one example of each.

8. Why does a hot air balloon rise? Relate your answer to the concepts of temperature and density.

9. What is the practical purpose of the “tare” button on a digital weighing balance?

10. Explain the special property of water that causes ice to float on its liquid form.

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Part 3: Quiz Answer Key

1. When forming a solution from two liquids, the substance present in the smaller amount is considered the solute. The substance present in the larger amount is the solvent, which is the component that does the dissolving.

2. A saturated solution is one in which the maximum possible amount of solute has been dissolved in a solvent at a specific temperature. If more solute is added, it will not dissolve and will instead settle at the bottom of the container.

3. The solubility of gases, like oxygen, decreases as water temperature increases. This means warm water holds less dissolved oxygen than cold water, which can negatively impact aquatic life that relies on this oxygen to survive.

4. Density is defined as the mass of a substance present in a unit of its volume. The mathematical formula to calculate it is Density = Mass / Volume.

5. To find the volume of an irregularly shaped object, first record the initial volume of water in a measuring cylinder. Then, fully submerge the object and record the final volume. The object’s volume is the difference between the final and initial water levels.

6. Air is classified as a solution because it is a uniform mixture of gases. Nitrogen is the solvent because it is present in the largest amount, while oxygen, argon, carbon dioxide, and other gases are considered the solutes.

7. A uniform mixture, or solution, has its components evenly distributed, such as salt dissolved in water. A non-uniform mixture has unevenly distributed components that can often be seen separately, such as sand mixed in water.

8. A hot air balloon rises because the air inside the balloon is heated, causing it to expand and increase in volume. This makes the hot air less dense than the cooler, denser air surrounding it, causing the balloon to float upwards.

9. The “tare” button on a digital weighing balance is used to reset the display to zero. This is practical for weighing an object in a container, as you can place the empty container on the balance, press “tare” to ignore its mass, and then add the object to measure only its mass.

10. Water has a unique property where its density is highest at 4°C. As it cools further and freezes into ice at 0°C, its particles arrange in a structure that takes up more space, causing it to expand. Because the same mass now occupies a larger volume, the ice becomes less dense than the liquid water and floats.

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Part 4: Essay Questions

Develop comprehensive answers for the following questions, synthesizing information from across the text.

1. Discuss the concept of solubility, explaining how it is measured and the key factors that influence it, specifically for solids and gases in liquid solvents. Use the examples of baking soda and dissolved oxygen to support your explanation.

2. Explain the concept of density in detail. Describe how to experimentally determine the density of both a regularly shaped solid (like a notebook) and an irregularly shaped solid (like a stone), outlining all necessary measurements, tools, and calculations.

3. Compare and contrast uniform and non-uniform mixtures. Define the components of a solution (solute, solvent) and explain how these definitions apply to different types of solutions, including solid-in-liquid, liquid-in-liquid, and gas-in-gas, using examples provided in the text.

4. Analyze the relationship between temperature, volume, and density for solids, liquids, and gases. Use the examples of a hot air balloon and ice floating on water to illustrate these principles and explain any exceptions to the general rule.

5. Drawing from the text’s examples of historical and traditional practices (e.g., Ayurvedic medicine, bamboo rafts, Ningel village salt production), discuss the practical applications of solutions, solvents, and density in human history and culture.

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Part 5: Glossary of Key Terms

Term	Definition
Balance	The instrument used to measure the mass of an object.
Concentration	The amount of solute present in a fixed quantity of a solution or solvent.
Concentrated Solution	A solution containing a relatively large amount of solute.
Density	The mass present in a unit volume of a substance, calculated as Mass / Volume.
Dilute Solution	A solution containing a relatively small amount of solute.
Mass	The quantity of matter present in any object.
Measuring Cylinder	A narrow, transparent cylindrical container with markings used to measure the volume of liquids.
Meniscus	The curved surface of a liquid inside a container like a measuring cylinder.
Non-uniform Mixture	A mixture in which the components are not evenly distributed.
Relative Density	A comparison of the density of a substance to the density of water at that temperature; it has no units.
Saturated Solution	A solution in which the maximum amount of solute has been dissolved at a particular temperature, and no more can be dissolved.
Solubility	The maximum amount of a solute that can be dissolved in a fixed quantity of a solvent at a particular temperature.
Solute	The component of a solution that dissolves in the solvent. It is typically the solid in a solid-liquid solution or the substance in the smaller amount in a liquid-liquid solution.
Solution	A uniform mixture formed when a solute dissolves in a solvent.
Solvent	The component of a solution that dissolves the solute. It is typically the liquid in a solid-liquid solution or the substance in the larger amount in a liquid-liquid solution.
Uniform Mixture	A mixture in which the components are evenly distributed throughout; also known as a solution.
Unsaturated Solution	A solution in which more solute can be dissolved at a given temperature.
Volume	The amount of space occupied by an object or a substance.