NCERT Science Notes - Class 9
Chapter 2 - Is Matter Around Us Pure?

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 9 | Science | Is Matter Around Us Pure?. In this chapter, students learn to differentiate between pure substances and mixtures. Pure substances have a single type of particle, such as elements and compounds. Mixtures, on the other hand, contain more than one type of particle and can be classified as homogeneous or heterogeneous. The chapter also delves into the methods of separating mixtures, like filtration, distillation, and chromatography. Students explore real-life examples and understand the significance of purity in substances. This foundational knowledge helps them grasp more complex chemical concepts and their applications in everyday life.

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NCERT Science Notes - Class 9
Chapter 2 - Is Matter Around Us Pure?

Questions that can be asked:

2.1 - What is a Mixture?

1. Definition:
   • Mixtures are made up of more than one kind of pure matter.
   • Example: Sodium chloride (table salt) is a pure substance but can be separated from water by evaporation, while sodium chloride itself cannot be physically separated into its chemical constituents.
2. Key Characteristics:
   • A mixture contains more than one pure substance and has distinct properties from those of its components.

1. Homogeneous Mixtures:
   • Mixtures with uniform composition throughout.
   • Example: Copper sulfate solution from Groups A and B in the activity. Both solutions have different colors due to the amount of copper sulfate used but are still homogeneous.
2. Heterogeneous Mixtures:
   • Mixtures that contain physically distinct parts and have non-uniform compositions.
   • Example: Mixtures of sodium chloride and iron filings, or oil and water.

Activity 2.1 : Understanding Homogeneous and Heterogeneous Mixtures:

1. What are the two types of mixtures?
2. What is a homogeneous mixture? Give two examples.
3. Can a homogeneous mixture have variable composition? Explain with an example.
4. What is a heterogeneous mixture? Give two examples.
5. How do homogeneous mixtures differ from heterogeneous mixtures in terms of uniformity of composition?

Activity 2.2: Understanding Solutions, Suspensions, and Colloidal Solutions

1. Experiment Setup:
   The class is divided into four groups (A, B, C, and D), each receiving different substances to mix with water. The goal is to observe the behavior of the mixtures and identify whether they form a solution, suspension, or colloidal solution.
2. Groups and Their Samples:
   • Group A: Few crystals of copper sulphate.
   • Group B: One spatula full of copper sulphate.
   • Group C: Chalk powder or wheat flour.
   • Group D: Few drops of milk or ink.
3. Key Observations:
   • Visibility of Particles:
     • Are the particles visible in the mixture after stirring?
   • Light Beam Test:
     • When a beam of light is directed through the mixture, is the path of the beam visible?
   • Stability of Mixture:
     • Does the mixture remain stable, or do the particles settle down after a while?
   • Filtration:
     • Is there any residue left on the filter paper after filtering the mixture?
4. Results of the Activity:
   • Group A and Group B (Copper Sulphate):
     • The particles of copper sulphate dissolve completely, and the mixture is clear and stable. The path of the light beam is not visible. There is no residue on the filter paper.
     • Conclusion: These groups have formed a solution.
   • Group C (Chalk Powder or Wheat Flour):
     • The particles remain suspended in water and do not dissolve. The mixture is cloudy, and the path of the light beam is visible. Over time, the particles begin to settle. There is residue on the filter paper after filtration.
     • Conclusion: This group has formed a suspension.
   • Group D (Milk or Ink):
     • The mixture appears milky or opaque. The path of the light beam is visible due to scattering of light by particles. The mixture remains stable over time, and there is no residue on the filter paper.
     • Conclusion: This group has formed a colloidal solution.
5. Next Steps:
   The activity will be followed by learning about solutions, suspensions, and colloidal solutions in more detail.

1. What did Groups A and B form when they mixed copper sulphate with water? Explain.
2. What type of mixture did Group C form? What observations support this?
3. How does the mixture formed by Group D differ from that of Group C?
4. What is the significance of the light beam test in this activity?
5. How can filtration help distinguish between solutions, suspensions, and colloidal solutions?

2.2 - What is a Solution?

1. Definition of a Solution:
   A solution is a homogeneous mixture of two or more substances, where the components are evenly distributed at the particle level. Examples of solutions include lemonade, soda water, and air.
2. Types of Solutions:
   • Liquid Solutions: These can contain a solid, liquid, or gas dissolved in a liquid.
     • Example: Lemonade (sugar in water).
   • Solid Solutions: Alloys, like brass, are solid solutions where two or more metals (or a metal and a non-metal) are mixed.
     • Example: Brass (30% zinc, 70% copper).
   • Gaseous Solutions: Air is a mixture of gases, primarily oxygen and nitrogen.
3. Components of a Solution:
   • Solvent: The substance that dissolves the other component and is usually present in a larger amount.
   • Solute: The substance that is dissolved in the solvent and is present in a smaller quantity.
4. Examples of Solutions:
   • Sugar in water: Sugar is the solute, and water is the solvent.
   • Tincture of iodine: Iodine (solid) is the solute, and alcohol (liquid) is the solvent.
   • Aerated drinks (soda water): Carbon dioxide (gas) is the solute, and water (liquid) is the solvent.
   • Air: A mixture of gases where oxygen (21%) and nitrogen (78%) are the main constituents.
5. Properties of a Solution:
   • Homogeneous Mixture: The solution has a uniform composition throughout.
   • Particle Size: The particles are smaller than 1 nm (10^-9 meters) in diameter, making them invisible to the naked eye.
   • No Scattering of Light: The small particle size prevents the solution from scattering light, so the path of a light beam is not visible when it passes through the solution.
   • Stable Mixture: The solute particles do not settle over time, making the solution stable.
   • Cannot be Filtered: The solute particles cannot be separated from the solvent by filtration.
6. More to Know: Alloys:
   • Alloys are mixtures of two or more metals, or a metal and a non-metal, which cannot be separated by physical methods.
   • Alloys, despite being mixtures, retain the properties of their components and can have variable compositions.
     • Example: Brass is an alloy made up of 30% zinc and 70% copper.

Questions and Answers:

1. What is a solution? Provide examples.
   • A solution is a homogeneous mixture of two or more substances. Examples include lemonade, soda water, and air.
2. What are the two main components of a solution? Explain their roles.
   • The two main components are:
     • Solvent: The component that dissolves the solute, usually present in a larger amount.
     • Solute: The component that is dissolved in the solvent, usually present in a smaller amount.
3. Give an example of a gas in liquid solution.
   • Aerated drinks (soda water) contain carbon dioxide (gas) as the solute and water (liquid) as the solvent.
4. What are the properties of a solution?
   • A solution is homogeneous, has very small particles that cannot be seen by the naked eye, does not scatter light, cannot be separated by filtration, and is stable.
5. Why can't the solute particles in a solution be seen by the naked eye?
   • The particles of a solution are smaller than 1 nm (10^-9 meters), which makes them too small to be seen by the naked eye.
6. What is an alloy, and why is it considered a mixture?
   • An alloy is a mixture of two or more metals or a metal and a non-metal. It is considered a mixture because it retains the properties of its constituents and can have variable compositions.
7. How does a solution differ from a heterogeneous mixture in terms of particle distribution?
   • In a solution, the particles are evenly distributed at the particle level, resulting in a uniform composition. In a heterogeneous mixture, the particles are not evenly distributed, and distinct parts can be observed.

Activity 2.3: Observing Solubility Changes with Temperature

Steps:

1. Take two beakers, each containing about 50 mL of water.
2. Add salt to one beaker and sugar or barium chloride to the other beaker. Stir continuously until no more solute dissolves. This is the point of saturation.
3. Heat the contents of the beakers, raising the temperature by around 5°C.
4. Add more solute after heating and observe what happens.

• When a solution reaches saturation, no more solute can dissolve at that temperature. However, when the temperature increases, the solubility of the solute usually increases as well, allowing more solute to dissolve. This is because higher temperatures provide more energy to the solvent molecules, helping them break down the solute particles further.
  
  

1. What is a saturated solution?
   • A saturated solution is one in which no more solute can dissolve at a given temperature.
2. What happens to the solubility of solutes like salt and sugar when the temperature is increased?
   • The solubility typically increases, meaning more solute can dissolve at higher temperatures.
3. Why does heating a solution allow more solute to dissolve?
   • Heating provides more energy to the solvent molecules, allowing them to break down and dissolve more solute particles.

Saturated and Unsaturated Solutions

1. Solubility:
   The amount of solute that can dissolve in a solvent at a specific temperature is called the solubility of the solute. Different substances have different solubilities, meaning salt, sugar, and barium chloride dissolve in water in different amounts at the same temperature.
2. Saturated Solution:
   A solution is called saturated when it contains the maximum amount of solute that can dissolve at a given temperature. Beyond this point, no more solute can dissolve in the solution.
3. Unsaturated Solution:
   If a solution contains less solute than its saturation level, it is called an unsaturated solution. More solute can be dissolved in such a solution without reaching saturation.
4. Effect of Cooling on a Saturated Solution:
   • If a saturated solution is cooled slowly, some of the dissolved solute may separate out (precipitate), as the solubility of most substances decreases with temperature.
   • This demonstrates that solubility changes with temperature.
5. Concentration of a Solution:
   The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. It can be expressed in three ways:
   • Mass by Mass Percentage: $$\text{Mass by Mass Percentage} = \left(\frac{\text{Mass of solute}}{\text{Mass of solution}}\right) \times 100$$
     • Example: If 10 grams of sugar are dissolved in 100 grams of water, the solution concentration by mass is: $$\left(\frac{10}{110}\right) \times 100 = 9.09\%$$
   • Mass by Volume Percentage: $$\text{Mass by Volume Percentage} = \left(\frac{\text{Mass of solute}}{\text{Volume of solution}}\right) \times 100$$
     • Example: If 10 grams of salt are dissolved in 100 mL of water, the concentration is: $$\left(\frac{10}{100}\right) \times 100 = 10\%$$
       
   • Volume by Volume Percentage: $$\text{Volume by Volume Percentage} = \left(\frac{\text{Volume of solute}}{\text{Volume of solution}}\right) \times 100$$
     
     • Example: If 50 mL of alcohol is mixed with 100 mL of water, the concentration is: $$\left(\frac{50}{150}\right) \times 100 = 33.33\%$$

1. Is the solubility of different substances the same at a given temperature?
   • No, different substances have different solubilities at the same temperature.
2. What is a saturated solution?
   • A solution that contains the maximum amount of solute that can be dissolved at a given temperature is called a saturated solution.
3. What happens to a saturated solution when it is cooled?
   • When a saturated solution is cooled, the solubility of the solute generally decreases, and some solute may precipitate out.
4. What are the three ways to express the concentration of a solution?
   • The concentration of a solution can be expressed as:
     1. Mass by mass percentage.
     2. Mass by volume percentage.
     3. Volume by volume percentage.
5. What is the concentration if 20 grams of salt are dissolved in 200 grams of water (mass by mass percentage)?
   • The mass by mass percentage would be: $$\left(\frac{20}{220}\right) \times 100 = 9.09\%$$
     

Example 2.1: Calculating the Mass by Mass Percentage of a Solution

Given:

• Mass of solute (salt) = 40 g
• Mass of solvent (water) = 320 g

Step 2: Apply the Mass by Mass Percentage Formula $$\text{Mass by Mass Percentage} = \left(\frac{\text{Mass of solute}}{\text{Mass of solution}}\right) \times 100$$ $$= \left(\frac{40}{360}\right) \times 100 = 11.11\%$$Answer:
The concentration of the solution in terms of mass by mass percentage is 11.11%.

2.2.2 - What is a Suspension?

A suspension is a heterogeneous mixture in which the solute particles are dispersed but do not dissolve in the solvent. The particles remain suspended in the medium, and the suspension may separate if left undisturbed.Properties of a Suspension:

1. Heterogeneous Mixture:
   A suspension is not uniform throughout; it has visible distinct parts.
2. Visible Particles:
   The particles in a suspension are large enough to be seen by the naked eye.
3. Light Scattering (Tyndall Effect):
   Suspensions scatter light, making the path of the beam visible when light passes through them.
4. Unstable:
   If left undisturbed, the particles in a suspension settle at the bottom. This indicates that suspensions are unstable mixtures.
5. Separation by Filtration:
   The particles of a suspension can be separated by filtration, as they are large enough to be caught by a filter.
6. Settling of Particles:
   When the particles settle down, the suspension breaks, and it no longer scatters light.

Questions and Answers:

1. What is a suspension?
   • A suspension is a heterogeneous mixture in which solute particles do not dissolve but remain suspended in the solvent. These particles are visible to the naked eye.
2. What happens to the particles in a suspension if left undisturbed?
   • The particles in a suspension settle down if left undisturbed, making the suspension unstable.
3. Can a suspension be separated by filtration?
   • Yes, the particles in a suspension can be separated from the solvent by the process of filtration.
4. Does a suspension scatter light?
   • Yes, the particles in a suspension scatter a beam of light, making its path visible (Tyndall effect).

2.2.3 - What is a Colloidal Solution?

What is a Colloidal Solution?

A colloidal solution (or colloid) is a type of heterogeneous mixture where tiny particles are uniformly distributed throughout a medium. Although it appears homogeneous due to the small size of the particles, it retains its heterogeneous nature.

Key Characteristics of a Colloidal Solution:

1. Particle Size:
   The particles in a colloid are larger than those in a solution but smaller than those in a suspension, typically ranging from 1 nm to 1000 nm.
2. Uniform Dispersion:
   Colloidal particles are evenly spread throughout the solution, giving it a homogeneous appearance.
3. Invisible Particles:
   Colloidal particles cannot be seen with the naked eye due to their small size.
4. Tyndall Effect:
   Colloids can scatter a beam of visible light, which is known as the Tyndall effect. This effect occurs when light passes through a colloidal solution, making the path of the light beam visible.
5. Examples of Colloidal Solutions:
   Common examples include:
   • Milk: A colloidal suspension of fat globules in water.
   • Fog: Water droplets dispersed in air.
   • Paint: Pigment particles dispersed in a liquid medium.

• The Tyndall effect can be observed when sunlight passes through a dense forest; the mist contains tiny water droplets that act as colloidal particles dispersed in the air.
• The scattering of light by dust and smoke particles in the air can also demonstrate the Tyndall effect, highlighting how light is scattered by small particles.

Questions and Answers:

1. What is a colloidal solution?
   • A colloidal solution is a heterogeneous mixture where tiny particles are uniformly spread throughout the medium, appearing homogeneous.
2. How do colloidal particles behave with light?
   • Colloidal particles scatter light, making the path of the light beam visible due to the Tyndall effect.
3. Can colloidal particles be seen with the naked eye?
   • No, colloidal particles are too small to be seen with the naked eye.
4. What is an example of a colloidal solution?
   • Milk is a common example of a colloidal solution, where fat globules are dispersed in water.

Properties of a Colloid

A colloid is a type of heterogeneous mixture with distinct properties that differentiate it from solutions and suspensions.

Key Properties of a Colloid:

1. Heterogeneous Mixture:
   A colloid consists of two distinct phases—the dispersed phase and the dispersion medium.
2. Particle Size:
   The particles in a colloid are too small to be seen with the naked eye, typically ranging from 1 nm to 1000 nm.
3. Light Scattering:
   Colloids are large enough to scatter a beam of light passing through them, making the light's path visible (Tyndall effect).
4. Stability:
   Colloidal particles do not settle down when left undisturbed, indicating that colloids are stable mixtures.
5. Separation:
   Colloids cannot be separated by standard filtration methods. However, they can be separated using centrifugation, a special technique.

• Dispersed Phase:
  The solute-like component or the particles dispersed throughout the mixture.
• Dispersion Medium:
  The component in which the dispersed phase is suspended (similar to a solvent in a solution).

1. What is the dispersed phase in a colloid?
   • The dispersed phase refers to the solute-like particles that are distributed throughout the colloidal solution.
2. What is the dispersion medium?
   • The dispersion medium is the substance (usually a liquid) in which the dispersed phase is suspended.
3. How can colloidal particles be separated?
   • Colloidal particles cannot be separated by filtration but can be separated using centrifugation.
4. What is the Tyndall effect?
   • The Tyndall effect is the scattering of light by colloidal particles, making the path of the light beam visible.

NCERT Science Notes - Class 9 | Chapter 2 - Is Matter Around Us Pure?

NCERT Science Notes - Class 9 | Chapter 2 - Is Matter Around Us Pure?