Chapter 2-Is Matter Around Us Pure?

1. Pure Substances vs. Mixtures

In everyday language, “pure” might mean something is not adulterated, like “pure ghee” or “pure milk.” But in science, it has a very specific meaning.

• 🧪 Pure Substance: A substance made up of only one type of particle. All the particles have the same chemical nature.
  • Examples: Salt (sodium chloride), sugar, and iron are pure substances because they are made of only one type of particle.
• 🤝 Mixture: A substance made up of two or more different types of pure substances mixed together.
  • Examples: Milk is a mixture of water, fats, and proteins. The air we breathe is a mixture of gases like oxygen, nitrogen, etc.

2. Types of Mixtures

Mixtures can be divided into two main types based on how their components are distributed.

Homogeneous mixtures have a uniform composition, while heterogeneous mixtures do not.

• Homogeneous Mixture: A mixture that has a uniform composition throughout. You can’t see the different components separately.
  • Another name for a homogeneous mixture is a solution.
  • Examples: Salt dissolved in water, sugar dissolved in water, or lemonade.
• Heterogeneous Mixture: A mixture that has a non-uniform composition. You can often see the different parts.
  • Examples: A mix of sand and water, oil and water, or a salad.

3. What is a Solution?

A solution is a homogeneous mixture of two or more substances. It has two parts:

• Solute: The substance that is dissolved (usually in a smaller amount).
• Solvent: The substance that does the dissolving (usually in a larger amount).

Example: In saltwater, salt is the solute and water is the solvent.

3.1 Properties of a Solution

• It’s a homogeneous mixture.
• The particles are extremely small (less than 1 nanometer) and can’t be seen with the naked eye.
• It is stable, meaning the solute particles don’t settle down.
• It does not scatter a beam of light, so the path of light is not visible through it.

4. What is a Suspension?

A suspension is a heterogeneous mixture where the solute particles do not dissolve but remain suspended in the solvent.

4.1 Properties of a Suspension

• It’s a heterogeneous mixture.
• The particles are large enough to be seen with the naked eye.
• It is unstable; the particles settle down if left undisturbed.
• It scatters a beam of light, making the path of light visible (this is called the Tyndall effect).

Example: Muddy water or chalk powder in water.

5. What is a Colloid?

A colloid is a mixture that appears homogeneous but is actually heterogeneous. The particle size is between that of a solution and a suspension.

Colloids and suspensions exhibit the Tyndall effect, while true solutions do not.

5.1 Properties of a Colloid

• It appears to be homogeneous.
• The particles are too small to be seen with the naked eye.
• It is stable; the particles do not settle down.
• It shows the Tyndall effect (scatters light).

Examples: Milk, fog, and shaving cream.

6. Physical vs. Chemical Changes

• 🏃‍♀️ Physical Change: A change where only the physical properties (like color, hardness, or state) of a substance change. No new substance is formed.
  • Examples: Melting ice, boiling water, or cutting a piece of paper.
• 🔥 Chemical Change: A change where a new substance with completely new properties is formed. This is also called a chemical reaction.
  • Examples: Burning wood, rusting of iron, or cooking an egg.

7. Types of Pure Substances

Pure substances are classified into two categories: elements and compounds.

• Elements: The most basic form of matter that cannot be broken down into simpler substances by chemical reactions.
  • Elements can be metals (like iron, gold), non-metals (like oxygen, hydrogen), or metalloids (which have properties of both, like silicon).
  • Example: Iron is an element. You can’t break it down into anything simpler.
• Compounds: A substance formed when two or more elements are chemically combined in a fixed proportion.
  • The properties of a compound are completely different from its constituent elements.
  • Example: Water (H₂O) is a compound made of hydrogen and oxygen. Its properties are very different from hydrogen (a combustible gas) and oxygen (a gas that supports combustion).

8. Extensive Practice Set

Here is a set of original questions to test your understanding of the chapter.

Part A: Multiple-Choice Questions (MCQs)

1. Which of the following is considered a pure substance in science?
   a) Milk
   b) Air
   c) Iron
   d) Lemonade
2. A mixture of oil and water is an example of:
   a) A homogeneous mixture
   b) A heterogeneous mixture
   c) A solution
   d) A colloid
3. The scattering of a beam of light by colloidal particles is known as:
   a) The Reflection Effect
   b) The Tyndall Effect
   c) The Dissolving Effect
   d) The Suspension Effect
4. Which of the following is a chemical change?
   a) Melting of wax
   b) Tearing a piece of paper
   c) Burning of a candle
   d) Dissolving sugar in water
5. Which of the following best describes an element?
   a) A substance made of two or more elements chemically combined.
   b) A substance that cannot be broken down into simpler substances by chemical means.
   c) A uniform mixture of two or more substances.
   d) A non-uniform mixture of two or more substances.

Part B: Short Answer Questions

6. Differentiate between a solute and a solvent, using the example of a sugar solution.
7. Give two properties of a suspension that make it different from a solution.
8. Why is milk considered a colloid and not a solution?
9. Classify the following into physical or chemical changes:
   • Rusting of an almirah
   • Boiling water to make steam
   • Making a fruit salad
   • Digestion of food
10. What is the difference between a mixture and a compound? Give one example of each.

Part C: Long Answer Questions

11. Explain the difference between homogeneous and heterogeneous mixtures with at least two examples for each.
12. Pragya wants to separate a mixture of sand, salt, and water. Describe the steps she should follow to separate all three components.
13. Describe the properties of a colloid with suitable examples. Why is a colloid considered a stable mixture?
14. You are given a colorless liquid. How would you determine if it is a pure substance (like distilled water) or a solution (like saltwater)?
15. What are elements and compounds? Explain how they are different from each other, focusing on their composition and properties. Give two examples of each.