Chapter 8: Nature of Matter

Probe and Ponder (Page 1)

Question: Which of the entities in the picture above consist of matter, and which of them do not?

Answer: The students, the books, the bags, the clothes, and the air around them consist of matter because they have mass and take up space. Things like thoughts, ideas, light, or the sound of their voices do not consist of matter.

Question: How can elements be combined to form a compound?

Answer: Elements can be combined to form a compound by reacting them chemically in fixed ratios. For example, hydrogen and oxygen combine to form water.

Question: How could the discovery of a compound that absorbs carbon dioxide from the air contribute to solving environmental challenges?

Answer: Such a compound could help reduce the amount of carbon dioxide in the air, which is a major pollutant and greenhouse gas. This would help clean the air and improve air quality.

In-text Questions and Activities

Activity 8.1: Let us experiment (Page 3)

Explanation of the Activity:

In this activity, we want to check if carbon dioxide is present in the air. We use a clear liquid called lime water (calcium hydroxide). When carbon dioxide mixes with lime water, it creates a white substance called calcium carbonate that makes the water look milky or cloudy. We leave a dish of lime water open to the air to see if this change happens.

Question: What do you observe?

Answer: You observe that the clear lime water turns milky after being exposed to air for a few hours.

Question: Can you explain why the solution has turned milky?

Answer: The solution turned milky because carbon dioxide present in the air reacted with the lime water (calcium hydroxide). This reaction formed calcium carbonate, which consists of tiny white particles that do not dissolve, making the water look cloudy.

Word Equation: Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water.

Activity 8.2: Let us explore (Page 4)

Explanation of the Activity:

This activity helps us see dust particles that are floating in the air but are usually invisible. By placing a clean black sheet of paper near an open window or in a garden, we can catch these particles as they settle down. The black background makes the light-colored dust easier to see.

Question: What do you observe?

Answer: You observe tiny particles settled on the surface of the black paper.

Question: What are these particles?

Answer: These are dust particles that were suspended in the air. This shows that air contains solid pollutants like dust.

Table 8.1: Different types of mixtures (Page 5)

Question: Complete the third column.

S.No.	Mixture-type	Examples	Uniform or non-uniform
1.	Gas and gas	Air	Uniform
2.	Gas and liquid	Aerated water (soda water)	Uniform
3.	Solid and gas	Carbon particles in air (Smoke/Dust)	Non-uniform
4.	Liquid and liquid	Oil and water	Non-uniform
5.	Solid and liquid	Sand and water (muddy water)	Non-uniform
		Seawater	Uniform
6.	Solid and solid	Baking powder	Uniform
		Alloys (like Brass or Steel)	Uniform

In-text Question (Page 6): According to science, how would you classify milk, packed fruit juice, baking soda, sugar, and soil—as mixtures or pure substances?

Answer:

• Mixtures: Milk (mixture of water, fats, proteins), Packed fruit juice (mixture of water, sugar, fruit pulp), Soil (mixture of sand, clay, minerals).
• Pure Substances: Baking soda (compound), Sugar (compound).

Activity 8.3: Let us experiment (Page 6-7)

Explanation of the Activity:

This activity demonstrates electrolysis. We pass electricity from a 9V battery through water (mixed with a little acid) to break it down. We place test tubes filled with water over the battery terminals. As electricity flows, it splits water into its building blocks: hydrogen gas and oxygen gas, which collect in the tubes.

Question: Do you observe the formation of any gas bubbles at both the terminals inside the test tubes?

Answer: Yes, gas bubbles form at both terminals inside the test tubes.

Question: Is the volume of the gas collected the same in both the test tubes?

Answer: No, the volume is not the same. One test tube has about double the gas compared to the other. This is because water (H₂O) has two parts hydrogen for every one part oxygen.

Question: What happens in each case? (Testing the gases)

Answer:

• Tube 1: When a burning candle is brought near the gas, it burns with a “pop” sound.
• Tube 2: When a burning candle is brought near the gas, the flame glows brighter.

Question: Which gas is present in each test tube?

Answer:

• The gas that made the “pop” sound is Hydrogen.
• The gas that made the flame brighter is Oxygen.

Question: Can these collected gases be water vapour?

Answer: No. If they were water vapour, they would have turned back into liquid water when cooled. Instead, they remained as gases.

In-text Question (Page 8): When electric current is passed through water, it breaks down into hydrogen and oxygen. Is this a chemical change or a physical change?

Answer: This is a chemical change because a new substance (hydrogen and oxygen gases) is formed from the original substance (water), and the properties of the new substances are different from water.

Activity 8.4: Let us experiment (Page 9-10)

Explanation of the Activity:

We want to see if sugar is an element or a compound. We heat dry sugar in a boiling tube. If it breaks down into simpler things, it is a compound. If it stays the same, it might be an element.

Question: What do you observe?

Answer: The sugar first turns brown and then turns into a black charred mass. Also, small water droplets appear on the upper, cooler parts of the tube.

Question: Can you predict what is left behind?

Answer: The black substance left behind is Charcoal (Carbon).

Question: Is sugar an element or a compound?

Answer: Sugar is a compound. It decomposed into Carbon and Water (which is made of Hydrogen and Oxygen) upon heating, proving it is made of simpler substances.

Activity 8.5: Let us experiment (Page 10-12)

Explanation of the Activity:

This activity compares a Mixture versus a Compound.

• Sample A (Mixture): We simply mix iron filings and sulfur powder together.
• Sample B (Compound): We heat the mixture of iron and sulfur until it turns into a black mass. This causes a chemical reaction, creating a new substance called Iron Sulfide.

Question: Is Sample A a uniform or a non-uniform mixture? Can you still observe both iron and sulfur as separate substances?

Answer: Sample A is a non-uniform mixture. You can distinctively see the yellow sulfur particles and the grey iron filings separately.

Question: Compare both the Samples A and B step by step and record your observations in Table 8.2.

Table 8.2: Comparison of Samples A and B

S.No.	Experiment	Sample A (Mixture)	Sample B (Compound)
1.	Appearance	Observations: Yellow powder and grey filings visible separately. (Non-uniform)	Observations: A black mass. The texture and colour are the same throughout. (Uniform)
2.	Magnet test	Observations: Iron filings are attracted to the magnet. Sulfur is left behind.	Observations: No effect. The material is not attracted to the magnet.
3.	Gas test (with dilute HCl)	Observations: (i) Odour: Odourless gas. (ii) Burning: Burns with a ‘pop’ sound (Hydrogen gas).	Observations: (i) Odour: Smell of rotten eggs (Hydrogen Sulfide gas).  (ii) Burning: Does not make a pop sound.

Discussion Points (Page 12):

• Can we separate the components of Samples A and B?
  • Sample A: Yes, physically (using a magnet).
  • Sample B: No, not by physical means (magnet doesn’t work).
• Why does the magnet have no effect on Sample B?
  • Because the iron has chemically reacted with sulfur to form a new compound (Iron Sulfide) which does not have magnetic properties.

Keep the curiosity alive (Exercises – Page 16-17)

1. Consider the following reaction where two substances, A and B, combine to form a product C: A + B → C. Assume that A and B cannot be broken down into simpler substances by chemical reactions. Based on this information, which of the following statements is correct?

Answer: (iv) A and B are elements, C is a compound, and has a fixed composition.

Reasoning: If A and B cannot be broken down, they must be elements. When elements combine chemically, they form a compound (C) with a fixed ratio.

2. Assertion: Air is a mixture. Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change.

Answer: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.

Reasoning: Air is made of gases like nitrogen, oxygen, and argon that retain their own properties and are not chemically combined.

3. Water, a compound, has different properties compared to those of the elements oxygen and hydrogen from which it is formed. Justify this statement.

Answer: Hydrogen is a fuel (it burns), and Oxygen supports burning. However, Water (the compound formed by them) neither burns nor supports burning; instead, it is used to extinguish fire. This shows the compound has completely different properties from its constituent elements.

4. In which of the following cases are all the examples correctly matched? Give reasons in support of your answers.

Answer: (iii) Pure substances- carbon dioxide, iron, oxygen, sugar.

Reasoning:

• (i) is wrong because “air” is a mixture.
• (ii) is wrong because “minerals” can be pure elements (like Gold) or compounds, not just mixtures. “Seawater” and “Air” are mixtures, but Bronze is an alloy (mixture).
• (iii) is correct: Carbon dioxide (compound), Iron (element), Oxygen (element), and Sugar (compound) are all pure substances because they consist of only one type of particle.
• (iv) is wrong because “brass” (alloy) and “air” are typically uniform mixtures.

5. Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify all the substances involved… as elements, compounds or mixtures, with justification.

Answer:

• Iron (Element): Cannot be broken down further.
• Moist Air (Mixture): Contains water vapour, oxygen, nitrogen, etc. mixed together.
• Iron Oxide (Compound): Formed by the chemical combination of iron and oxygen.
• Magnesium (Element): A pure metal.
• Oxygen (Element): A pure non-metal gas.
• Magnesium Oxide (Compound): Formed by the chemical combination of magnesium and oxygen in a fixed ratio.

6. Classify the following as elements, compounds, or mixtures in Table 8.3.

Answer:

Elements	Compounds	Mixtures
Aluminium	Carbon dioxide	Sand
Gold	Magnesium oxide	Seawater
Oxygen	Glucose	Muddy water
Nitrogen	Water	Air
Sulfur	Sodium chloride	Fruit juice
Hydrogen	Baking soda	Rust (hydrated iron oxide)
	Iron sulfide

Note: Rust acts like a compound (Iron Oxide), but rust flakes are often considered a mixture of oxides and hydroxides. In this context, it is usually classified as a compound or product of reaction.

Identify pure substances amongst these:

Answer: The pure substances are all the Elements and Compounds listed above (Aluminium, Gold, Oxygen, Nitrogen, Sulfur, Hydrogen, Carbon dioxide, Magnesium oxide, Glucose, Water, Sodium chloride, Baking soda, Iron sulfide).

7. What new substance is formed when a mixture of iron filings and sulfur powder is heated, and how is it different from the original mixture? Also, write the word equation.

Answer: The new substance formed is Iron Sulfide.

• Difference: The original mixture was magnetic (iron was attracted) and yellow/grey in color. The new substance (Iron Sulfide) is a black mass, is not magnetic, and has different chemical properties (e.g., smell of rotten eggs with acid).
• Equation: Iron + Sulfur → Iron Sulfide (on heating).

8. Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.

Answer: No. An element is made of only one kind of atom and cannot be broken down. A compound is made of two or more different elements combined chemically. A substance cannot be both single and combined at the same time.

9. How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?

Answer:

1. It would be a gas, not a liquid, so we couldn’t drink it.
2. It would be highly dangerous because hydrogen burns explosively and oxygen helps burning. Lighting a match near “water” would cause an explosion instead of putting out a fire.

10. Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.

Answer:

• Gas A: Hydrogen gas.
• Explanation: The figure shows Iron filings reacting with Dilute hydrochloric acid. This reaction produces Iron chloride and Hydrogen gas.
• Equation: Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas.

11. Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.

Answer:

1. Water (Hydrogen + Oxygen): Used for drinking and cleaning.
2. Carbon Dioxide (Carbon + Oxygen): Used by plants for photosynthesis and in fire extinguishers.

12. How can gold be classified as both a mineral and a metal?

Answer:

• As a Metal: Gold is an element with properties like shininess and malleability, so it is chemically a metal.
• As a Mineral: Gold is found naturally in the earth’s crust in its solid, pure form (native mineral). Since minerals are natural solid substances, gold is also a mineral.