Questions Answer Chapter 8 Science Curiosity Class 8 CBSE Board

Nature of Matter: Elements, Compounds, and Mixtures

Keep the curiosity alive

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?

(i) A, B, and C are all compounds and only C has a fixed composition.

(ii) C is a compound, and A and B have a fixed composition.

(iii) A and B are compounds, and C has a fixed composition.

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

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

2. Assertion: Air is a mixture.

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

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

(ii) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion.

(iii) Assertion is true, but Reason is false.

(iv) Assertion is false, but Reason is true.

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

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: Water, a compound, has different properties compared to hydrogen and oxygen because it is formed by a chemical reaction creating a new substance. Hydrogen and oxygen are gases with no fixed shape or volume; hydrogen is flammable, and oxygen supports burning. Water is a liquid with a definite volume, does not burn, and extinguishes fires. These differences arise from the chemical bonding in water molecules (two hydrogen atoms and one oxygen atom), giving it unique physical and chemical properties unlike its constituent elements.

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

(i) Elements – water, nitrogen, iron, air.

(ii) Uniform mixtures- minerals, seawater, bronze, air.

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

(iv) Non-uniform mixtures – air, sand, brass, muddy water

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

Justification: Carbon dioxide (CO₂) and sugar (sucrose) are compounds, made of multiple elements chemically combined with fixed composition. Iron and oxygen (O₂) are elements, also with fixed composition. All are pure substances, as they have uniform properties and cannot be broken down physically.

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

Answer:

• Iron: Element
• Moist air: Mixture
• Iron oxide: Compound
• Magnesium: Element
• Oxygen: Element
• Magnesium oxide: Compound

Justification:

1. Iron: Iron is an element because it is made of only iron atoms and cannot be broken down into simpler substances by chemical reactions, as elements are described as the simplest substances with identical particles.

2. Moist air: Moist air is a mixture because it consists of air (a uniform mixture of gases like nitrogen, oxygen, and carbon dioxide) combined with water vapor, physically mixed without chemical changes, as air is identified as a mixture of gases.

3. Iron oxide: Iron oxide is a compound because it is formed by the chemical reaction of iron and oxygen (from moist air), creating a new substance with a fixed composition and different properties, similar to how water is a compound of hydrogen and oxygen.

4. Magnesium: Magnesium is an element because it consists of only magnesium atoms, like iron, and cannot be broken down into simpler substances.

5. Oxygen: Oxygen is an element because it exists as O₂ molecules made of only oxygen atoms, which cannot be broken down further by chemical reactions.

6. Magnesium oxide: Magnesium oxide is a compound because it is formed by the chemical reaction of magnesium and oxygen, resulting in a new substance with a fixed composition and different properties, like other compounds such as water or iron sulfide.

6. Classify the following as elements, compounds, or mixtures in Table 8.3.Carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur,hydrogen, baking soda.

Answer:

Pure substances:

• Aluminium
• Gold
• Oxygen
• Nitrogen
• Sulfur
• Hydrogen
• Carbon dioxide
• Magnesium oxide
• Rust
• Iron sulfide
• Glucose
• Water
• Sodium chloride
• Baking soda

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 for the reaction.

Answer:- When a mixture of iron filings and sulfur powder is heated, a new substance called iron sulfide is formed.

This is different from the original mixture because:

1. Appearance: The original mixture (Sample A) shows separate black iron filings and yellow sulfur particles, making it a non-uniform mixture. Iron sulfide (Sample B) is a black mass with a uniform texture and color throughout.

2. Magnetic Properties: In the original mixture, iron filings are attracted to a magnet, allowing separation of iron and sulfur. Iron sulfide is not attracted to a magnet, showing that iron and sulfur can no longer be separated.

3. Chemical Properties: In the original mixture, iron reacts with dilute hydrochloric acid to produce hydrogen gas (colorless, odorless, burns with a pop sound), while sulfur does not react. Iron sulfide reacts with dilute hydrochloric acid to produce hydrogen sulfide gas (colorless, with a rotten egg-like odor), indicating it is a new substance with different properties.

Word Equation for the Reaction: Iron + Sulfur → Iron sulfide

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

Answer:- No, a substance cannot be classified as both an element and a compound.

Explanation:

1. An element is a pure substance made of only one type of atom and cannot be broken down into simpler substances by chemical reactions. Examples include hydrogen, oxygen, and iron.

2. A compound is a pure substance formed when two or more different elements combine chemically in a fixed ratio, with properties different from its constituent elements. Examples include water (made of hydrogen and oxygen) and iron sulfide (made of iron and sulfur).

Since a substance is either made of one type of atom (element) or multiple types of atoms chemically bonded (compound), it cannot be both at the same time. For example, water is a compound, not an element, because it consists of hydrogen and oxygen atoms bonded in a 2:1 ratio. Similarly, iron is an element, not a compound, because it is made of only iron atoms.

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

Answer:- If water were a mixture of hydrogen and oxygen instead of a compound, it would be dangerous to use. Hydrogen is flammable and oxygen supports burning, so the mixture could catch fire or explode. We would not be able to use it safely for drinking, cooking, bathing, or putting out fires.

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

Answer : Gas (A) is Hydrogen gas.

Word equation for the reaction:

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 : Two compounds made only from non-metals are:

1. Carbon Dioxide (CO₂) Uses:

• It is used in fire extinguishers to put out fires because it does not support combustion.
• It is used in carbonated drinks like soda to give them fizz.

2. Water (H₂O) Uses:

• It is essential for drinking and keeping our bodies hydrated.
• It is used for cooking food and cleaning purposes.

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

Answer : Gold can be classified as both a mineral and a metal because:

1. As a mineral, gold is found in nature in a pure solid form. It is a native mineral, meaning it occurs naturally without being combined with other elements.

2. As a metal, gold is a shiny, malleable, and ductile element that conducts electricity and heat. It is used in making jewellery, coins, and electronic components.

Thus, gold is a naturally occurring mineral and also a useful metal due to its physical properties