Nature of Matter : Elements, Compounds, and MixturesClass 8 Science NCERT Solutions
12 Solutions
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Solution 1 of 12
Q1Keep the curiosity alive
Consider the following reaction where two substances, A and B , combine to form a product 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)
, 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.
Solution
The correct statement is (iv): A and B are elements, C is a compound, and has a fixed composition.
Elements are substances that cannot be broken down into simpler substances by chemical reactions. Compounds are formed when different elements combine in fixed ratios.
Q2Keep the curiosity alive
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.
Solution
The correct answer is (i): Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
Air is a mixture of gases like nitrogen, oxygen, argon, and carbon dioxide. A mixture is formed when two or more substances are mixed together, where each substance retains its properties and does not react chemically with each other.
Q3Keep the curiosity alive
Water, a compound, has different properties compared to those of the elements oxygen and hydrogen from which it is formed. Justify this statement.
Solution
Water is a compound formed from the elements hydrogen and oxygen. Hydrogen is a flammable gas, and oxygen supports combustion. However, water extinguishes fire. Therefore, the properties of water are different from the properties of the elements that form it. Compounds are formed when different elements combine in fixed ratios to form something entirely new, and the properties of compounds are different from those of elements forming that compound.
Q4Keep the curiosity alive
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.
Solution
The examples are correctly matched in case (iii): Pure substances - carbon dioxide, iron, oxygen, sugar.
- Elements are the simplest substances that cannot be broken down further. Nitrogen and iron are elements, but water and air are not.
- Uniform mixtures have components that are evenly distributed and cannot be distinguished. Seawater and air are uniform mixtures, but minerals and bronze can be non-uniform.
- Pure substances consist of the same type of particles. Carbon dioxide, iron, oxygen, and sugar are pure substances.
- Non-uniform mixtures have components that are generally visible. Sand and muddy water are non-uniform mixtures, but air and brass are uniform mixtures.
Q5Keep the curiosity alive
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.
Solution
- Iron: Element, as it cannot be broken down into simpler substances.
- Moist air: Mixture, as it is a combination of gases and water vapor.
- Iron oxide (rust): Compound, as it is formed by the chemical combination of iron and oxygen.
- Magnesium: Element, as it cannot be broken down into simpler substances.
- Oxygen: Element, as it cannot be broken down into simpler substances.
- Magnesium oxide: Compound, as it is formed by the chemical combination of magnesium and oxygen.
Q6Keep the curiosity alive
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. Identify pure substances amongst these and list them below.
Solution
Table 8.3
| Elements | Compounds | Mixtures |
|---|---|---|
| Aluminium | Carbon dioxide | Sand |
| Gold | Magnesium oxide | Seawater |
| Oxygen | Rust | Muddy water |
| Nitrogen | Iron sulfide | Air |
| Sulfur | Glucose | Fruit juice |
| Hydrogen | Water | Baking soda |
| Sodium chloride |
Pure substances:
Aluminium, Gold, Oxygen, Nitrogen, Sulfur, Hydrogen, Carbon dioxide, Magnesium oxide, Rust, Iron sulfide, Glucose, Water, Sodium chloride
Q7Keep the curiosity alive
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.
Solution
When a mixture of iron filings and sulfur powder is heated, a new substance called iron sulfide is formed. Iron sulfide is different from the original mixture because iron and sulfur are chemically combined, and the properties of the compound are different from those of the elements.
In the original mixture, iron and sulfur retain their individual properties, such as iron being attracted to a magnet. In iron sulfide, iron is no longer attracted to a magnet, and the mixture appears the same throughout.
Word equation: Iron + Sulfur → Iron sulfide
Q8Keep the curiosity alive
Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.
Solution
It is not possible for a substance to be classified as both an element and a compound.
Elements are the simplest substances that cannot be broken down into simpler substances by chemical means. Compounds are formed when two or more elements are chemically combined in a fixed ratio. Therefore, a substance can only be either an element or a compound, but not both.
Q9Keep the curiosity alive
How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?
Solution
If water were a mixture of hydrogen and oxygen instead of a compound, our daily lives would be significantly different. Since mixtures retain the properties of their constituents, water would exhibit the properties of both hydrogen and oxygen. This would mean:
- Water would be flammable due to the presence of hydrogen.
- It would support combustion due to the presence of oxygen.
- It would not be effective for extinguishing fires.
- The chemical properties would be unstable, and it would be difficult to control or use in many applications where water is essential.
Q10Keep the curiosity alive
Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.
Solution
Based on the context of the chapter (Activity 8.3), Fig 8.24 likely depicts the electrolysis of water. In this process, Gas A, which occupies twice the volume of the other gas, is Hydrogen.
The word equation for the chemical reaction is:
Water → Hydrogen + Oxygen
Q11Keep the curiosity alive
Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.
Solution
-
Carbon dioxide (CO2):
- Uses: Used in fire extinguishers to put out fires. Used by plants for photosynthesis.
-
Water (H2O):
- Uses: Essential for life and used for drinking, cleaning, and agriculture. Used as a solvent in many chemical reactions.
Q12Keep the curiosity alive
How can gold be classified as both a mineral and a metal?
Solution
Gold can be classified as both a mineral and a metal because:
- Mineral: Gold occurs naturally in the Earth's crust as a solid substance with a definite chemical composition. Some of the minerals are called native minerals, which are pure elements and not compounds. These can be metals, such as gold, silver, copper, etc., or non-metals like sulfur, carbon, etc.
- Metal: Gold exhibits metallic properties such as luster, conductivity, malleability, and ductility. Elements can be classified into metals and non-metals. Gold, silver, magnesium, iron, and aluminium are metals.