Students can use Curiosity Class 8 Science Book Solutions Chapter 8 Nature of Matter: Elements, Compounds, and Mixtures Class 8 Question and Answer as a quick reference guide.
Class 8 Science Chapter 8 Nature of Matter: Elements, Compounds, and Mixtures Question Answer
Science Class 8 Chapter 8 Question Answer Nature of Matter: Elements, Compounds, and Mixtures
Nature of Matter: Elements, Compounds, and Mixtures Class 8 Question Answer (InText)
Question 1.
Probe and Ponder (Page 116)
- Which of the entities in the picture (refer NCERT page 116) consist of matter, and which of them do not?
- How can elements be combined to form a compound?’
- How could the discovery of a compound that absorbs carbon dioxide from the air contribute to solving environmental challenges?
- Share your questions
……………………………………………………..
…………………………………………………….. ?
Answer:
(Questions may vary)
(i) How might the use of alloys instead of pure metals benefit industries like construction or medicine?
(ii) How does knowing the difference between uniform and non-uniform mixtures help in designing better food, medicine, or cosmetics?
Question 2.
Can you identify a few more examples of nonuniform mixtures around you? (Page 117)
Answer:
Sprout salad, soil, muddy water, fruit salad, pizza and dust in the air.
Question 3.
Can you list a few uniform mixtures? (Page 117)
Answer:
Salt water, sugar solution, air, alloys, soda water, lemonade and stainless steel.
Question 4.
Is air a mixture? What kind of mixture is it? (Page 118)
Answer:
Air is a uniform mixture. It consists of gases like nitrogen, oxygen, carbon dioxide, argon, and water vapour that are physically combined. The composition of air is consistent throughout, and its components are evenly distributed.
Question 5.
Have you ever noticed tiny shining particles moving in a beam of sunlight entering a dark room through a small opening? What are these particles? (Page 119)
Answer:
Yes, the tiny shining particles seen moving in a beam of sunlight entering a dark room are dust particles. These particles are suspended in the air and become visible when sunlight passes through them. They are not part of the air itself but are pollutants present in it.
Question 6.
According to science, how would you classify milk, packed fruit juice, baking soda, sugar, and soil – as mixtures or pure substances? (Page 121)
Answer:
According to science, milk, packed fruit juice, and soil are mixtures because they contain different substances physically combined without fixed composition. Baking soda and sugar are pure substances; baking soda is a compound with a definite chemical composition, and sugar is also a compound made of carbon, hydrogen, and oxygen in a fixed ratio.
Question 7.
When an electric current is passed through water, it breaks down into hydrogen and oxygen. Is this a chemical change or a physical change? (Page 123)
Answer:
When electric current is passed through water and it breaks down into hydrogen and oxygen, it is a chemical change. This is because new substances with different properties are formed, and the water moiecules are chemically decomposed into hydrogen and oxygen gases.
Question 8.
Why cannot we separate hydrogen and oxygen present in water by physical means? (Page 124)
Answer:
We cannot separate hydrogen and oxygen present in water by physical means because they are chemically combined in a fixed ratio to form a compound. Physical methods like filtration or evaporation only separate mixtures, not compounds.
NCERT Class 8 Science Chapter 8 Question Answer Nature of Matter: Elements, Compounds, and Mixtures (Exercise)
Keep the curiosity alive(Pages 131-132)
Question 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 cannot be broken down into simpler substances by chemical reactions. Hence, A and B are elements.
C is formed by the chemical combination of A and B. This makes C a compound. Compounds have a fixed composition (a specific ratio of elements). So, statement (iv) is correct.
Question 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.
Question 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 is formed when hydrogen and oxygen combine chemically in a fixed ratio (2: 1). This process results in a new substance with entirely different properties from its components.
Comparison of properties:
| Substance | Properties |
| Hydrogen | A flammable gas that can catch fire easily. |
| Oxygen | A gas that supports combustion (helps things burn). |
| Water | A liquid at room temperature that is used to extinguish fire. It does not burn or support burning. |
Question 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:
(i) Elements-water, nitrogen, iron, air Water is a compound, nitrogen is an element, iron is an element and air is a mixture of gases.
(ii) Uniform mixtures-minerals, seawater, bronze, air
Minerals can be compounds or elements, not mixtures, seawater is a uniform mixture (soluble solid + liquid), bronze is an alloy (uniform solid mixture of copper and tin) and air is a uniform gas mixture.
(iii) Pure substances-carbon dioxide, iron, oxygen, sugar
Carbon dioxide is a compound (pure substance), iron is an element (pure substance), oxygen is an element (pure substance) and sugar is a compound (pure substance made of carbon, hydrogen, and oxygen).
(iv) Non-uniform mixtures-air, sand, brass, muddy water
Air is a uniform mixture (gases are evenly mixed), sand is a non-uniform mixture, brass is an alloy, which is a uniform mixture, and muddy water is a nonuniform mixture (insoluble solid + liquid).
Question 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:
Reaction 1: Iron reacts with moist air to form iron oxide,
Iron + Moist air → Iron oxide.
- Iron is an element as it consists of only one type of atom and cannot be broken down further.
- Moist air is a mixture as it consists of a uniform mixture of gases (nitrogen, oxygen, etc.)
- Iron oxide is a compound formed by a chemical combination of iron and oxygen in a fixed ratio and has different properties from its elements.
Reaction 2: Magnesium burns in oxygen to form magnesium oxide,
Magnesium + Oxygen → Magnesium oxide.
- Magnesium is an element, as it is a metal made of one type of atoms.
- Oxygen is an element as it is a non-metallic gas made of oxygen atoms.
- Magnesium oxide is a compound formed by the chemical reaction between magnesium and oxygen. It has properties different from its elements.
Question 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 sulphide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulphur, hydrogen, baking soda.
Identify pure substances amongst these and list them below.
Answer:
Table 8.3: Classification of Substances
All the elements and compounds are generally pure substances.
|
Pure substances |
| Gold, Oxygen, Nitrogen, Sulphur, Hydrogen, Carbon dioxide, Magnesium oxide,Rust, Iron sulphide, Glucose, Water, Sodium chloride, Baking soda |
Question 7.
What new substance is formed when a mixture of iron filings and sulphur 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 sulphur powder is heated, a new substance called iron sulphide is formed. The differences between iron sulphide and original mixture are as follows:
| Original Mixture | Iron Sulphide (Product) | ||
| (i) | Physical mixture of iron and sulphur | A chemical compound | |
| (ii) | Components can be separated by physical process. | Cannot be separated by physical process. | |
| (iii) | Iron retains its magnetic property. | Becomes nonmagnetic after reaction | |
| (iv) | No new substance is formed. | New substance with different properties is formed. | |
Word equation for the reaction:
Iron + Sulphur → Iron sulphide
Question 8.
Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.
Answer:
No, it is not possible for a substance to be classified as both an element and a compound. An element is a pure substance made of only one kind of atoms, and a compound is a pure substance made from two or more different elements chemically combined in a fixed ratio.
Question 9.
How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?
Answer:
Water is a stable compound made by chemical combination of hydrogen and oxygen gases in 2:1 ratio. It has unique properties, like it puts out fires, supports life and has a stable boiling and freezing point.
If water were just a mixture of hydrogen and oxygen. It would not support life as a mixture of hydrogen and oxygen would not be a liquid, and it would be toxic.
It would be dangerous and unstable, as hydrogen is a highly explosive gas, and oxygen supports combustion. Our entire planet’s ecosystem would collapse.
Question 10.
Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.
Answer:
When iron reacts with dilute hydrochloric acid, hydrogen gas is released.
So, Gas A is hydrogen.
Word equation for the reaction:
Question 11.
Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.
Answer:
(i) Carbon dioxide is made of carbon and oxygen (both non-metals).
- It is used in fire extinguishers to put out fires, as it does not support combustion.
- It is used by plants in photosynthesis to make food and release oxygen.
(ii) Water is made of hydrogen and oxygen (both non-metals).
- It is essential for drinking and for the survival of all living organisms.
- It is used as a solvent in industries and labs to dissolve many substances.
Question 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, in nature, gold is found in the earth’s crust in its native form. A mineral is a naturally occurring solid with a definite chemical composition and crystal structure. Gold is also classified as a metal because of its physical and chemical properties. It is shiny (lustrous), a good conductor of heat and electricity, malleable and ductile.
Class 8 Particulate Nature of Matter: Elements, Compounds, and Mixtures Question Answer (Activities)
Activity 8.1: Let Us Experiment (Pages 118-119)
Aim: To make lime water and to confirm the presence of carbon dioxide in the air.
Observation: When a small amount of calcium oxide is slowly added to water, it reacts vigorously and releases heat. On continuous stirring, a solution of calcium hydroxide is formed. This solution, called lime water, is colourless after filtering. When left in a petri dish for a few hours and stirred at regular intervals, the solution turns milky.
Conclusion: The lime water turns milky when exposed to air because carbon dioxide in the air reacts with calcium hydroxide to form calcium carbonate, which appears as tiny white particles.
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
This demonstrates the presence of carbon dioxide in the air.
Activity 8.2: Let Us Explore (Pages 119-120)
Aim: To observe dust particles suspended in the air using a black sheet of paper.
Observation: When a clean black sheet of paper is placed near an open window or in a garden for a few hours, tiny particles settle on its surface. Using a magnifying glass, these particles can be seen more clearly.
Conclusion: The particles settled on the black sheet show that dust particles are suspended in the air. These dust particles are not a part of the air and are considered as pollutants. The amount and type of dust particles in the air can change depending on the time and location.
Activity 8.3: let us experiment (demonstration activity) (pages 121-122)
Aim: To pass electricity through water and observe the gases produced, showing that water is composed of hydrogen and oxygen.
Observation:
- Gas bubbles were observed forming at both terminals inside the test tubes.
- Unequal volumes of gas were collected in the two test tubes after 10-15 minutes.
- When a burning candle was brought near the mouth of one test tube, a pop sound was observed, indicating the presence of hydrogen gas.
- In the other test tube, the candle flame glowed brighter, confirming the presence of oxygen gas.
Conclusion:
The experiment shows that water is made up of two different gases-hydrogen and oxygen. These gases are not water vapour, as they do not condense back to form water. When an electric current passes through water, it splits into these gases viz., hydrogen and oxygen. This confirms that water is a compound formed by the chemical combination of two elements, hydrogen and oxygen.
Activity 8.4: Let Us Experiment (Pages 124-125)
Aim: To heat sugar and observe its decomposition into its constituent elements.
Observation: When sugar is gently heated in a boiling tube, it first turns brown and then black as it starts to char. Small droplets of water appear near the open end of the boiling tube. After complete heating, a black solid (charcoal or carbon) is left behind in the tube.
Conclusion: This activity shows that sugar breaks down into water and carbon when heated. The water droplets came from the sugar itself, not from the air. This proves that sugar is not an element, but a compound made up of carbon, hydrogen, and oxygen.
Activity 8.5: let us experiment (demonstration activity) (pages 125-128)
Aim: To observe the difference between a mixture and a compound of iron and sulphur.
Answer:
Conclusion: Sample A is a mixture of iron and sulphur. We can see both substances clearly. Iron keeps its magnetic property and reacts with dilute hydrochloric acid to produce a gas that burns with a ‘pop’ sound (hydrogen). Sulphur does not react and stays at the bottom. The components can be separated by using a magnet.
Sample B is a compound called iron sulphide. It is black and looks the same throughout. It does not get attracted to a magnet. It reacts with dilute hydrochloric acid and produces a gas with a rotten egg smell (hydrogen sulphide). The properties are completely different from iron and sulphur. The components cannot be separated.