Nature of Matter Elements Compounds and Mixtures Class 8 Question Answer Science Chapter 8

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Class 8 Science Curiosity Chapter 8 Question Answer

Class 8 Science Ch 8 Nature of Matter Elements Compounds and Mixtures Question Answer

Class 8 Science Chapter 8 Nature of Matter Elements Compounds and Mixtures Question Answer (InText)

Question 1.
How can elements be combined to form a Compound? (Page 116)
Answer:
Elements combine to form a compound through a chemical reaction. They join in a fixed ratio to make a new substance with different properties.

Question 2.
How could the discovery of a compound that absorbs carbon dioxide from the air contribute to solving environmental challenges? (Page 116)
Answer:
The compound could reduce carbon dioxide levels in the air, helping to fight global warming. It would make the air cleaner and slow down climate change.

Question 3.
List a few uniform mixtures. (Page 117)
Answer:
Salt water, sugar dissolved in water, air, vinegar and brass (a mixture of copper and zinc).

Question 4.
Is air a mixture? What kind of mixture is it? (Page 118)
Answer:
Yes, air is a mixture. It is a uniform mixture of gases like nitrogen, argon, water vapour, oxygen and carbon dioxide.

Question 5.
Apart from gases, have you ever observed anything else present in the air? What are these particles?
(Page 119)
Answer:
Yes, sometimes dust, smoke and soot particles can be seen in the air. These are tiny solid or liquid dust particles.

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:
Milk – Mixture
Packed fruit juice – Mixture
Baking soda – Pure substance (compound)
Sugar – Pure substance (compound)
Soil – Mixture

Question 7.
When electricity is passed through water some gases are evolved. Can these collected gases be water vapour? (Page 122)
Answer:
No, these gases cannot be water vapour. If they were, they would condense back to form water. These gases are likely hydrogen and oxygen formed during electrolysis of water.

Question 8.
When 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:
This is a chemical change, because new substances (hydrogen and oxygen gases) are formed which are entirely different from water.

Question 9.
Why cannot we separate hydrogen and oxygen present in water by physical means? (Page 124)
Answer:
We cannot separate hydrogen and oxygen in water by physical means because water is a compound. Only a chemical change can break water into its elements.

Question 10.
Are common salt and sugar elements or compounds? (Page 124)
Answer:
Common salt and sugar are made of more than one element chemically combined. So, they are classified as compounds.

Question 11.
Is it possible to separate sodium chloride into its elements by physical processes? (Page 124)
Answer:
No, sodium chloride (common salt) is a compound, and compounds cannot be separated by physical processes. They can only be separated by chemical methods.

Nature of Matter Elements Compounds and Mixtures Class 8 Questions and Answers (Exercise)

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?
(a) A, B and Care all compounds and only Chas a fixed composition.
(b) C is a compound, and A and B have a fixed composition.
(c) A and B are compounds, and C has a fixed composition.
(d) A and B are elements, Cis a compound, and has a fixed composition.
Answer:
(d) Consider the reaction A + B → C. A and B cannot be broken into simpler substances. So, they are elements. C is formed by a chemical reaction, so it is a compound and has a fixed composition.

Question 2.
Assertion (A) Air is a mixture.
Reason (R) A mixture is formed when two or more susbtances are mixed, without undergoing any chemical change.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true, but R is false
(d) A is false, but R is true
Answer:
(a) Both A and R are true and R is the correct explanation for A. Air is a mixture and it contains gases like oxygen, nitrogen, carbon dioxide, etc. mixed physically and not chemically.

Question 3.
Water, a compound, has different properties compared to those of the elements oxygen and hydrogen form which it is formed. Justify this statement.
Answer:
Water is formed by the chemical combination of hydrogen and oxygen. Hydrogen is a gas which burns easily and oxygen also supports burning. But water is a liquid and puts out fire. So, water has new properties that are very different from the elements that make it.

Question 4.
In which of the following cases are all the examples correctly matched? Give reasons in support of your answers.
(a) Elements — water, nitrogen, iron, air.
(b) Uniform mixtures — minerals, seawater, bronze, air.
(c) Pure substances — carbon dioxide, iron, oxygen, sugar.
(d) Non-uniform mixtures — air, sand, brass, muddy water.
Answer:
(c) Nitrogen and iron.are elements, but water and air are not.

Seawater, bronze, and air are uniform mixtures. Minerals are not always uniform, they are usually mixtures of different substances.
All are either elements (iron, oxygen) or compounds (carbon dioxide, sugar). So, this option (c) is correct as all the of the examples given are pure substances.
Sand and muddy water are non-uniform. Air and brass are uniform mixtures.

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:
Iron – Element
Air – Mixture
Iron oxide – Compound
Magnesium – Element
Oxygen – Molecule of an element
Magnesium oxide – Compound

Question 6.
Classify the following as elements, compounds, or mixtures in given table.
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.

Elements	Compounds	Mixtures

Answer:

Elements	Compounds	Mixtures
Aluminium	Carbon dioxide	Sand
Gold	Magnesium oxide	Seawater
Oxygen	Rust	Muddy water
Sulphur	Glucose	Air
Nitrogen	Sodium chloride	Fruit juice
Hydrogen	Baking soda
	Iron sulphide
	Water

Question 7.
What new substance is formed when a mixture of iron filings and sulphur powder is heated, and how is it different form the original mixture? Also, write the word equation for the reaction.
Answer:
New substance formed from heating iron filings and sulphur powder is iron sulphide. It is a compound and has new properties.

In the original mixture, iron and sulphur could be separated easily. But after heating, they react chemically to form iron sulphide, which is black and cannot be separated easily.

Word equation:
Iron + Sulphur → Iron sulphide

Question 8.
It is 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 both an element and a compound. Element is made of only one kind of atom. Whereas, compound is made of two or more elements chemically combined. So, they are different types of substances.

Question 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 just a mixture of hydrogen and oxygen then it could explode when lit (as hydrogen is flammable). We would not be able to drink or use it safely. Its properties would not be fixed. So, life would be impossible without water as a compound.

Question 10.
Analyse given figure. Identify Gas A. Also, write the word equation of the chemical reaction.

Answer:
Hydrogen gas (Gas A) is produced when iron reacts with dilute hydrochloric acid.
Word Equation:
Iron + Hydrochloric acid → Iron chloride + Hydrogen gas

Question 11.
Write the names of any two compounds made only form non-metals, and also mention two uses of each of them.
Answer:
Two compounds made only from non-metals and their uses are as follows :
(i) Carbon dioxide (CO₂): Used in fire extinguishers. Plants also use it for photosynthesis.
(ii) Water (H₂O): Essential for life and used for drinking and cleaning.

Question 12.
How can gold be classified as both a mineral and a metal?
Answer:
Gold is classified as a mineral because it is found in nature in rocks, in the form of native mineral. After refining, gold shows properties of metals like lustre, ductility, and good conductor of electricity. So, gold is a metal in pure form and a mineral when mined from earth.

Class 8 Science Chapter 8 Question Answer (Activities)

Activity 1 (Page 118)

Aim
To prepare lime water and demonstrate the presence of carbon dioxide in air.

Materials Required
Water, glass tumbler, calcium oxide (quick-lime), stirring rod, filter paper, petri dish, marker.

Procedure

1. Fill a glass tumbler halfway with water.
2. Slowly add a small amount of calcium oxide into the water.
3. Stir continuously until a clear solution forms (lime water).
4. Filter the solution to remove undissolved solids and collect the clear filtrate.
5. Pour the colourless lime water into a clean petri dish.
6. Leave it open at room air for a few hours, stirring at regular intervals. .

Observation
Initially, calcium oxide reacts vigorously with water to form calcium hydroxide and releases heat. Later, the solution becomes clear and colourless. After some time, the solution becomes milky.

Conclusion
The solution turns milky because carbon dioxide in the air reacts with calcium hydroxide to form insoluble calcium carbonate. This demonstrates the presence of C02 gas in the environment.
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Viva Questions

1. What is produced when calcium oxide reacts with water?
2. Why does lime water turn milky when exposed to air?

Activity 2 (Page 119)

Aim
To demonstrate that air contains suspended dust particles.

Materials Required
Black sheet of paper, open window or outdoor environment and magnifying glass.

Procedure

1. Ensure the black sheet of paper is clean and free from visible dust.
2. Place it undisturbed near an open window or outdoors for several hours.
3. Observe the surface, and use a magnifying glass to examine any particles more closely.

Observation
Tiny particles settle on the black paper and visible as spots especially under magnification.

Conclusion
These particles are dust suspended in the air. They do not form integral part of air, and their amount and nature vary with environment and time.

Viva Questions

1. Where do dust particles in the air come from?
2. What will be the observation if the experiment is performed indoors?

Activity 3 (Page 121)

Aim
To show that water is made of hydrogen and oxygen as elements.

Materials Required
Beaker or glass tumbler, two small test tubes, water, dilute sulphuric acid (a few drops), 9V battery, test-tube stand, candle matchstick/lighter.

Procedure

1. Fill the beaker 2/3 with water and add a few drops of dilute sulphuric acid.
2. Fill two small test tubes completely with this water taken from the beaker, and invert them in the stand.
3. Place the 9V battery in the beaker without spilling water.
4. Carefully place each test tube one on each battery terminal and keeping them submerged.
5. Wait 10 – 15 minutes, observing gas bubbles collecting in the test tubes.
6. Remove the tubes one by one and test the gases using a burning candle held near each mouth.

Observation
One tube produces a pop sound when a candle is brought close, indicating hydrogen gas. The other tube makes the candle flame glow brighter, indicating oxygen gas.

Conclusion
Passing electric current through water decomposes it into hydrogen and oxygen gases. This confirms that water consists of these two distinct elements.
Water → Hydrogen + Oxygen

Viva Questions

1. Why is sulphuric acid added to the water?
2. Can the gases produced be water vapour?
3. Is this change chemical or physical? Explain your answer.

Activity 4 (Page 124)

Aim
To show that sugar is made up of carbon, hydrogen and oxygen as elements.

Materials Required
Boiling tube, sugar (1 teaspoon), test tube holder, spirit lamp, watch glass, matchstick/lighter.

Procedure

1. Take a teaspoon of sugar and place it in a clean and dry boiling tube.
2. Hold the boiling tube with a test tube holder.
3. Gently heat the bottom of the tube over a spirit lamp.
4. Observe any changes in the sugar and the walls of the boiling tube.
5. Once heating is complete, collect the black residue in a watch glass.

Observation
The sugar first melts and turns brown, then black as it chars. Water droplets appear near the open end of the tube. A black substance, charcoal (carbon) is left behind in the tube.

Conclusion
Sugar decomposes on heating, releasing water vapour and leaving behind carbon. This shows that sugar is a compound made of carbon, hydrogen and oxygen.
Sugar → Carbon + Water (vapour)

Viva Questions

1. Why do water droplets appear on the inside of the boiling tube?
2. What is the black substance left behind after heating sugar?
3. Is sugar an element or a compound? Why?

Activity 5 (Page 125)

Aim
To distinguish between a mixture and a compound using iron filings and sulphur powder.

Materials Required
Iron filings (5.6 g), sulphur powder (3.2 g), two watch glasses, magnet, china dish, burner, pestle and mortar, dilute hydrochloric acid, two test tubes and test tube stand.

Procedure

1. Mix 5.6 g iron filings and 3.2 g sulphur powder thoroughly in a watch glass. Label this as Sample A.
2. Observe the mixture carefully (colour, texture). Test with a magnet.
3. Take half of Sample A in a china dish and heat it while stirring contiguously until a black mass forms.
4. Allow it to cool, grind it with a pestle and label it as Sample B.
5. Compare Sample A and Sample B in terms of appearance, magnetism and reaction with dilute hydrochloric acid.
6. Test the gases released during the acid reaction by wafting and bringing a burning splinter near the mouth of the test tube.

Observation

Experiment	Sample A	Sample B
Appearance	Yellow and black, powdery mixture	Uniform black solid
Magnet Test	Iron fillings attracted to magnet	No effect of magnet
Gas Test	Colourless, pop sound (H2)	Rotten egg smell (H2S gas)

Conclusion
Sample A is a physical mixture. Its components (iron and sulphur) retain their individual properties and can be separated.

Sample B is a new substance (iron sulphide), a compound with properties different from its components.
The reaction between iron and sulphur upon heating forms iron sulphide compound.
Iron + Sulphur → iron sulphide

Viva Questions

1. What difference did you observe when using a magnet on Sample A and Sample B?
2. What does the rotten egg smell in Sample B indicate?
3. Why can’t iron and sulphur be separated from Sample B?