Question 1. Which property distinguishes a mixture from a compound?
A. A mixture always contains two elements
B. Components of a mixture retain their individual properties
C. A mixture always has a fixed composition
D. Components of a mixture cannot be separated
Answer: B. Components of a mixture retain their individual properties
Question 2. Which statement about the composition of a mixture is correct?
A. It always has a fixed ratio of components
B. Its composition can vary
C. It always contains equal amounts of components
D. Its composition depends only on temperature
Answer: B. Its composition can vary
Question 3. Which of the following is a homogeneous mixture?
A. Sand and water
B. Oil and water
C. Salt solution
D. Iron filings and sulphur
Answer: C. Salt solution
Question 4. A non-uniform mixture is also known as a:
A. Homogeneous mixture
B. Heterogeneous mixture
C. Pure substance
D. Compound
Answer: B. Heterogeneous mixture
Question 5. Which method can be used to separate iron filings from sulphur powder without forming a new substance?
A. Evaporation
B. Filtration
C. Using a magnet
D. Distillation
Answer: C. Using a magnet
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Question 6. Sugar is completely dissolved in water. The resulting mixture is:
A. Heterogeneous
B. Homogeneous
C. An element
D. A compound
Answer: B. Homogeneous
Question 7. Which statement correctly describes a compound?
A. Its constituents are physically mixed in any proportion
B. It is formed when elements chemically combine in a fixed ratio
C. Its components retain all their original properties
D. It can always be separated by physical methods
Answer: B. It is formed when elements chemically combine in a fixed ratio
Question 8. Which of the following is a compound?
A. Air
B. Brass
C. Soil
D. Carbon dioxide
Answer: D. Carbon dioxide
Question 9. Which of the following is an example of an element?
A. Water
B. Sodium chloride
C. Iron
D. Air
Answer: C. Iron
Question 10. Why is water classified as a compound?
A. It contains only one type of atom
B. Hydrogen and oxygen are chemically combined in a fixed ratio
C. Hydrogen and oxygen are physically mixed
D. Its composition can vary
Answer: B. Hydrogen and oxygen are chemically combined in a fixed ratio
Question 11. What is the ratio of hydrogen to oxygen atoms in a water molecule?
A. 1 : 1
B. 1 : 2
C. 2 : 1
D. 2 : 2
Answer: C. 2 : 1
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Question 12. Which pair correctly represents an alloy and its constituents?
A. Brass — copper and zinc
B. Brass — copper and tin
C. Bronze — copper and zinc
D. Steel — copper and zinc
Answer: A. Brass — copper and zinc
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Question 13. Bronze is mainly a mixture of:
A. Copper and zinc
B. Copper and tin
C. Iron and carbon
D. Sodium and chlorine
Answer: B. Copper and tin
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Question 14. Which statement correctly distinguishes an element from a compound?
A. An element contains different types of atoms chemically combined
B. A compound contains only one type of atom
C. An element consists of one kind of atom, whereas a compound contains chemically combined elements
D. Both elements and compounds can always be separated physically
Answer: C. An element consists of one kind of atom, whereas a compound contains chemically combined elements
Question 15. Which statement about pure substances is correct?
A. They can always be separated by physical methods
B. Their composition is variable
C. Elements and compounds are pure substances
D. They always contain more than one type of substance
Answer: C. Elements and compounds are pure substances
Question 1. What is a mixture? Give two examples.
Answer:
A mixture is formed when two or more substances are physically combined without undergoing a chemical reaction. The components retain their individual properties, and their proportions can vary.
Examples:
Air
Soil
Question 2. Why is air classified as a mixture?
Answer:
Air contains several gases, including nitrogen, oxygen, carbon dioxide and water vapour. These gases are physically mixed and do not chemically combine to form a new substance. Therefore, air is considered a mixture.
Question 3. What is meant by a pure substance?
Answer:
A pure substance contains only one kind of particle and has a fixed composition and definite characteristic properties. Elements and compounds are classified as pure substances.
Question 4. State any three differences between a mixture and a compound.
Answer:
| Mixture | Compound |
| ------------------------------------------------ | ---------------------------------------------------------- |
| Components are physically mixed. | Elements are chemically combined. |
| Composition can vary. | Composition is fixed. |
| Components retain their individual properties. | A new substance with different properties is formed. |
| Components can be separated by physical methods. | Physical methods cannot separate the constituent elements. |
Question 5. What happens when iron and sulphur are heated together?
Answer:
When iron and sulphur are heated together, they chemically combine to form iron sulphide. Iron sulphide is a compound and has properties different from those of iron and sulphur.
Question 6. What are alloys? Give one example.
Answer:
Alloys are homogeneous mixtures formed by combining two or more elements, generally metals, without forming a new compound through a chemical reaction.
Example: Brass, which is a mixture of copper and zinc.
Other examples mentioned are bronze and steel.
Question 7. Explain why water is classified as a compound.
Answer:
Water is formed when hydrogen and oxygen chemically combine in a fixed ratio of 2:1. The resulting substance has properties different from those of hydrogen and oxygen. Therefore, water is a compound.
Question 8. Why cannot the components of a compound usually be separated by physical methods?
Answer:
In a compound, the constituent elements are chemically combined and new chemical bonds are formed. A new substance with its own identity and properties is produced. Therefore, physical methods cannot separate the constituent elements.
In contrast, the components of a mixture are only physically combined and can be separated using physical methods.
Question 9. Why do the properties of a compound differ from those of its constituent elements?
Answer:
When elements combine chemically, new chemical bonds are formed and a new substance is produced. The arrangement and bonding of its atoms are different from those in the original elements. Hence, the compound has different properties.
For example, hydrogen and oxygen are gases, whereas the compound water is a liquid.
Question 10. What are minerals? Give one example.
Answer:
Minerals are naturally occurring substances obtained from the Earth's crust. They may occur as elements or compounds and can contain impurities.
Example: Bauxite, which is an ore from which aluminium can be extracted.
Question 11
A student is given three samples labelled P, Q and R.
Sample P contains only one type of atom.
Sample Q contains two elements chemically combined in a fixed ratio and has properties different from both elements.
Sample R contains two substances physically mixed together, and their relative amounts can be changed.
a) Identify P, Q and R as an element, compound or mixture.
b) Which sample can be separated into its components using physical methods?
c) Which sample has a variable composition?
Answer:
a)
P → Element
Q → Compound
R → Mixture
b) Sample R, because its components are physically mixed.
c) Sample R, because the composition of a mixture can vary.
Question 12.
A student has two white powders, A and B. Powder A is made by physically mixing two substances. Each substance continues to show its original properties. Powder B is formed when two elements react chemically in a fixed ratio. The new substance has properties different from both starting elements.
Which powder is a mixture and which is a compound? Explain your reasoning.
Answer:
Powder A is a mixture because its components are only physically combined and retain their individual properties.
Powder B is a compound because its elements have chemically combined in a fixed ratio to form a new substance with different properties.
The key difference is physical mixing in a mixture versus chemical combination in a compound.
Question 13.
A student argues:
> “Brass must be a compound because it looks like a single uniform substance.”
Another student disagrees and says that brass is a mixture.
Who is correct? Give two observations that support your Answer.
Answer:
The second student is correct. Brass is a homogeneous mixture of copper and zinc.
Two supporting observations are:
1. Copper and zinc are physically combined rather than chemically forming a new compound.
2. The composition of brass can vary, while the components retain their individual properties.
Therefore, uniform appearance alone does not make a substance a compound.
Question 14.
A student performs the following investigation:
Hydrogen gas and oxygen gas are separately collected.
They are allowed to chemically combine in the required proportion.
A liquid with properties very different from the two gases is produced.
The student concludes that the liquid is a mixture because it contains hydrogen and oxygen.
Do you agree? Give a reason for your Answer.
Answer:
No, the conclusion is incorrect. The liquid is water, a compound.
Hydrogen and oxygen have chemically combined in a fixed 2:1 ratio to form a new substance. The resulting water has properties different from those of hydrogen and oxygen.
If they were merely physically mixed, the components would retain their original properties.
Question 15.
A laboratory contains the following samples:
A: Iron filings mixed with sulphur
B: Water
C: Sand mixed with water
D: Brass
A student wants to separate the components of each sample.
a) Which samples can be separated using physical methods?
b) Which sample is a compound and therefore cannot be separated into its constituent elements by ordinary physical methods?
c) Why can iron and sulphur be separated before heating but not after they are chemically combined?
Answer:
a) Samples A, C and D are mixtures and can be separated using appropriate physical methods.
b) B (water) is a compound. Its hydrogen and oxygen are chemically combined and cannot be separated by ordinary physical methods.
c) Before heating, iron and sulphur are only physically mixed, so each retains its identity and can be separated physically. On heating, they chemically combine to form iron sulphide, a new compound with different properties.