Reactions of metals with oxygen MCQs for Grade 9 Exam

Metallic properties refer to the characteristic features of metals. Metals are a group of elements that possess certain common properties such as electrical conduction, thermal conduction, malleability, ductility, and a lustrous appearance. However, one property that is generally not shown by metals is dullness.

Dullness refers to the lack of shine or luster in an object. In the context of metals, dullness implies that the surface of the metal does not reflect light and appears matte or non-reflective. This property is typically associated with non-metals or non-metallic substances.

Explanation:

- Reflectivity and Luster:
Metals are known for their characteristic luster or shine. When light falls on the surface of a metal, it gets reflected uniformly, resulting in a shiny appearance. This property is due to the presence of free electrons in the metal lattice, which allows for the easy movement of light waves. As a result, metals are excellent reflectors of light and have a high degree of luster.

- Electrical Conduction:
Metals are excellent conductors of electricity. This property is due to the presence of delocalized electrons in the metal lattice. These free electrons are not bound to any specific atom and can move freely within the metal structure. When a voltage is applied across a metal, these free electrons can easily move and carry the electric current.

- Sonorous Nature:
Metals also possess the property of being sonorous, which means they produce a ringing sound when struck. This property is again related to the presence of free electrons in the metal lattice. When a metal is struck, the force of the impact causes the metal atoms to vibrate. These vibrations are transmitted through the lattice, resulting in the production of sound waves.

- Ductility:
Ductility is the property of a material to be drawn into thin wires without breaking. Metals are highly ductile due to the presence of metallic bonds. Metallic bonds are formed by the sharing of valence electrons between atoms. This arrangement allows the atoms to slide over each other when a force is applied, making the metal malleable and ductile.

In conclusion, metals generally do not exhibit the property of dullness. They are highly reflective and possess a characteristic luster. Other properties exhibited by metals include electrical conduction, sonorous nature, and ductility.

Iron can react with steam to form different oxides depending on the reaction conditions. The most likely oxide that would be obtained on prolonged reaction of iron with steam is Fe3O4 (iron(II,III) oxide), option C.

Here's an explanation of why Fe3O4 is the correct answer:

1. Reaction of iron with steam:
When iron reacts with steam, a redox reaction takes place. The iron is oxidized, while the steam is reduced.
The general equation for the reaction is:
Iron (Fe) + Steam (H2O) → Iron Oxide (FeOx) + Hydrogen Gas (H2)

2. Formation of Fe3O4:
Under prolonged reaction with steam, iron can form a mixture of FeO (iron(II) oxide) and Fe3O4 (iron(II,III) oxide). However, Fe3O4 is the more stable and predominant oxide that forms.

3. Fe3O4 structure:
Fe3O4 is a mixed oxide consisting of both Fe2+ and Fe3+ ions. It has a crystal structure where Fe2+ ions occupy one-half of the octahedral sites, while Fe3+ ions occupy one-eighth of the tetrahedral sites.
This arrangement gives rise to the formula Fe3O4, indicating the presence of both iron(II) and iron(III) ions.

4. Color and properties:
Fe3O4 is a black or dark brown solid. It is a magnetic material and has a high melting point. The presence of both Fe2+ and Fe3+ ions in the structure contributes to its unique properties.

5. Other options:
a) FeO (iron(II) oxide) is a possible product, but it is less likely to be obtained on prolonged reaction. FeO is a black solid and is less stable compared to Fe3O4.
b) Fe2O3 (iron(III) oxide) is not formed on prolonged reaction with steam. It is a red-brown solid and can be obtained through different reactions but not in this case.
d) A mixture of Fe2O3 and Fe3O4 is possible, but Fe3O4 would still be the predominant oxide formed.

In conclusion, Fe3O4 is the most likely oxide obtained on prolonged reaction of iron with steam due to its stability and the presence of both iron(II) and iron(III) ions in its structure.

Explanation of the Reaction
When metals react with acids, their reactivity determines whether they will dissolve. In this scenario, we have three different test tubes containing various acids.
Test Tubes and Their Contents
- Test Tube A: Concentrated HCl
- Test Tube B: Concentrated HNO3
- Test Tube C: Mixture of concentrated HCl and HNO3 (3:1 ratio)
Observations of the Metal's Behavior
- Test Tubes A and B: No change occurred
- Test Tube C: The metal dissolved
Analysis of the Metal
- Test Tube A (HCl) does not react with noble metals like gold (Au) and platinum (Pt), which explains the lack of change.
- Test Tube B (HNO3) typically reacts with metals but does not dissolve noble metals like Au and Pt either.
- Test Tube C combines both HCl and HNO3, forming aqua regia, a powerful solvent capable of dissolving gold.
Conclusion
Given that the metal dissolved only in Test Tube C, it can be inferred that the metal is likely Gold (Au). The reaction with aqua regia highlights its unique chemical properties, making it the only metal among the options that can dissolve in that specific acid mixture.
Summary of Key Points
- Test Tube A: No reaction - Gold and Platinum are unreactive.
- Test Tube B: No reaction - HNO3 does not dissolve Gold or Platinum.
- Test Tube C: Reaction occurs - Only Gold dissolves in aqua regia.
Thus, the correct answer is option C: Au (Gold).

Understanding Ionic and Covalent Compounds
Ionic compounds are formed by the transfer of electrons from one atom to another, typically between metals and non-metals, resulting in the formation of charged ions. Covalent compounds, on the other hand, involve the sharing of electrons between atoms.
Analyzing the Compounds
Let's evaluate each compound in the list:

• (i) KCl - Potassium chloride is an ionic compound. It consists of potassium ions (K+) and chloride ions (Cl-).
• (ii) HCl - Hydrochloric acid is a covalent compound. Although it can dissociate into ions in solution, it is primarily a molecular compound formed by the sharing of electrons between hydrogen and chlorine.
• (iii) CCl4 - Carbon tetrachloride is a covalent compound. It is composed of one carbon atom sharing electrons with four chlorine atoms, forming a stable molecular structure.
• (iv) NaCl - Sodium chloride is another ionic compound. It is formed from sodium ions (Na+) and chloride ions (Cl-).

Conclusion
Based on the analysis, the compounds that are not ionic are:
- HCl (ii)
- CCl4 (iii)
Thus, the correct answer is option b) (ii) and (iii) as they are covalent compounds, while KCl and NaCl are ionic. Understanding these distinctions is crucial in chemistry to predict the behavior of substances in different chemical reactions.

Explanation:

When a more reactive metal displaces a less reactive metal from its salt solution, it is known as a displacement reaction. In this case, we need to determine which metal can displace the other three metals from their salt solutions.

To determine the reactivity of the metals, we can refer to the reactivity series of metals. The reactivity series is a list of metals arranged in order of their reactivity, with the most reactive metal at the top and the least reactive metal at the bottom.

The reactivity series of metals from most reactive to least reactive is as follows:
Potassium (K) - Sodium (Na) - Calcium (Ca) - Magnesium (Mg) - Aluminum (Al) - Zinc (Zn) - Iron (Fe) - Lead (Pb) - Hydrogen (H) - Copper (Cu) - Silver (Ag) - Gold (Au)

Analysis of the options:
a) Cu (Copper) - Copper is less reactive than silver, magnesium, and zinc. It cannot displace any of the other three metals from their salt solutions.

b) Ag (Silver) - Silver is less reactive than magnesium and zinc but more reactive than copper. It can displace copper from its salt solution but cannot displace magnesium or zinc.

c) Mg (Magnesium) - Magnesium is more reactive than copper, silver, and zinc. It can displace copper and silver from their salt solutions but cannot displace zinc.

d) Zn (Zinc) - Zinc is more reactive than copper, silver, and magnesium. It can displace copper, silver, and magnesium from their salt solutions.

Conclusion:
Based on the reactivity series, the metal that can be displaced from the solution of its salts by the other three metals is silver (Ag). Therefore, the correct answer is option b) Ag.

Introduction to Safety Fuse Wire
Safety fuse wires are crucial components in electrical circuits, designed to protect against overcurrent. Their primary function is to melt and break the circuit when excessive current flows, preventing potential hazards like electrical fires.
Material Composition of Fuse Wire
The correct answer is that safety fuse wire is made from an alloy of tin and lead. Here’s why:
Key Properties of Tin and Lead Alloy
- Low Melting Point:
- The alloy of tin and lead has a lower melting point compared to pure metals like copper or silver. This allows the fuse to melt quickly when the current exceeds safe levels.
- Good Conductivity:
- While not as conductive as copper or silver, the alloy still provides adequate conductivity for normal operating conditions.
- Cost-Effectiveness:
- Tin and lead alloys are less expensive than precious metals like platinum and silver, making them more practical for widespread use in fuse wires.
Why Not Other Metals?
- Platinum:
- Extremely expensive and has a high melting point, making it unsuitable for fuse applications.
- Silver:
- While highly conductive, silver is not cost-effective for fuse wires and has a higher melting point compared to the tin-lead alloy.
- Copper:
- Though conductive, copper does not melt as easily as the tin-lead alloy, making it less effective for fuse applications.
Conclusion
In summary, the safety fuse wire is made from an alloy of tin and lead due to its optimal melting point, adequate conductivity, and cost-effectiveness. This combination ensures reliable operation in protecting electrical circuits.

Reaction of Aluminium with Sodium Hydroxide
When aluminium reacts with sodium hydroxide (NaOH), it undergoes a chemical reaction that produces hydrogen gas. Here’s a detailed explanation:
Nature of the Reaction
- Aluminium is a reactive metal and can react with alkalis like sodium hydroxide.
- The reaction is an example of a metal reacting with a base.
Products of the Reaction
- When aluminium (Al) is added to sodium hydroxide solution, it produces:
- Hydrogen gas (H2)
- Sodium aluminate (NaAlO2)
Chemical Equation
- The balanced chemical equation for this reaction is:
2 Al + 2 NaOH + 6 H2O → 2 NaAl(OH)4 + 3 H2↑
- In this equation, you can see that hydrogen gas is released as a byproduct.
Why Other Options are Incorrect
- Oxygen is evolved (Option A): This is incorrect as hydrogen gas is produced, not oxygen.
- Water is produced (Option C): Water is present in the reaction but is not a product; it acts as a solvent.
- No reaction takes place (Option D): This is false as aluminium reacts vigorously with sodium hydroxide.
Conclusion
- The correct answer is option B: Hydrogen is produced when aluminium is added to sodium hydroxide solution. This reaction is notable for demonstrating the reactivity of metals with bases and the generation of hydrogen gas.

Understanding the Incorrect Statement about Magnesium Metal
Magnesium is a reactive metal with several notable properties. Among the statements provided, option 'B' is incorrect. Here’s why:
Reaction with Cold Water
- Magnesium does not react with cold water.
- Instead, it reacts very slowly, if at all, and does not produce significant amounts of magnesium oxide or hydrogen gas.
Correct Reactions
- Burning in Oxygen:
- Magnesium burns brightly in oxygen, producing magnesium oxide with a dazzling white flame.
- Reaction with Steam:
- When magnesium is heated in steam, it reacts to form magnesium hydroxide and hydrogen gas.
- Reaction with Hot Water:
- Magnesium can react with hot water, producing magnesium hydroxide and hydrogen gas as well.
Summary of Correct Behavior
- Magnesium's reactivity increases with temperature.
- Cold water does not provide enough energy for a significant reaction to occur, which is why option 'B' is incorrect.
Conclusion
Recognizing the conditions under which magnesium reacts is crucial for understanding its chemical behavior. While it does react with steam and hot water, cold water does not facilitate a notable reaction, marking option 'B' as the incorrect statement.

Reason for the Formation of Green Coating on Copper:

When copper is exposed to air, it slowly loses its shining brown surface and gains a green coating. This green coating is known as copper carbonate (CuCO3) or copper(II) carbonate hydroxide (Cu2CO3(OH)2), which is commonly known as verdigris. The formation of this green coating is primarily due to the reaction of copper with carbon dioxide and moisture present in the air.

Explanation:

1. Reaction with Carbon Dioxide:
- Copper reacts with carbon dioxide present in the air to form copper(II) oxide (CuO).
- The reaction can be represented as:
2Cu + O2 + CO2 → 2CuO + CO
- Copper(II) oxide is a black compound.

2. Reaction with Moisture:
- Copper(II) oxide further reacts with moisture present in the air to form copper(II) hydroxide (Cu(OH)2).
- The reaction can be represented as:
CuO + H2O → Cu(OH)2
- Copper(II) hydroxide is a blue compound.

3. Reaction with Carbon Dioxide and Moisture:
- Copper(II) hydroxide reacts with carbon dioxide present in the air to form copper carbonate (CuCO3).
- The reaction can be represented as:
Cu(OH)2 + CO2 → CuCO3 + H2O
- Copper carbonate is a green compound.

Conclusion:

The formation of the green coating on copper is due to the reaction of copper with carbon dioxide and moisture present in the air. Initially, copper reacts with carbon dioxide to form copper(II) oxide, which further reacts with moisture to form copper(II) hydroxide. Finally, copper(II) hydroxide reacts with carbon dioxide to form copper carbonate. This copper carbonate coating gives the characteristic green color to the surface of the copper.

Understanding Electromagnetic Separation
Electromagnetic separation is a method used to concentrate ores based on their magnetic properties. This technique is effective for separating magnetic minerals from non-magnetic ones.
Types of Ores Mentioned
- Chromite:
- Chromite is the main ore for chromium and is magnetic in nature, making it suitable for electromagnetic separation.
- Cuprite:
- Cuprite (Cu2O) is a copper oxide mineral and is non-magnetic, which means it cannot be effectively concentrated using electromagnetic methods.
- Magnetite:
- Magnetite is a strongly magnetic iron ore and is excellent for concentration by electromagnetic separation.
- Pyrolusite:
- Pyrolusite (MnO2) is a manganese ore that is also magnetic, allowing it to be concentrated using this method.
Conclusion
The correct answer is option 'B' (Cuprite) because it is a non-magnetic mineral. In contrast, the other ores listed (Chromite, Magnetite, Pyrolusite) possess magnetic properties, thus making them suitable candidates for concentration through electromagnetic separation. Understanding the magnetic characteristics of minerals is crucial for selecting the appropriate separation techniques in mineral processing.