Metals, Non-Metals, and Reactivity Order of Metals
Metals, Non-Metals, and Reactivity Order of Metals MCQ Questions
13.
Which of the following alkali metals can be cut easily with an ordinary knife due to its extreme softness?
Approach
The question asks to identify an alkali metal that is soft enough to be cut with an ordinary knife. This requires knowledge of the physical properties of alkali metals and other common metals.
Step-by-step
- Understand Alkali Metals: Alkali metals are a group of chemical elements in Group 1 of the periodic table (lithium, sodium, potassium, rubidium, caesium, and francium). They are known for being highly reactive and having low densities and low melting points.
- Softness of Alkali Metals: A key characteristic of alkali metals is their extreme softness. This property is due to their metallic bonding, which involves a relatively weak attraction between the delocalized electrons and the metal cations, and their large atomic radii. This allows the layers of atoms to slide past each other easily.
- Evaluate the Options:
- Magnesium: Magnesium is an alkaline earth metal (Group 2). While relatively light, it is harder than alkali metals and cannot be easily cut with a knife.
- Sodium: Sodium is an alkali metal. It is famously known for its extreme softness, silvery-white appearance when freshly cut, and high reactivity. It can indeed be easily cut with an ordinary knife.
- Iron: Iron is a transition metal. It is a hard, strong metal widely used in construction and manufacturing, and it cannot be cut with a knife.
- Copper: Copper is also a transition metal. It is ductile and malleable but much harder than alkali metals and cannot be cut with a knife.
- Conclusion: Among the given options, only sodium is an alkali metal and possesses the characteristic extreme softness that allows it to be cut with a knife.
Correct Option:
B) Sodium — Sodium is an alkali metal known for its exceptional softness. It has a low melting point and can be easily cut with a knife, revealing a shiny surface that quickly tarnishes upon exposure to air.
Incorrect Options
- A) Magnesium — Magnesium is an alkaline earth metal. While it is relatively light, it is significantly harder than alkali metals and cannot be cut with an ordinary knife.
- C) Iron — Iron is a transition metal and is known for its strength and hardness. It is impossible to cut iron with a knife.
- D) Copper — Copper is a transition metal. It is malleable and ductile but is a relatively hard metal and cannot be cut with an ordinary knife.
14.
'Tensile strength' of a metal refers to its ability to:
A.
Withstand a stretching force without breaking
B.
Produce a ringing sound
C.
Conduct electric current
D.
Reflect light from its surface
ANSWER :
A. Withstand a stretching force without breaking
Approach
The question asks for the definition of 'tensile strength' in the context of metals. This requires understanding fundamental material properties.
Step-by-step
- Define Tensile Strength: Tensile strength is a material property that measures the maximum stress a material can withstand while being stretched or pulled before breaking. It's a crucial indicator of a material's resistance to fracture under tension.
- Analyze the Options:
- A) "Withstand a stretching force without breaking" directly aligns with the definition of tensile strength. When a material is subjected to a tensile (stretching) force, its tensile strength determines how much of that force it can endure before it yields or fractures.
- B) "Produce a ringing sound" relates to the acoustic properties or elasticity of a material, not its tensile strength.
- C) "Conduct electric current" refers to electrical conductivity, which is a different physical property altogether.
- D) "Reflect light from its surface" describes reflectivity or luster, an optical property, not related to mechanical strength.
- Conclusion: Based on the definition, option A accurately describes tensile strength.
Correct Option: A) Withstand a stretching force without breaking
Incorrect Options
- B) Produce a ringing sound — This property is related to a material's elasticity and density, often referred to as its acoustic properties. It has no direct relation to tensile strength, which is a measure of resistance to breaking under tension.
- C) Conduct electric current — This refers to electrical conductivity, which is a material's ability to allow the flow of electric charge. It is a distinct electrical property and not related to mechanical strength.
- D) Reflect light from its surface — This describes the optical property of reflectivity or luster. While many metals are reflective, this characteristic is unrelated to their ability to withstand mechanical forces like stretching.
15.
Most metals have comparatively high melting and boiling points. Which of these metals is a notable exception with a low melting point?
Approach
The question asks to identify a metal that has an unusually low melting point compared to most other metals. This requires knowledge of the general properties of metals and specific exceptions.
Step-by-step
- Recall that metals are generally characterized by high melting and boiling points due to strong metallic bonding.
- Consider the options provided and their typical melting points.
- Identify the metal among the options that deviates significantly from the general trend of high melting points.
Correct Option:
B) Gallium
Gallium (Ga) is a metal with a remarkably low melting point of approximately \(29.76^\circ \text{C}\). This means it can melt in the palm of your hand. This property makes it a notable exception to the general rule that metals have high melting points.
Incorrect Options
- A) Iron: Iron (Fe) is a transition metal with a high melting point of about \(1538^\circ \text{C}\). It is a typical example of a metal with strong metallic bonds, requiring significant energy to break and thus having a high melting point.
- C) Platinum: Platinum (Pt) is a noble metal known for its extremely high melting point, around \(1768^\circ \text{C}\). It is one of the most refractory metals.
- D) Copper: Copper (Cu) is another common metal with a relatively high melting point of about \(1085^\circ \text{C}\). It also exhibits strong metallic bonding typical of most metals.
16.
Why is sodium metal kept immersed in kerosene oil for storage?
A.
To prevent it from melting at room temperature
B.
To increase its overall weight
C.
To improve its metallic lustre permanently
D.
To prevent its reaction with oxygen and moisture in air
ANSWER :
D. To prevent its reaction with oxygen and moisture in air
Approach
This question tests knowledge about the chemical properties and storage of alkali metals, specifically sodium. Sodium is a highly reactive metal, and its storage method is designed to prevent unwanted chemical reactions with common atmospheric components.
Step-by-step
- Identify the nature of sodium: Sodium (Na) is an alkali metal, located in Group 1 of the periodic table. Alkali metals are known for their high reactivity due to having a single valence electron, which they readily lose to form a positive ion.
- Consider atmospheric components: Air contains oxygen (\(\text{O}_2\)) and moisture (\(\text{H}_2\text{O}\)).
- Evaluate sodium's reaction with oxygen: Sodium reacts vigorously with oxygen in the air to form sodium oxide (\(\text{Na}_2\text{O}\)) or sodium peroxide (\(\text{Na}_2\text{O}_2\)). This reaction is often exothermic and can lead to tarnishing or even ignition. The reaction is:
\[ 4\text{Na(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{Na}_2\text{O(s)} \] - Evaluate sodium's reaction with moisture: Sodium reacts even more violently with water (moisture) to produce sodium hydroxide (\(\text{NaOH}\)) and hydrogen gas (\(\text{H}_2\)). This reaction is highly exothermic, releasing significant heat, which can ignite the hydrogen gas, leading to an explosion. The reaction is:
\[ 2\text{Na(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)} + \text{Heat} \] - Determine the purpose of kerosene oil: Kerosene oil is a hydrocarbon and is non-reactive with sodium. It is also immiscible with water and less dense than sodium, allowing the sodium to remain submerged. By immersing sodium in kerosene, it is isolated from both oxygen and moisture in the air, preventing these dangerous reactions.
Correct Option:
D) To prevent its reaction with oxygen and moisture in air — Sodium is highly reactive and readily reacts with oxygen and moisture (water vapor) present in the air. These reactions are vigorous and can be dangerous. Kerosene oil provides an inert environment, preventing contact with air and thus preventing these reactions.
Incorrect Options
- A) To prevent it from melting at room temperature — Sodium has a melting point of \(97.8^\circ\text{C}\), which is well above typical room temperatures. Kerosene oil does not significantly lower the ambient temperature to prevent melting.
- B) To increase its overall weight — Storing sodium in kerosene oil does not increase its intrinsic weight. The purpose is chemical stability, not altering its mass.
- C) To improve its metallic lustre permanently — Sodium quickly tarnishes upon exposure to air due to reaction with oxygen. Kerosene prevents this tarnishing, thus preserving its metallic lustre as long as it's submerged, but it doesn't improve it permanently in the sense of an inherent change to the metal itself. The primary reason for storage is safety and preventing degradation, not aesthetics.
17.
When sodium metal burns in air, it primarily forms:
Approach
The question asks about the primary product formed when sodium metal burns in air. Air is primarily composed of nitrogen (approximately 78%) and oxygen (approximately 21%). We need to consider the reactivity of sodium with these components under burning conditions.
Step-by-step
- Reactivity of Sodium with Oxygen: Sodium is a highly reactive alkali metal. When it burns in air, it reacts vigorously with oxygen. The primary product formed under normal burning conditions (i.e., in excess air) is sodium oxide. The reaction is:\[ 4\text{Na(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{Na}_2\text{O(s)} \]
- Formation of Peroxide/Superoxide: While sodium can form sodium peroxide (\(\text{Na}_2\text{O}_2\)) and even sodium superoxide (\(\text{NaO}_2\)) with oxygen, the primary product when burning in *limited* air or under controlled conditions is sodium oxide. However, when simply stated as "burns in air," sodium oxide is the most common and stable primary product, especially at higher temperatures. In excess oxygen, sodium peroxide is often formed. But typically, when just "burns in air" is mentioned, the most straightforward and stable oxide is considered the primary product.
- Reactivity of Sodium with Nitrogen: Sodium does not readily react with nitrogen at room temperature or even upon burning in air to form sodium nitride. Nitrogen is a very unreactive gas due to the strong triple bond between its atoms. Only very reactive metals like lithium can react directly with nitrogen at high temperatures to form nitrides.
- Other components of air: Air also contains trace amounts of carbon dioxide, noble gases, and water vapor. Sodium reacts with water vapor to form sodium hydroxide and hydrogen gas, but this is not the primary product of burning. Reaction with carbon dioxide is also not the primary outcome.
Therefore, the primary product of sodium burning in air is sodium oxide.
Correct Option:
A) Sodium oxide is the primary product formed when sodium metal burns in air, reacting with the oxygen present.
Incorrect Options
- B) Sodium carbide: Sodium does not react with carbon dioxide in the air to form sodium carbide under normal burning conditions. Carbides are typically formed under very specific, high-temperature conditions or with carbon sources.
- C) Sodium nitride: Sodium does not readily react with nitrogen in the air to form sodium nitride. Nitrogen is quite unreactive, and only a few very active metals (like lithium) form nitrides directly upon heating in nitrogen.
- D) Sodium hydride: Sodium hydride is formed by the reaction of sodium metal with hydrogen gas, usually at elevated temperatures. Hydrogen is present in air only in trace amounts (from water vapor decomposition or other sources) and is not a primary reactant for forming the main product when sodium burns in air.
18.
Which protective oxide layer forms on the surface of aluminium, preventing it from further corrosion?
ANSWER :
D. Aluminium oxide
Approach
The question asks about the protective layer that forms on aluminium to prevent further corrosion. This involves understanding the chemical reactivity of aluminium with oxygen in the atmosphere.
Step-by-step
- Aluminium is a highly reactive metal, but it exhibits excellent corrosion resistance.
- When fresh aluminium is exposed to air, it immediately reacts with oxygen.
- This reaction forms a very thin, dense, and tenacious layer on its surface.
- The chemical formula for aluminium is \(\text{Al}\) and for oxygen is \(\text{O}_2\).
- The reaction between aluminium and oxygen forms aluminium oxide. The balanced chemical equation is:
\[ 4\text{Al(s)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{Al}_2\text{O}_3\text{(s)} \]- This layer of aluminium oxide (\(\text{Al}_2\text{O}_3\)) acts as a barrier, preventing further oxygen from reaching the underlying metal. This phenomenon is known as passivation.
- This protective oxide layer is chemically stable and adheres strongly to the aluminium surface, making aluminium resistant to corrosion even though it is a reactive metal.
Correct Option: D) Aluminium oxide
Incorrect Options
- A) Aluminium hydroxide: While aluminium hydroxide (\(\text{Al(OH)}_3\)) can be formed from aluminium oxide in the presence of water, it is not the primary protective layer formed directly upon exposure to air. The initial protective layer is the oxide.
- B) Aluminium carbide: Aluminium carbide (\(\text{Al}_4\text{C}_3\)) is formed when aluminium reacts with carbon at high temperatures. It is not a protective layer formed by atmospheric corrosion.
- C) Aluminium nitride: Aluminium nitride (\(\text{AlN}\)) is a compound formed when aluminium reacts with nitrogen, typically at high temperatures. It is not the protective layer formed on the surface of aluminium in air.
