CBSE Class 10 Science Chapter 3: Metals and Non-Metals NCERT Solutions

Solution: A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound. We need to consider the reactivity series of metals. The reactivity series generally lists metals in order of decreasing reactivity. A metal higher in the series can displace a metal lower in the series from its salt solution.

Let's analyze the options:

1. Sodium (Na) is much more reactive than Copper (Cu). NaCl is a stable salt, and copper cannot displace sodium from it.
2. Magnesium (Mg) is more reactive than Aluminium (Al). Aluminium cannot displace magnesium from MgCl₂ solution.
3. Iron (Fe) is more reactive than Silver (Ag). Iron can displace silver from AgNO₃ solution, but here we have FeSO₄ and silver metal. Silver is less reactive than iron, so it cannot displace iron from FeSO₄ solution.
4. Copper (Cu) is more reactive than Silver (Ag). Therefore, copper metal can displace silver from silver nitrate (AgNO₃) solution. The reaction would be: $Cu(s)\; +\; 2AgNO\_3(aq)\; \backslash rightarrow\; Cu(NO\_3)\_2(aq)\; +\; 2Ag(s)$

Answer: The pair that will give displacement reactions is AgNO₃ solution and copper metal.

Solution: Rusting is the process of corrosion of iron, which occurs when iron is exposed to moisture and oxygen. Several methods can prevent rusting:

1. Applying grease or oil creates a barrier between iron and air/moisture.
2. Applying paint also forms a protective layer, preventing contact with the environment.
3. Applying a coating of zinc (galvanization) is another effective method. Zinc is more reactive than iron and corrodes preferentially, protecting the iron underneath (sacrificial protection).

However, for an iron frying pan, methods like applying grease or paint are not ideal. These coatings can be easily washed away or destroyed by the heat generated during cooking and subsequent cleaning. Therefore, applying a coating of zinc (galvanization) is a more suitable and durable method for preventing rust on an iron frying pan, although it's less common for cookware due to aesthetic and potential health concerns if the zinc layer degrades significantly. Answer: While all methods can prevent rusting in general, applying a coating of zinc is considered the most suitable for a frying pan among the given options, considering the limitations of grease and paint with heat and washing.

Solution: Let's analyze the properties of the oxides formed by the given elements:

1. Calcium (Ca): Calcium is a metal. It reacts with oxygen to form calcium oxide (CaO). Calcium oxide is an ionic compound with a very high melting point. It reacts with water to form calcium hydroxide (Ca(OH)₂), which is soluble in water. $2Ca(s)\; +\; O\_2(g)\; \backslash rightarrow\; 2CaO(s)$ $CaO(s)\; +\; H\_2O(l)\; \backslash rightarrow\; Ca(OH)\_2(aq)$
2. Carbon (C): Carbon is a non-metal. It forms oxides like carbon monoxide (CO) and carbon dioxide (CO₂). CO₂ is a gas with a low melting point and dissolves in water to form carbonic acid, but the oxide itself doesn't have a high melting point.
3. Silicon (Si): Silicon is a metalloid. It forms silicon dioxide (SiO₂), which has a very high melting point but is insoluble in water.
4. Iron (Fe): Iron is a metal. It forms oxides like Fe₂O₃, which have high melting points but are insoluble in water.

Based on the properties described (high melting point compound, soluble in water), calcium is the most likely element. Answer: The element is likely to be calcium.

Solution: Food cans are typically made of iron or steel, which are prone to rusting. To protect them, they are coated with a layer of another metal. The choice of coating metal depends on its reactivity relative to iron and its ability to protect the can.

Tin (Sn) and Zinc (Zn) are commonly used for coating iron.

Reactivity Comparison: Zinc is significantly more reactive than tin. Both zinc and tin are more reactive than iron.

Protection Mechanism:

• Tin Coating: Tin is less reactive than iron. It forms a protective layer. If the tin coating is scratched, exposing the iron, the iron will rust because it is more reactive than tin and will corrode preferentially.
• Zinc Coating (Galvanization): Zinc is more reactive than iron. It provides protection in two ways: barrier protection and sacrificial protection. If the zinc coating is scratched, the more reactive zinc corrodes preferentially, protecting the iron underneath from rusting.

Reason for using Tin in Food Cans: While zinc offers sacrificial protection, it is undesirable for food cans because if the coating is breached, the more reactive zinc can react with the food, potentially making it toxic or altering its taste. Tin, being less reactive than iron, acts primarily as a barrier. Even if scratched, the iron will rust, but the tin itself does not react readily with food. Therefore, tin is preferred for food cans to ensure food safety, despite zinc offering better sacrificial protection against rust in other applications. The question implies a reason why tin is chosen *over* zinc, and the key difference is their reactivity with food. Answer: Food cans are coated with tin and not with zinc because zinc is more reactive than tin, and this higher reactivity could lead to undesirable reactions with the food contents if the coating is compromised.

Solution:

(a) Distinguishing between metals and non-metals:

1. Using the hammer (Test for Malleability):

   Take a small piece of the sample. Place it on a hard surface and strike it firmly with the hammer.

   • If the sample can be beaten or hammered into thin sheets without breaking, it exhibits malleability and is likely a metal.
   • If the sample breaks into pieces or crumbles upon hammering, it is likely a non-metal.

2. Using the battery, bulb, wires, and switch (Test for Electrical Conductivity):

   Set up a simple electrical circuit. Connect the battery, bulb, switch, and wires in series. Insert the sample to be tested into the circuit, either by connecting its ends to the terminals or by placing it between two points in the circuit. Close the switch.

   • If the bulb glows, it indicates that the sample conducts electricity, meaning it is a metal.
   • If the bulb does not glow, it indicates that the sample does not conduct electricity (or conducts very poorly), meaning it is a non-metal.

(b) Usefulness of these tests:

These tests are based on the characteristic physical properties of metals and non-metals, making them quite useful for distinguishing between them, especially for solid samples at room temperature.

• Malleability: Most metals are malleable, allowing them to be shaped into thin sheets. This is a key distinguishing physical property.
• Electrical Conductivity: Metals are generally excellent conductors of electricity, while non-metals are poor conductors (insulators). This is another fundamental difference.

These tests are practical and do not require complex chemical reactions. However, it's important to note that some elements might exhibit intermediate properties (like metalloids), and some metals might be brittle (like Manganese) or have low conductivity under certain conditions. Despite these exceptions, these physical property tests are highly effective for a general classification.

Solution:

Amphoteric oxides are a special class of chemical compounds, specifically oxides, that exhibit dual chemical behavior. They possess the ability to react with both acids and bases, behaving as either an acid or a base depending on the nature of the reactant they are paired with. This dual reactivity is a characteristic property of certain metal oxides.

When an amphoteric oxide reacts with an acid, it acts as a base. When it reacts with a base, it acts as an acid.

Examples of amphoteric oxides:

1. Aluminium oxide (Al₂O₃):

   It reacts with acids to form aluminium salts and water:

   $$Al\_2O\_3(s)\; +\; 6HCl(aq)\; \backslash rightarrow\; 2AlCl\_3(aq)\; +\; 3H\_2O(l)$$

   It also reacts with strong bases like sodium hydroxide to form a soluble complex salt, sodium aluminate:

   $$Al\_2O\_3(s)\; +\; 2NaOH(aq)\; +\; 3H\_2O(l)\; \backslash rightarrow\; 2Na[Al(OH)\_4](aq)$$ (or $2NaAlO\_2(aq)\; +\; 2H\_2O(l)$)

2. Zinc oxide (ZnO):

   It reacts with acids to form zinc salts and water:

   $$ZnO(s)\; +\; 2HCl(aq)\; \backslash rightarrow\; ZnCl\_2(aq)\; +\; H\_2O(l)$$

   It also reacts with strong bases like sodium hydroxide to form sodium zincate:

   $$ZnO(s)\; +\; 2NaOH(aq)\; +\; H\_2O(l)\; \backslash rightarrow\; Na\_2[Zn(OH)\_4](aq)$$ (or $2NaZnO\_2(aq)\; +\; H\_2O(l)$)

Other examples include lead oxide (PbO) and tin oxide (SnO).