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). Copper cannot displace sodium from NaCl solution.
2. Aluminium (Al) is more reactive than magnesium (Mg). Aluminium cannot displace magnesium from MgCl₂ solution.
3. Iron (Fe) is more reactive than silver (Ag). Iron can displace silver from AgNO₃ solution, but the question asks about FeSO₄ solution and silver metal. Silver is less reactive than iron, so silver metal cannot displace iron from FeSO₄ solution.
4. Copper (Cu) is more reactive than silver (Ag). Therefore, copper metal can displace silver from AgNO₃ solution.

   $$Cu(s)\; +\; 2AgNO\_3(aq)\; \backslash rightarrow\; Cu(NO\_3)\_2(aq)\; +\; 2Ag(s)$$

Answer: The pair that will give a displacement reaction 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. Methods to prevent rusting involve creating a barrier between iron and the environment or making iron less susceptible to corrosion.

Let's evaluate the given options for an iron frying pan:

1. Applying grease: Grease forms a protective layer that prevents moisture and oxygen from reaching the iron surface, thus preventing rust. However, for a frying pan, this method is impractical as the grease would burn off during heating and be washed away during cleaning.
2. Applying paint: Paint also forms a barrier against moisture and oxygen. Similar to grease, paint is not suitable for a frying pan because it would peel off or burn due to the high temperatures involved in cooking and repeated washing.
3. Applying a coating of zinc (Galvanization): Zinc is more reactive than iron. When iron is coated with zinc, the zinc acts as a sacrificial metal. It corrodes preferentially, protecting the iron even if the coating is scratched. This is a very effective method for preventing rust.
4. All of the above: While grease, paint, and zinc coating are general methods to prevent rusting, only applying a coating of zinc is practically suitable for a frying pan in the long run, considering its use.

The question asks for a suitable method. While all methods can prevent rusting in general, the context of a frying pan makes grease and paint unsuitable. Galvanization (coating with zinc) is a suitable and durable method.

Answer: Applying a coating of zinc is the most suitable method among the choices for long-term protection of an iron frying pan, although grease and paint can prevent rusting in other contexts.

Solution: When an element reacts with oxygen, it forms an oxide. We are given that the resulting oxide has a high melting point and is soluble in water.

Let's consider the options:

1. Calcium (Ca): Calcium is a metal. It reacts with oxygen to form calcium oxide (CaO). Calcium oxide is a basic oxide with a very high melting point. Calcium oxide reacts with water to form calcium hydroxide (Ca(OH)₂), which is soluble in water (though sparingly soluble, it dissolves enough to form a solution).

   $$2Ca(s)\; +\; O\_2(g)\; \backslash rightarrow\; 2CaO(s)$$

   $$CaO(s)\; +\; H\_2O(l)\; \backslash rightarrow\; Ca(OH)\_2(aq)$$

2. Carbon (C): Carbon is a non-metal. It reacts with oxygen to form carbon dioxide (CO₂). CO₂ is a gas at room temperature and dissolves in water to form carbonic acid (H₂CO₃), but the oxide itself does not have a high melting point.
3. Silicon (Si): Silicon is a metalloid. It reacts with oxygen to form silicon dioxide (SiO₂), which has a very high melting point but is insoluble in water.
4. Iron (Fe): Iron is a metal. It reacts with oxygen to form iron oxides (like Fe₂O₃). Iron oxides have high melting points but are generally 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 the food from contamination and the can from corrosion, they are coated with a less reactive metal. This process is called tinning.

We need to compare the reactivity of zinc and tin:

According to the reactivity series of metals, zinc is more reactive than tin. Both zinc and tin are more reactive than iron.

If a food can were coated with zinc, and the coating got scratched, exposing the iron underneath, the more reactive zinc would react preferentially with any acidic food components or moisture, protecting the iron. However, this reaction could also lead to the dissolution of zinc into the food, which can be harmful. Furthermore, if the can is dented or scratched, the zinc coating will corrode preferentially, protecting the iron. This is called sacrificial protection.

Tin, being less reactive than zinc, also provides a protective barrier. If the tin coating is scratched, the iron is exposed. Since tin is less reactive than iron, it does not offer sacrificial protection. Instead, if both tin and iron are exposed to an electrolyte, iron (being more reactive than tin) will corrode faster. However, tin is non-toxic and does not react with most food substances, making it a safer choice for coating food containers, even if it doesn't provide sacrificial protection.

The primary reason for choosing tin over zinc for food cans is that zinc is more reactive than tin. While zinc offers sacrificial protection, its higher reactivity can lead to undesirable reactions with food if the coating is compromised. Tin provides a safer, non-reactive barrier.

Answer: Food cans are coated with tin and not with zinc because zinc is more reactive than tin.

Solution: We can use the given materials to test for two characteristic physical properties of metals: malleability and electrical conductivity.

(a) Distinguishing between metals and non-metals:

1. Test for Malleability: Take a small piece of the sample and strike it firmly with the hammer.
   • If the sample can be beaten into thin sheets without breaking, it exhibits malleability and is likely a metal. For example, metals like copper, aluminium, and iron can be hammered into sheets.
   • If the sample breaks into pieces or crumbles upon hammering, it is brittle and likely a non-metal. For example, coal or sulphur will break apart when hammered.
2. Test for Electrical Conductivity: Set up a simple circuit using the battery, bulb, wires, and switch. Connect the sample to be tested in series with the bulb and battery.
   • If the bulb glows when the sample is in the circuit, it means the sample conducts electricity and is therefore a metal. Metals like copper, iron, and aluminium are good conductors.
   • If the bulb does not glow, it means the sample does not conduct electricity and is likely a non-metal. Non-metals like wood, plastic, or coal are poor conductors (insulators).

(b) Usefulness of these tests:

These tests are quite useful in distinguishing between most metals and non-metals because they are based on fundamental physical properties that differ significantly between the two categories.

• Malleability: Most metals are malleable, while most non-metals are brittle. This test is straightforward and requires only a hammer.
• Electrical Conductivity: Metals are generally excellent conductors of electricity, whereas non-metals are poor conductors (insulators). This test is also simple to perform with basic electrical components.

These tests rely on physical properties, avoiding the need for chemical reactions, which can sometimes be complex or require specific reagents. Therefore, they provide a practical and effective way to differentiate between metals and non-metals in a laboratory or even in a field setting.

Solution: Amphoteric oxides are a special class of chemical compounds, specifically oxides, that exhibit dual chemical behaviour. They can react with both acids and bases, acting as either an acid or a base depending on the reactant.

This dual nature arises because these oxides contain elements that can exhibit variable oxidation states or have properties that allow them to interact with both acidic and basic substances.

Examples of amphoteric oxides:

1. Aluminium oxide (Al₂O₃): Aluminium oxide reacts with acids to form aluminium salts and water, behaving as a base. It also reacts with strong bases to form aluminates and water, behaving as an acid.

   Reaction with an acid (like Hydrochloric acid, HCl):

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

   In this reaction, Al₂O₃ acts as a base.

   Reaction with a base (like Sodium hydroxide, NaOH):

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

   Or, more simply represented as:

   $$Al\_2O\_3(s)\; +\; 2NaOH(aq)\; \backslash rightarrow\; 2NaAlO\_2(aq)\; +\; H\_2O(l)$$

   In this reaction, Al₂O₃ acts as an acid.

2. Zinc oxide (ZnO): Zinc oxide also displays amphoteric properties.

   Reaction with an acid (like Sulfuric acid, H₂SO₄):

   $$ZnO(s)\; +\; H\_2SO\_4(aq)\; \backslash rightarrow\; ZnSO\_4(aq)\; +\; H\_2O(l)$$

   Here, ZnO acts as a base.

   Reaction with a strong base (like Sodium hydroxide, NaOH):

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

   Or, more simply represented as:

   $$ZnO(s)\; +\; 2NaOH(aq)\; \backslash rightarrow\; Na\_2ZnO\_2(aq)\; +\; H\_2O(l)$$

   Here, ZnO acts as an acid.

Other examples include lead oxides (PbO, PbO₂) and tin oxides (SnO, SnO₂).