Metals and Non-metals

Concept: A more reactive metal displaces a less reactive metal from its oxide. The reactivity order is: Magnesium > Zinc > Copper.

Analysis of each combination:

1. Zinc oxide + Zinc → No reaction (same metal).
2. Zinc oxide + Magnesium → Displacement occurs because Mg is more reactive than Zn.
$$\text{ZnO} + \text{Mg} \rightarrow \text{MgO} + \text{Zn}$$
3. Zinc oxide + Copper → No reaction because Cu is less reactive than Zn.
4. Magnesium oxide + Zinc → No reaction because Zn is less reactive than Mg.
5. Magnesium oxide + Magnesium → No reaction (same metal).
6. Magnesium oxide + Copper → No reaction because Cu is less reactive than Mg.
7. Copper oxide + Zinc → Displacement occurs because Zn is more reactive than Cu.
$$\text{CuO} + \text{Zn} \rightarrow \text{ZnO} + \text{Cu}$$
8. Copper oxide + Magnesium → Displacement occurs because Mg is more reactive than Cu.
$$\text{CuO} + \text{Mg} \rightarrow \text{MgO} + \text{Cu}$$
9. Copper oxide + Copper → No reaction (same metal).

Conclusion: Displacement reactions will take place in the following cases:
- Zinc oxide heated with Magnesium
- Copper oxide heated with Zinc
- Copper oxide heated with Magnesium

Answer: Metals that are least reactive (noble metals) do not corrode easily.

Examples: Gold (Au), Platinum (Pt), and Silver (Ag) do not corrode easily because they are very low in the reactivity series and do not react readily with oxygen, moisture, or other substances in the environment.

Definition: An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal.

Examples:
- Brass – a mixture of copper and zinc.
- Steel – a mixture of iron and carbon.
- Bronze – a mixture of copper and tin.

Significance: Alloys are made to improve the properties of metals, such as hardness, strength, and resistance to corrosion.

Correct Answer: (d) AgNO₃ solution and copper metal.

Concept: A more reactive metal displaces a less reactive metal from its salt solution. Reactivity order: Mg > Al > Zn > Fe > Cu > Ag.

(a) NaCl + Cu: Copper is less reactive than sodium, so no displacement occurs.

(b) MgCl₂ + Al: Aluminium is less reactive than magnesium, so no displacement occurs.

(c) FeSO₄ + Ag: Silver is less reactive than iron, so no displacement occurs.

(d) AgNO₃ + Cu: Copper is more reactive than silver, so copper displaces silver from silver nitrate solution.
$$\text{Cu} + 2\text{AgNO}_3 \rightarrow \text{Cu(NO}_3)_2 + 2\text{Ag}$$
This is a displacement reaction.

Correct Answer: (c) Applying a coating of zinc.

Justification: A frying pan is used for cooking at high temperatures. Grease and paint would burn off at high temperatures and are therefore not suitable. Applying a coating of zinc (galvanisation) is a durable method that protects iron from rusting even at higher temperatures, as zinc forms a protective oxide layer. Hence, option (c) is the most suitable method for a frying pan.

Correct Answer: (a) calcium.

Justification: Calcium reacts with oxygen to form calcium oxide (CaO), which has a very high melting point (~2613°C). Calcium oxide dissolves readily in water to form calcium hydroxide:
$$\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2$$
The oxides of carbon (CO₂) and silicon (SiO₂) are not highly soluble in water in the same manner, and iron oxide has a high melting point but is not soluble in water. Hence, the element is calcium.

Correct Answer: (c) zinc is more reactive than tin.

Justification: Zinc is more reactive than tin and would react with the food contents (especially acidic foods), making the food unsafe for consumption. Tin is less reactive and does not react with food substances, making it safe for coating food cans.

(a) Method to distinguish metals from non-metals:

Test 1 – Using the hammer (Malleability test):
- Strike the sample with the hammer.
- If the sample is beaten into a thin sheet (malleable), it is a metal.
- If the sample breaks into pieces (brittle), it is a non-metal.

Test 2 – Using the battery, bulb, wires and switch (Conductivity test):
- Set up a simple electric circuit: connect the battery, bulb, switch, and two open wire ends in series.
- Place the sample between the two open wire ends and close the switch.
- If the bulb glows, the sample conducts electricity → it is a metal.
- If the bulb does not glow, the sample does not conduct electricity → it is a non-metal.

(b) Assessment of usefulness of these tests:

These tests are useful but have limitations:
- Malleability test: Most metals are malleable and most non-metals are brittle, so this test works well in general. However, some metals like sodium are soft and may not behave like typical metals under a hammer.
- Conductivity test: This is a reliable test for most metals (good conductors) and non-metals (poor conductors). However, graphite (a non-metal/allotrope of carbon) is an exception — it conducts electricity and the bulb would glow, which could lead to incorrect identification.
- Therefore, both tests together give a more reliable result, but exceptions exist.

Definition: Oxides that react with both acids and bases to form salt and water are called amphoteric oxides.

Examples:
1. Aluminium oxide (Al₂O₃)
- Reacts with acid: $\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$
- Reacts with base: $\text{Al}_2\text{O}_3 + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O}$

2. Zinc oxide (ZnO)
- Reacts with acid: $\text{ZnO} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O}$
- Reacts with base: $\text{ZnO} + 2\text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2\text{O}$

Concept: Metals placed above hydrogen in the activity series can displace hydrogen from dilute acids. Metals below hydrogen cannot.

Two metals that WILL displace hydrogen from dilute acids:
1. Zinc (Zn): $\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$
2. Magnesium (Mg): $\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\uparrow$

Two metals that will NOT displace hydrogen from dilute acids:
1. Copper (Cu) — placed below hydrogen in the activity series.
2. Silver (Ag) — placed below hydrogen in the activity series.

Given: A metal M is to be refined by electrolytic refining.

In electrolytic refining:

| Component | Material Used |
|-----------|---------------|
| Anode | Thick block/slab of impure metal M |
| Cathode | Thin strip of pure metal M |
| Electrolyte | An aqueous solution of a soluble salt of metal M (e.g., if M = copper, then copper sulphate solution CuSO₄ is used) |

Process: When electric current is passed, the impure metal at the anode dissolves into the electrolyte, and pure metal gets deposited at the cathode. The impurities settle below the anode as anode mud.

Given: Sulphur is burned/heated and the gas evolved is collected.

Identification of gas: When sulphur burns in air/oxygen, it forms sulphur dioxide (SO₂).

(a) Action of the gas:

(i) Dry litmus paper:
SO₂ gas has no effect on dry litmus paper because the gas needs water to form an acid. Dry litmus paper will not change colour.

(ii) Moist litmus paper:
SO₂ dissolves in water present on the moist litmus paper to form sulphurous acid (H₂SO₃), which is acidic. Therefore, moist litmus paper will turn red (blue litmus turns red).
$$\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3$$

(b) Balanced chemical equation for the combustion of sulphur:
$$\text{S} + \text{O}_2 \xrightarrow{\Delta} \text{SO}_2$$

Two ways to prevent rusting of iron:

1. Painting/Oiling/Greasing: Applying a coat of paint, oil, or grease on the surface of iron prevents it from coming in contact with moisture and oxygen, thereby preventing rusting.

2. Galvanisation: Coating iron with a layer of zinc (a more reactive metal) protects it from rusting. Even if the zinc coating is scratched, zinc being more reactive than iron, gets oxidised preferentially and protects the iron beneath. This process is called galvanisation.

Answer: When non-metals combine with oxygen, they form acidic oxides or neutral oxides.

Acidic oxides: These dissolve in water to form acids.
- Example: $\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3$ (sulphurous acid)
- Example: $\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3$ (carbonic acid)

Neutral oxides: These are neither acidic nor basic.
- Example: Carbon monoxide (CO), Water (H₂O), Nitric oxide (NO).

(a) Platinum, gold and silver are used to make jewellery:
These metals are highly lustrous (shiny and attractive), malleable (can be beaten into desired shapes), and are least reactive (noble metals). They do not tarnish or corrode easily when exposed to air or moisture, so jewellery made from them retains its shine and beauty for a very long time.

(b) Sodium, potassium and lithium are stored under oil:
Sodium, potassium and lithium are extremely reactive metals. They react vigorously with oxygen and moisture present in air, and even catch fire spontaneously. To prevent them from reacting with air and water, they are stored under kerosene oil (or any inert oil), which acts as a barrier.

(c) Aluminium is highly reactive yet used to make cooking utensils:
Although aluminium is highly reactive, when exposed to air, it immediately reacts with oxygen to form a thin, hard, and stable layer of aluminium oxide (Al₂O₃) on its surface. This oxide layer is non-reactive and prevents further corrosion or reaction. This property makes aluminium safe and durable for making cooking utensils. It is also lightweight and a good conductor of heat.

(d) Carbonate and sulphide ores are converted into oxides during extraction:
It is easier to obtain metals from their oxides than from their carbonates or sulphides. Metal oxides can be directly reduced to metals using reducing agents like carbon (coke) at high temperatures. Therefore, carbonate ores are first heated (calcination) and sulphide ores are roasted (roasting) to convert them into their respective metal oxides before reduction:
- Calcination: $\text{ZnCO}_3 \xrightarrow{\Delta} \text{ZnO} + \text{CO}_2$
- Roasting: $2\text{ZnS} + 3\text{O}_2 \xrightarrow{\Delta} 2\text{ZnO} + 2\text{SO}_2$

Explanation:

Copper vessels, when exposed to moist air, get coated with a layer of copper carbonate (a green dull layer), which is a basic compound. This makes the vessels look tarnished and dull.

Lemon juice and tamarind juice contain organic acids (citric acid and tartaric acid respectively). When these acidic substances are applied to the tarnished copper vessel, the acid reacts with the basic copper carbonate and dissolves it:
$$\text{Copper carbonate} + \text{Acid} \rightarrow \text{Salt} + \text{Water} + \text{CO}_2$$

This removes the dull green layer and exposes the shiny copper surface underneath, making the vessel look clean and bright. This is why sour substances (which are acidic) are effective in cleaning tarnished copper vessels.

Differences between Metals and Non-metals based on Chemical Properties:

| Property | Metals | Non-metals |
|---|---|---|
| Reaction with oxygen | Form basic oxides (e.g., CuO, MgO) | Form acidic or neutral oxides (e.g., SO₂, CO₂, CO) |
| Reaction with water | React with water/steam to form metal hydroxide/oxide and hydrogen gas (e.g., $2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2$) | Generally do not react with water |
| Reaction with dilute acids | React with dilute acids to produce hydrogen gas (e.g., $\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2$) | Do not react with dilute acids to produce hydrogen |
| Reaction with hydrogen | Do not react with hydrogen | React with hydrogen to form hydrides (e.g., H₂S, HCl, NH₃) |
| Ion formation | Form positive ions (cations) by losing electrons | Form negative ions (anions) by gaining electrons |
| Displacement | More reactive metals displace less reactive metals from salt solutions | Non-metals do not displace hydrogen from acids |
| Reducing/Oxidising nature | Act as reducing agents (lose electrons) | Act as oxidising agents (gain electrons) |

Detective Analysis:

Observation: The gold bangles sparkled like new (became shiny) but their weight was drastically reduced.

Deduction: The solution used by the man was Aqua Regia (Royal Water).

What is Aqua Regia? It is a freshly prepared mixture of concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO₃) in the ratio of 3:1 (by volume).

Explanation: Aqua regia is one of the very few substances that can dissolve gold. When the bangles were dipped in aqua regia, some gold from the surface dissolved in the solution. This removed the dull outer layer and exposed the shiny gold beneath, making the bangles sparkle. However, since some gold was dissolved away, the weight of the bangles was drastically reduced.

$$\text{Au} + 3\text{HNO}_3 + 4\text{HCl} \rightarrow \text{AuCl}_3 + 3\text{NO}_2 + 3\text{H}_2\text{O}$$

Conclusion: The fraudulent goldsmith used aqua regia to clean the bangles, but in doing so, he dissolved a significant amount of gold from the bangles, reducing their weight and thus cheating the lady.

Reason:

Copper is preferred over steel for making hot water tanks because of the following reasons:

1. Copper does not react with water: Copper is a less reactive metal and does not react with hot or cold water. Steel (iron alloy), on the other hand, reacts with water and oxygen (especially hot water) to form rust (iron oxide), which weakens the tank over time.

2. Corrosion resistance: Copper is highly resistant to corrosion by water. Iron (steel) corrodes readily in the presence of water and oxygen, a process called rusting:
$$4\text{Fe} + 3\text{O}_2 + x\text{H}_2\text{O} \rightarrow 2\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O (rust)}$$

3. Good conductor of heat: Copper is an excellent conductor of heat, which is useful for heating water efficiently.

Conclusion: Since copper does not corrode with water and is a good conductor of heat, it is more suitable than steel for making hot water tanks.