Chemistry 2025 Jamb Examination

In the coordination compound [Cu(NH₃)₄]Cl₂, there are different types of bonding present. The copper ion and the ammonia ligands are held together by dative (coordinate) covalent bonds within the complex ion [Cu(NH₃)₄]²⁺.
However, the bond between this large positively charged complex ion and the negatively charged chloride ions (Cl^-) is established through electrostatic attraction. This attraction between oppositely charged ions is the definition of an ionic bond.

A reducing agent is a substance that loses electrons to another reactant, thereby reducing the other reactant and becoming oxidised in the process.
In this reaction, the oxidation state of Magnesium (Mg) increases from 0 to +2 (in MgCl₂). This increase in oxidation state indicates that Magnesium has lost electrons. Since it provides electrons to the hydrogen ions in HCl, it acts as the reducing agent and is itself oxidised.

A physical change is a reversible change where no new chemical substance is formed.
When sodium chloride (table salt) is dissolved in water, the salt crystals break apart into ions, but the chemical identity of the salt remains unchanged. The process is easily reversed by evaporating the water to recover the solid salt. In contrast, the other options involve chemical reactions that produce new substances (calcium hydroxide, iron oxide, or carbon dioxide).

To isolate specific components of air, different reagents are used to absorb specific gases:
1. Potash (KOH) absorbs Carbon(IV) oxide (CO₂).
2. Alkaline pyrogallol absorbs Oxygen (O₂).
3. Fused calcium chloride acts as a desiccant to absorb water vapour.
After passing through these reagents, the only major components of air that remain unabsorbed are Nitrogen (which is relatively inert) and the noble gases (like Argon).

The group number of an element in the periodic table is determined by the number of electrons in its outermost (valence) shell.
Looking at the configuration 1s² 2s² 2p⁶ 3s² 3p⁵, the outermost shell is the 3rd shell (n=3). The number of electrons in this shell is 2 (from 3s) + 5 (from 3p) = 7 electrons. Therefore, the element belongs to Group 7 (the Halogens).

A covalent bond involves the sharing of a pair of electrons between two atoms. A 'polar' covalent bond occurs when there is a significant difference in electronegativity between the two atoms.
The more electronegative atom pulls the shared electron pair closer to itself, leading to an unequal sharing of the electron pair and the creation of partial positive and negative charges (dipoles).

Standard cell notation follows the convention: Anode | Anode Ion || Cathode Ion | Cathode.
1. At the anode (oxidation): Mg is oxidised to Mg²⁺, so we write Mg | Mg²⁺.
2. At the cathode (reduction): Pb²⁺ is reduced to Pb, so we write Pb²⁺ | Pb.
Connecting them with a double bar for the salt bridge gives: Mg | Mg²⁺ || Pb²⁺ | Pb.

According to Graham's Law of Diffusion, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. This means heavier gases diffuse slower, and lighter gases diffuse faster.
Molar masses:
- SO₂ = 32 + (16 \(\times\) 2) = 64
- O₂ = 16 \(\times\) 2 = 32
- NH₃ = 14 + (1 \(\times\) 3) = 17
- H₂ = 1 \(\times\) 2 = 2
Increasing rate means from heaviest to lightest: SO₂ (64) < O₂ (32) < NH₃ (17) < H₂ (2).

Litmus paper is an indicator used to identify acidic or basic substances. Red litmus turns blue in the presence of a base (alkaline solution).
On the pH scale:
- pH < 7 is acidic.
- pH = 7 is neutral.
- pH > 7 is basic/alkaline.
Of the given values, only 8.0 is greater than 7, meaning it is the only basic solution capable of turning red litmus blue.

Corrosion is the slow deterioration of metals due to chemical reactions with their environment. For iron and its alloys (like steel), this specific oxidation process in the presence of oxygen and moisture is called rusting. [Image showing the process of iron rusting]
The result is the formation of hydrated iron(III) oxide, a reddish-brown flaky solid. Galvanizing and alloying are actually methods used to *prevent* corrosion.