Explain briefly each of the following observations: Ammonium trioxocarbonate(IV) leaves no residue when heated strongly. (ii) Solid ionic compounds do not conduct electric current. (iii) The melting point of sodium chloride crystals is higher than that of paraffin wax.

Here are the solutions to the questions you've provided.

Question 1:

a) (i) Three properties of elements which generally increase across a period in the periodic table are:

• Electronegativity
• Ionization energy
• Electron affinity

a) (ii) For electronegativity:
As you move across a period, the nuclear charge increases due to more protons in the nucleus, while the number of electron shells remains the same. This increased nuclear attraction pulls the valence electrons closer to the nucleus, leading to a stronger attraction for shared electrons in a covalent bond, hence increasing electronegativity.

b) (i) To arrange K, Li, and Na in order of increasing size:
These are Group 1 elements. Atomic size increases down a group because the number of electron shells increases, and the outermost electrons are further from the nucleus, experiencing less effective nuclear charge due to shielding.
Order: $\boxed{\text{Li} < \text{Na} < \text{K}}$

b) (ii) To arrange $\text{Cl}^-$, $\text{Ca}^{2+}$, and $\text{K}^+$ in order of increasing size:
These are isoelectronic species, meaning they all have the same number of electrons (18 electrons, like Argon). For isoelectronic species, the size decreases as the nuclear charge (number of protons) increases, because a greater positive charge pulls the same number of electrons more strongly.

• $\text{Cl}^-$: 17 protons, 18 electrons
• $\text{K}^+$: 19 protons, 18 electrons
• $\text{Ca}^{2+}$: 20 protons, 18 electrons Order: $\boxed{\text{Ca}^{2+} < \text{K}^+ < \text{Cl}^-}$

c) Three characteristic properties of transition metals are:

• They exhibit variable oxidation states.
• They form colored compounds (or ions) in solution and in solid state.
• They can act as catalysts.
• They form complex ions.

d) (i) An alpha particle:

• Mass: 4 atomic mass units (approximately)
• Charge: +2

d) (ii) Gamma rays are more injurious to health than alpha particles because:
Gamma rays are high-energy electromagnetic radiation with very high penetrating power, meaning they can pass through significant amounts of matter and cause damage deep within tissues and organs. Alpha particles, while highly ionizing, have very low penetrating power and can be stopped by a sheet of paper or the outer layer of skin, making them less harmful unless ingested or inhaled.

Question 2:

a) (i) A mole is the amount of substance that contains as many elementary entities (atoms, molecules, ions, etc.) as there are atoms in 12 grams of carbon-12. This number is Avogadro's constant, approximately $6.022 \times 10^{23}$.

a) (ii) A standard solution is a solution of accurately known concentration. It is typically prepared by dissolving a precisely weighed amount of a primary standard substance in a known volume of solvent.

b) Hydrogen chloride gas dissolved in:

• (i) Water: Forms hydrochloric acid ($\text{HCl(aq)}$), which is a strong acid and dissociates completely into $\text{H}^+$ and $\text{Cl}^-$ ions. These mobile ions allow the solution to conduct electricity.
• (ii) Benzene: $\text{HCl}$ remains as covalent molecules in benzene and does not ionize. Therefore, the solution will not conduct electricity. The solution in water will conduct electricity.

c) (i) To arrange equimolar solutions of sodium chloride, sucrose, and ethanoic acid in their increasing ability to conduct electricity:

• Sucrose ($\text{C}_{12}\text{H}_{22}\text{O}_{11}$): A non-electrolyte; it dissolves but does not ionize, so it has no ability to conduct electricity.
• Ethanoic acid ($\text{CH}_3\text{COOH}$): A weak electrolyte; it partially ionizes in water, producing a low concentration of ions and thus has a poor ability to conduct electricity.
• Sodium chloride ($\text{NaCl}$): A strong electrolyte; it completely dissociates into $\text{Na}^+$ and $\text{Cl}^-$ ions in water, producing a high concentration of ions and thus has a strong ability to conduct electricity. Order: $\boxed{\text{Sucrose} < \text{Ethanoic acid} < \text{Sodium chloride}}$

c) (ii) The solution with the lowest pH is ethanoic acid.

• Ethanoic acid is a weak acid, so its solution will be acidic (pH < 7).
• Sodium chloride is a salt of a strong acid and a strong base, so its solution is neutral (pH = 7).
• Sucrose is a non-ionic molecular compound, so its solution is neutral (pH = 7).

d) To calculate the atomic mass of zinc: