(a) Intracellular fluid is the solution between the cells in the human body. Table 2.1 shows the masses, in mg, of some ions in \(100cm^3\) of intracellular fluid.
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 8.2 — Group I properties (Part (a)(i), (e))
• Topic 8.1 — Arrangement of elements / Periodic Table (Part (a)(ii))
• Topic 12.5 — Identification of ions and gases (Part (b))
• Topic 2.1 — Elements, compounds and mixtures (Part (c))
• Topic 7.3 — Preparation of salts / solubility rules (Part (d))
• Topic 2.2 — Atomic structure and the Periodic Table / reactions (Part (f))
(a)(i)
Calcium (Ca²⁺)
The table shows the mass of calcium ions is 0.1 mg, which is significantly lower than potassium (150 mg), sodium (10 mg), or magnesium (30 mg). A lower mass in the same volume corresponds directly to a lower concentration of that positive ion.
(a)(ii)
Hydrogen carbonate (HCO₃⁻)
Group IV of the Periodic Table contains Carbon (C). Hydrogen carbonate ions are the only ions listed in Table 2.1 that contain a Group IV element (carbon) within their molecular structure.
(b)
Add dilute hydrochloric acid (or nitric acid) followed by barium chloride solution; a white precipitate (barium sulfate) confirms sulfate ions.
Acidifying first removes carbonate or sulfite impurities that could also form precipitates with barium. The formation of an insoluble white precipitate of BaSO₄ is a positive, unambiguous test for the sulfate anion (SO₄²⁻).
(c)
NH₄Cl
Ammonium (NH₄⁺) carries a 1+ charge, and chloride (Cl⁻) carries a 1- charge. Therefore, they combine in a 1:1 ratio to form the neutral ionic compound ammonium chloride.
(d)
Lead chloride (PbCl₂)
According to general solubility rules, most chlorides are soluble except those of lead, silver, and mercury(I). Sodium chloride, ammonium chloride, and magnesium chloride are all highly soluble in water, unlike lead chloride.
(e)
Hardness: between 0.25 and 0.65 MPa (e.g., 0.4 MPa). Observations: Extremely rapid reaction producing a lilac flame, or explosion/bubbles form more rapidly than potassium.
Group I hardness decreases down the group (lithium 0.6, sodium 0.4, so potassium ~0.2-0.3). Reactivity increases down the group; rubidium reacts more violently than potassium, producing hydrogen that ignites instantly.
(f)
2Na + H₂ → 2NaH
Hydrogen gas (H₂) exists as a diatomic molecule. The equation requires 2 sodium atoms to balance the 2 sodium atoms in 2NaH, and the 2 hydrogen atoms from one H₂ molecule provide the hydrogen for the two formula units.
