Compound L decomposes on heating. One of the products is gas M. M reacts with unburned hydrocarbons to form peroxyacetyl nitrate, PAN. What could be the formula of L?
▶️ Answer/Explanation
Ans: B
To identify compound L, we analyze the given conditions:
- Decomposition of L produces gas M: The gas M reacts with hydrocarbons to form PAN (peroxyacetyl nitrate), which is a component of smog. This suggests M is NO₂, as NO₂ is involved in smog formation.
- Possible decomposition reactions:
- Ca(NO₃)₂ decomposes on heating to produce CaO, NO₂, and O₂.
- MgCO₃ decomposes to MgO and CO₂, but CO₂ does not form PAN.
Since NO₂ is required for PAN formation, the correct compound L must be Ca(NO₃)₂ (Option B).
V and W are two compounds. Each one contains a different Group 2 element. A sample of each solid is added to water, shaken, and the pH of the resulting solution is measured.
Which row could identify V and W? 
▶️ Answer/Explanation
Ans: C
Group 2 oxides (e.g., MgO, CaO) react with water to form hydroxides, producing alkaline solutions (pH > 7). The pH increases down the group as solubility increases. V (pH 9) likely contains a less soluble oxide (e.g., MgO), while W (pH 12) contains a more soluble oxide (e.g., BaO). Option C matches this trend, as BaO (higher pH) and MgO (lower pH) fit the observed data.
X and Y are elements in Period 3 of the Periodic Table. Y has a greater atomic number than X. The stable ion formed by Y has a greater radius than the stable ion formed by X. The stable ion formed by Y has 18 electrons. Which row is correct?
▶️ Answer/Explanation
Ans: A
Given that Y has a greater atomic number than X and forms an ion with 18 electrons, Y must be a Group 16 or 17 element (e.g., S or Cl). The ion with 18 electrons suggests Y forms \( \text{Y}^{2-} \) (if S) or \( \text{Y}^- \) (if Cl). Since the ion of Y is larger than that of X, X must form a smaller cation (e.g., \( \text{Na}^+ \) or \( \text{Mg}^{2+} \)). Comparing the options, Row A (X = Na, Y = S) fits because \( \text{S}^{2-} \) has 18 electrons and a larger radius than \( \text{Na}^+ \).
Sodium and sulfur are burned separately in oxygen. Each reaction has a distinctive coloured flame. Which row is correct?
▶️ Answer/Explanation
Ans: C
When sodium burns in oxygen, it produces a yellow flame due to the emission spectrum of sodium. When sulfur burns in oxygen, it produces a blue flame due to the formation of sulfur dioxide (\(SO_2\)). Only row C correctly matches these flame colors, making it the correct answer.