JEE Main 2021 · 24 February · evening · Single correct · +4 / −1
Match List - I with List - II.
Choose the correct answer from the options given below :
| List I (Salt) | List II (Flame colour wavelength) | ||
|---|---|---|---|
| (a) | LiCl | (i) | 455.5 nm |
| (b) | NaCl | (ii) | 670.8 nm |
| (c) | RbCl | (iii) | 780.0 nm |
| (d) | CsCl | (iv) | 589.2 nm |
- (a) (a)-(ii), (b)-(i), (c)-(iv), (d)-(iii)
- (b) (a)-(iv), (b)-(ii), (c)-(iii), (d)-(i)
- (c) (a)-(i), (b)-(iv), (c)-(ii), (d)-(iii)
- (d) (a)-(ii), (b)-(iv), (c)-(iii), (d)-(i)
Answer and solution
Answer (d) (a)-(ii), (b)-(iv), (c)-(iii), (d)-(i)
Solution
Chemistry ma’am · 3 steps
Turn each flame colour into its place on the spectrum, and every wavelength finds its salt.
- Why flames glow. Heat excites the valence electron of the metal atom. When it falls back, it gives out light of a fixed wavelength, so each alkali metal has its own flame colour.
- Colour to wavelength. In the visible spectrum, blue lies near 450–490 nm, yellow near 580–590 nm and red near 620–750 nm. A line at 780 nm sits at the deep-red edge, just past the visible range.
Salt Flame colour Wavelength Match LiCl crimson red 670.8 nm (a)-(ii) NaCl golden yellow 589.2 nm (b)-(iv) RbCl red-violet 780.0 nm (c)-(iii) CsCl blue 455.5 nm (d)-(i) - So. The matching is (a)-(ii), (b)-(iv), (c)-(iii), (d)-(i), which is option (d). Lithium at 670.8 nm and sodium at 589.2 nm alone already rule out every other option.
The idea to keep. Learn the flame colours, not just the wavelengths: Li crimson, Na yellow, K violet, Rb red-violet, Cs blue. Then place each colour on the spectrum, blue short and red long.
Answer from the NTA final answer key. Solution by PrepWiser.