CAPF AC 2020 Q9: Science & Technology – Chemistry
- (a) Strontium salts burn with a red flame
- (b) Barium metal is used in medicine to examine the alimentary canal of a patient by X-ray
- (c) Green plants need magnesium in order to make the chlorophyll that trap the energy from sunlight in photosynthesis
- (d) Red colour of gemstone Ruby is due to the element beryllium present in it
Answer: (d) Red colour of gemstone Ruby is due to the element beryllium present in it
Explanation
Alkaline earth metals and the colour of ruby
How to think: three statements are standard NCERT lines on Group 2 metals.
The fourth mixes up two gemstones.
Beryllium belongs to beryl (emerald), not to ruby.
The alkaline earth metals are the Group 2 elements: beryllium, magnesium, calcium, strontium, barium and radium.
| Statement | Verdict | Fact |
|---|---|---|
| (a) Strontium salts burn with a red flame | True | Crimson red flame; used in red fireworks |
| (b) Barium is used to examine the alimentary canal by X-ray | True | A barium meal (barium sulphate) blocks X-rays |
| (c) Green plants need magnesium to make chlorophyll | True | Magnesium is the central atom of chlorophyll |
| (d) Red colour of gemstone Ruby is due to the element beryllium | NOT true | The red colour is due to chromium |
Ruby and emerald:
| Gemstone | Mineral | Colour comes from |
|---|---|---|
| Ruby | Corundum (aluminium oxide) | Chromium |
| Sapphire | Corundum (aluminium oxide) | Iron and titanium |
| Emerald | Beryl (contains beryllium) | Chromium |
Flame colours: calcium is brick red, strontium is crimson red, barium is apple green.
Answer: (d) Red colour of gemstone Ruby is due to the element beryllium present in it.
Why each option is right or wrong
- (a) Incorrect. This statement is true: strontium salts give a crimson red flame.
- (b) Incorrect. This statement is true: a barium meal is used for X-ray of the alimentary canal.
- (c) Incorrect. This statement is true: magnesium is at the centre of chlorophyll.
- (d) CORRECT. This is not true: ruby is aluminium oxide coloured red by chromium, with no beryllium.
Key point
Ruby = aluminium oxide (corundum) coloured by chromium. Beryllium is in emerald (beryl). Strontium: red flame; barium: X-ray meal; magnesium: chlorophyll.
Remember
Ruby is red because of chromium, not beryllium.
Common mistakes
- Linking beryllium with ruby; it belongs to emerald and aquamarine.
- Thinking the barium meal is barium metal; it is insoluble barium sulphate.
- Mixing up flame colours: strontium is red, barium is green.
Exam tip
Ruby and sapphire are both corundum (aluminium oxide). Beryllium points to beryl, which is emerald.
Sources
- NCERT Class XI Chemistry — The s-Block Elements (Group 2 elements)
- NCERT Class XII Chemistry — The d- and f-Block Elements (colour of transition metal ions)