How many of the following metals can be detected in the laboratory through flame test
i) Be
ii) Mg
iii) Na
iv) K
v) Ba

Answer

To determine how many of the listed metals can be detected in the laboratory through a flame test, we will analyze each metal one by one.

1. **Beryllium (Be)**: Beryllium does not impart any color to the flame. This is because its valence electrons are tightly bound to the nucleus due to its small atomic size and high ionization energy. Therefore, it cannot be detected through a flame test.

2. **Magnesium (Mg)**: Similar to beryllium, magnesium also does not produce a flame color. The reason is the same: the electrons in magnesium are strongly attached to the nucleus, requiring a significant amount of energy for excitation, which is not provided by the flame. Thus, magnesium cannot be detected through a flame test.

3. **Sodium (Na)**: Sodium produces a bright yellow color when subjected to a flame test. This is due to the excitation of its electrons, which emit light at a specific wavelength when they return to their ground state. Therefore, sodium can be detected through a flame test.

4. **Potassium (K)**: Potassium gives a lilac or light purple color in a flame test. Similar to sodium, the electrons in potassium can be excited by the heat of the flame, allowing it to be detected. Therefore, potassium can also be detected through a flame test.

5. **Barium (Ba)**: Barium produces a green color in a flame test. The electrons in barium can be excited by the flame, allowing it to be detected. Thus, barium can also be detected through a flame test.

Now, summarizing the findings:
- Be: Not detectable
- Mg: Not detectable
- Na: Detectable
- K: Detectable
- Ba: Detectable

**Conclusion**: Out of the five metals listed, three can be detected through a flame test (Na, K, and Ba).

**Final Answer**: 3 metals can be detected through a flame test.

---