The correct option is Gallium and caesium.
Explanation
Metals generally possess high melting and boiling points due to strong metallic bonding. However, exceptions exist where the interatomic forces are weaker, resulting in melting points near or below standard room and body temperatures.
Option Analysis
- Gallium and caesium is correct: Gallium (Ga) has a melting point of approximately 29.76°C (302.91 K), and Caesium (Cs) has a melting point of approximately 28.44°C (301.59 K). Since the normal human body temperature is roughly 37°C (310 K), both metals absorb sufficient heat when placed on the palm to transition from a solid to a liquid state.
- Sodium and potassium is incorrect: Sodium (Na) and Potassium (K) are alkali metals with relatively low melting points (Na: ~98°C, K: ~63°C) compared to most other metals, but they remain solid at human body temperature.
- Iron and copper is incorrect: Iron (Fe) and Copper (Cu) are transition metals with very high melting points (Fe: ~1538°C, Cu: ~1085°C) due to strong metallic bonds involving d-electrons.
- Zinc and aluminium is incorrect: Zinc (Zn) and Aluminium (Al) have melting points of approximately 419.5°C and 660.3°C, respectively, which are far above body temperature.
Key Takeaway
Gallium and Caesium are distinct among metals for having melting points just below human body temperature (37°C), causing them to melt upon contact with the palm, unlike other low-melting alkali metals like Sodium or Potassium.