Calorific value is the energy released per unit mass or volume of fuel. Common units: kJ/kg, kJ/g, J/g, kcal/kg, Btu/lb. It measures fuel efficiency. Hydrogen has about 141.9 MJ/kg (higher), petrol ~47 MJ/kg. Temperature (°C) is not a unit for calorific value. Pressure and concentration are not appropriate.
Explanation:
Sodium is a soft alkali metal that can be easily cut with a knife, revealing a silvery surface that tarnishes quickly in air. Potassium is also soft. Iron, zinc, and copper are much harder. Alkali metals have low density and low melting points. They are stored under kerosene to prevent reaction with moisture and oxygen.
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