Plaster of Paris is CaSO₄·½H₂O. It is made by heating gypsum (CaSO₄·2H₂O) at 373 K. It sets with water to form gypsum again. Used for plastering fractured bones, making statues.
Consider the following statements about the modern periodic table: 1. Elements are arranged by atomic mass. 2. It has 18 groups and 7 periods. 3. The f-block elements are called inner transition metals. Which are correct?
Explanation:
The modern periodic table is based on atomic number, not atomic mass (Mendeleev's table was based on mass). It consists of 18 vertical groups and 7 horizontal periods. The f-block elements, placed at the bottom, are indeed called inner transition metals (lanthanides and actinides). Thus, statements 2 and 3 are correct.
Explanation:
Burning of a candle involves combustion of wax (hydrocarbons) with oxygen to produce CO₂ and H₂O, a chemical change. The melting of wax is a physical change. Cutting wood is physical (size change). Freezing water is physical (state change). A chemical change results in new substances with different properties. Often irreversible.
Explanation:
CNG stands for Compressed Natural Gas. It is primarily methane stored at high pressure. It is considered a cleaner fuel than petrol or diesel because it produces fewer pollutants like carbon monoxide and unburnt hydrocarbons, and it leaves no residue upon combustion.
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