The method of purifying water by passing it through a semipermeable membrane under pressure is called: MCQ with Answer and Explanation

Explanation:
Teflon is polytetrafluoroethylene (PTFE) polymerized from tetrafluoroethene (CF₂=CF₂). CH₂=CH₂ gives polythene; CH₂=CHCl gives PVC; CH₂=CH-C₆H₅ (styrene) gives polystyrene. The C-F bond is extremely strong, giving Teflon its chemical inertness and thermal stability.

Explanation:
Cement sets by hydration reactions forming C-S-H gel and Ca(OH)₂, even underwater. It's a chemical process, not drying. Gypsum controls the setting rate.

Explanation:
Nitrogen has 5 valence electrons; it needs 3 more to complete the octet. It shares three electrons with three hydrogen atoms, forming three covalent bonds. Therefore, valency is 3. In the case of nitrogen, covalency is 3, while its electrovalency could be 5 (losing all 5). For covalent molecules, valency = number of shared electron pairs. In N₂, valency is also 3 (triple bond).