ANS:A - floating hollow polypropylene spheres

Insulation of a liquid surface can be achieved by floating hollow polypropylene spheres on top of its surface. Explanation:

• Floating hollow polypropylene spheres are commonly used as insulation materials in various industrial applications, including liquid storage tanks, ponds, and reservoirs.
• These spheres are lightweight, buoyant, and have low thermal conductivity, making them effective insulators when placed on the surface of a liquid.
• When the spheres are spread across the liquid surface, they create a barrier that reduces heat transfer between the liquid and the surrounding environment.
• The trapped air within the hollow spheres provides additional insulation by minimizing conduction and convection of heat.
• Other methods such as spraying alumina powder or putting cement on the liquid surface may not provide effective insulation and can even be detrimental to the liquid's properties or intended use.
• Therefore, floating hollow polypropylene spheres are the preferred choice for insulating liquid surfaces in many industrial settings.