Q: Why does a liquid remain hot or cold for a long time inside a thermos flask?
Answer: Do you know that thermos flask has double glass wall? The air is trapped in between this double wall of glass. When a liquid hot or cold is put into a thermos then this air does not allow the heat to loose through it. Do you know why? Yes, you are right, because the air is an insulator or poor conductor of heat.
This is the only simple reason that trapped air in double glass will not allow the heat to loose and this is the reason that a liquid remain hot or cold for a long time inside a thermos.
Q: Why is cooking quicker in a pressure cooker?
Answer: The boiling point of a liquid increases with increase in the pressure. When the pressure inside a pressure cooker is increased, the boiling point of the water is increased. More the boiling point, easier will be the cooking. Therefore due to this reason, cooking is quicker in a pressure cooker.
Q: Why does ice float on water but sink in alcohol?
Answer: It is simple as we all know that heavier things sink and lighter things float in liquids. Therefore, as ice is lighter than water so it floats in water and ice is heavier than alcohol thus it sinks in alcohol.