Heat a test tube of water near the top with a weighted ice cube near the bottom.even when the water at the top starts boiling,the ice cube does not melt.why
Answers
Water has a very large heat capacity, so it takes a lot of energy to change the temperature. Warmer water will be less dense and move upward to the top of the test tube to boil while the colder water can stay at the bottom. Water does not conduct heat through it very much and ice conducts the heat through much less. This can be shown by a pond of alcohol that can burn when lit on fire sitting on a block of ice that would slowly melt.
Since the conductivity of water allows t. he bottom layer of water to remain cool and stationary the ice block stays there while the upper layer of water takes heat from a flame and boils.
Vertical movement of water by convection is the best way for heat transfer to mix water. so that it is at the same temperature. This is seen by boiling bubbles at the bottom of a stove moving up as a pan heats up.
A demonstration of convection is possible by a dropper of food coloring placed near the bottom of a cold water glass. Also an ice cube that had food coloring put in can be placed at the bottom of the glass. When the water is cold such as at 38 deg F, the color droplet just sits there and does not move. If it is placed in hot water such as at 100 deg F, the color dissipates through the entire glass fairly fast. Try looking at the glass in the microwave or with a burner put under it.
Another example of temperature stratification in water can be felt when swimming in a pond that does not allow sunlight to penetrate very far because of silt in the murky water. It can be felt that your feet are in much colder water just five feet down than the water that the sun heats at the surface of the pond.
Although the water thermal conductivity is fairly low, dry air has a much lower conductivity so air provides excellent insulation compared to water. Metal, especially copper and aluminum have a much higher thermal conductivity than water. Placing a copper wire in the test tube with the ice cube at the bottom will make the ice melt much faster,