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A mixture of ice and water is contained in a piston cylinder arrangement as shown in figure. If heat  is added in the system and the piston is allowed to move as the ice melts. The Heat Q and the work W for the system ( water + ice ) are:

A mixture of ice and water is contained in a piston cylinder arrangement as shown in figure. If heat  is added in the system and the piston is allowed to move as the ice melts. The Heat Q and the work W for the system ( water + ice ) are: Correct Answer Q > 0, W < 0

Explanation:

  • Heat is added to the system which is taken as positive, i.e. Q > 0, if Heat added to the system.
  • Water exhibits an anomalous behavior, it contracts on heating between 0°C and 4°C.
  • The volume of a given amount of water decreases as it is cooled from room temperature until its temperature reaches 4°C. Below 4 °C, the volume increases, and therefore the density decreases.
  • This means that water has a maximum density at 4°C
  • As explained above, when we cool water below 4°C the water molecules present in ice form have a lot of empty space within the ice crystal.
  • As the ice melt, the inter-molecular distance decreases, when the water comes to 4 °C then the density of the water will increase first and then decrease as shown below.

​Mistake Points

  • On increasing temperature volume is not always increases, here behavior of water is anonymously different from other material.
  • Here generally students marked as W > 0 when heat is added to the system which is incorrect.

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