Question

blackboard dries up after wiping it why

Answer 1

- A blackboard is not porous, i.e. it actually never takes up much water from the sponge in the first place
 - Yet the surface is hydrophilic, i.e. the water that does stay on the board forms a very thin film instead of droplets and together with the slightly rough texture this makes for a large surface area to only a very small volume of water.
- The board is mounted vertically. That's the ideal configuration for convection: water vapour has a lower density than air, so close to the surface the air rises up, and because the entire surface is aligned in the same direction and air can efficiently stream along the surface from below (turbulence helps further), there's a steady supply of unsaturated air into which more water can evaporate unhindered.
 - The bulk of the board is usually metallic, i.e. it has good thermal conductivity.

hope this helps....