Why does paper curl on burning

The upper or flame side of the paper loses water faster compared with the lower side. This is because paper is not a good conductor of heat and hence both sides are not heated to the same extent by the flame. That only the upper side curls can be further tested by the turning the burning paper by 180 degrees. The curling reverses and does not continue to be in the direction of the side that is now the lower side.

Another interesting way to check the direction of curling is by wetting only one side of the paper. The side of the paper that is dry will tend to curl.

Paper tends to curl basically because the expansion of the fibers on both sides of the paper is different when it either brunt or wetted on one side only. In the case of the burning paper, the upper side loses water faster and hence the curling is seen on that side. In the case where the paper is wetted on one side only, the side that is dry tends to curl. Curling is not seen when both sides of the paper gets wet at the same time as when it is immersed in water.

One can control the way direction of curling in a paper that is kept with neither sides being the upper or lower side by gently curling it in one direction. The curling is seen in the direction that is concave.

The nature and extend of curling when the paper is burnt can be checked using papers of different thickness. The thinner the paper, the lesser the curling seen in one direction.