Wax is coated on the inner wall of a
capillary tube and the tube is then dipped
water. Then, compared to the unwaxed
capillary, the angle of contact theta and the height h Upto which water rises change.
These changes are:
1)Theta increases and h also increases
2)Theta decreases and h also decreases
3)Theta increases and h decreases
4)Theta decreases and h increases
may the adhesion forces between the water molecules and glass molecules decreases so capillarity height will decrease.
The correct answer is 3) Theta increases and h decreases.
When a capillary tube is coated with wax, it becomes hydrophobic, which means it repels water. As a result, the angle of contact between the water and the wax-coated capillary wall increases. The angle of contact is the angle formed between the tangent to the liquid surface at the point of contact and the plane of the solid surface underneath.
When the angle of contact increases, it implies that the adhesive forces between the water molecules and the wax-coated capillary wall are weak, while the cohesive forces between water molecules are stronger. This weak adhesion and stronger cohesion cause the water to rise to a lesser height (h) in the wax-coated capillary compared to an unwaxed capillary, where the angle of contact is smaller.
Therefore, the angle of contact (theta) increases and the height to which water rises (h) decreases in a wax-coated capillary tube compared to an unwaxed capillary tube.
To understand the changes in the angle of contact (theta) and the height to which water rises (h) when a capillary tube is coated with wax and then dipped in water, we need to consider the effect of wax on the interaction between water, capillary tube, and air.
The angle of contact (theta) measures the interaction between the liquid (water), the solid (capillary tube), and the surrounding air. It determines whether the liquid will wet the solid surface or not. A larger angle of contact means a lesser wetting of the solid surface, and a smaller angle of contact signifies more wetting.
Now, let's analyze the options:
1) Theta increases and h also increases: If the angle of contact increases, it means the liquid is less inclined to wet the solid surface (capillary tube). In this case, the water will not rise as high as before. Therefore, Option 1 is incorrect.
2) Theta decreases and h also decreases: If the angle of contact decreases, it signifies that the liquid is more inclined to wet the solid surface. This increased wetting will cause the water to rise higher in the capillary tube. Hence, Option 2 is incorrect.
3) Theta increases and h decreases: This option matches the explanation provided for Option 1, where an increase in the angle of contact leads to a decrease in the height to which water rises. Therefore, Option 3 is incorrect.
4) Theta decreases and h increases: As explained above, when the angle of contact decreases, the liquid is more inclined to wet the solid surface. Consequently, the water will rise to a greater height in the capillary tube. This matches the explanation given for Option 2. Thus, Option 4 is the correct answer.
In summary, when wax is coated on the inner wall of a capillary tube and then dipped into water, the angle of contact (theta) decreases, and the height (h) to which water rises increases.