Wax is coated on the inner wall of a capillary tube and the tube is then dipped in water. Then, compared to the unwaxed capillary, the angle of contact θ and the height h upto which water rises change. These changes are:

CONCEPT:

Capillary Action and Contact Angle

• When a liquid rises or falls in a narrow tube due to the adhesive forces between the liquid and the tube wall and the cohesive forces within the liquid, it is known as capillary action.
• The angle of contact (θ) is the angle formed between the tangent to the liquid surface and the solid surface inside the tube.
• Water typically has a low contact angle with materials it can wet (hydrophilic materials), leading to capillary rise.
• If the tube is made hydrophobic (e.g., by coating with wax), the contact angle increases, leading to a decrease in the height of the liquid rise.

EXPLANATION:

• Let's examine the given options:
  • Option 1: θ increases and h also increases
    • This option is incorrect because if the contact angle θ increases (due to wax coating), the height h up to which water rises decreases.
  • Option 2: θ decreases and h also decreases
    • This option is incorrect because if the contact angle θ decreases, the height h up to which water rises increases.
  • Option 3: θ increases and h decreases
    • This option is correct because if the contact angle θ increases (due to wax coating), the height h up to which water rises decreases.
  • Option 4: θ decreases and h increases
    • This option is incorrect because if the contact angle θ decreases, the height h up to which water rises increases.

Therefore, the correct answer is option 3: θ increases and h decreases.