Capillarity is the phenomenon in which a liquid in a narrow tube rises or falls relative to the outside liquid because the curved meniscus creates a pressure difference across the surface.

• Wetting liquids rise and non-wetting liquids fall.
• The OCR examples include blotting paper, towels, lamp wicks, moisture in soil, and wood swelling.
• Do not describe capillarity as suction by the tube; the root cause is the pressure difference across the curved surface.

For a liquid that wets the tube, pressure due to the liquid column balances the surface-tension pressure difference, giving h = (2T cosθ)/(rdg). For a fixed liquid-solid pair, this becomes Jurin's law, h proportional to 1/r.

• Smaller radius gives larger rise because h varies inversely with r.
• The more accurate OCR form including meniscus weight is h = (2T cosθ)/(rdg) - r/3.
• The trap is forgetting the cosθ factor or replacing r by diameter without adjusting the formula.

Concave meniscus with theta < 90° gives rise, plane meniscus with theta = 90° gives no change, and convex meniscus with theta > 90° gives depression. For two unequal radii, h = (2T cosθ/dg)[1/r1 - 1/r2].

• In a tilted capillary, the vertical height h remains unchanged, but the length of liquid inside the tube is l = h/cosα.
• If the capillary tube is shorter than the natural rise, the liquid reaches the top and adjusts the meniscus radius instead of overflowing.
• Students often confuse the measured length along the tube with the actual vertical rise.

Whether a liquid forms a drop or spreads over the surface depends on the balance TSL + TLA cosθ = TSA, or equivalently cosθ = (TSA - TSL)/TLA.

• If the solid attracts the liquid strongly, theta becomes acute and spreading is favoured.
• If liquid molecules attract one another more strongly than the solid attracts them, theta becomes obtuse and the drop stays beaded.
• The common mistake is to memorise spreading as a separate chapter fact rather than as a consequence of the same contact-angle balance.