You find the surface tension of water by measuring the force needed to overcome its surface layer and then dividing it by the length over which that force is applied.
Understanding Surface Tension
Surface tension is the property of a liquid's surface that allows it to resist an external force. It causes the surface of the liquid to act like an elastic sheet. In water, this occurs due to the cohesive forces between water molecules. These forces are stronger at the surface, where molecules are not surrounded by others on all sides.
The Formula for Surface Tension
The general formula for measuring surface tension, denoted by the Greek letter gamma (γ), is:
γ = F / d
Where:
- γ (gamma) is the surface tension, typically measured in Newtons per meter (N/m) or dynes per centimeter (dyn/cm).
- F is the force applied, measured in Newtons (N) or dynes (dyn).
- d is the length along which the force is felt, measured in meters (m) or centimeters (cm).
Methods to Measure Surface Tension
Several methods can be employed to measure surface tension, all following the core principle of this formula. Here are a couple:
1. Wilhelmy Plate Method
- A thin plate, usually made of platinum or glass, is partially immersed into the liquid (like water).
- The force required to pull the plate from the liquid's surface is precisely measured using a sensitive balance.
- The length (d) used in the surface tension calculation is the perimeter of the plate in contact with the liquid.
- This is a reliable method for determining surface tension accurately.
2. Du Noüy Ring Method
- A platinum ring is submerged into the liquid.
- The force needed to detach the ring from the liquid's surface is measured by a tensiometer.
- The length(d) used here will be twice the circumference of the ring.
- This method is commonly used in labs because of its easy implementation and reliability.
Example calculation:
Let's say using the Wilhelmy Plate method, you measure a force of 0.0072 N needed to detach the plate from water and the perimeter of the plate in contact with the water was 0.1 m. Using the formula:
γ = F / d
γ = 0.0072 N / 0.1 m
γ = 0.072 N/m
The surface tension of the water would be 0.072 N/m.
Practical Insights
- The surface tension of water is affected by temperature. As temperature increases, surface tension typically decreases.
- Adding substances (like soap) to water reduces its surface tension, allowing it to spread and wet surfaces more easily.
- Surface tension is why small insects can walk on water, and why water droplets form spherical shapes.
