Surface tension is a property of liquids that arises due to the cohesive forces between liquid molecules. It is the force that allows the liquid surface to resist external forces and maintain its shape. Surface tension varies with temperature and impurity concentration in the following ways:1. Temperature: As the temperature of a liquid increases, the surface tension typically decreases. This is because the increased thermal energy causes the liquid molecules to move more rapidly, reducing the strength of the cohesive forces between them. As a result, the surface tension decreases with increasing temperature. However, this relationship is not always linear, and the rate of decrease may vary depending on the specific liquid.2. Impurity concentration: The presence of impurities, such as dissolved salts or surfactants, can significantly affect the surface tension of a liquid. In general, the addition of impurities can either increase or decrease the surface tension, depending on the nature of the impurity and its interaction with the liquid molecules. For example, the addition of surfactants surface-active agents can significantly reduce the surface tension by disrupting the cohesive forces between the liquid molecules at the surface.To measure surface tension accurately, two common methods are used: the drop weight method and the capillary rise method.1. Drop weight method: This method involves measuring the force required to detach a droplet of liquid from the end of a thin tube or needle. The surface tension is calculated by dividing the measured force by the circumference of the tube or needle. This method is particularly useful for measuring the surface tension of liquids with high viscosity or those that do not wet the tube material.To perform the drop weight method, follow these steps:a. Clean and dry the tube or needle to ensure no impurities are present.b. Fill the tube with the liquid to be tested.c. Allow a droplet to form at the end of the tube and measure the force required to detach it.d. Repeat the process several times to obtain an average value.e. Calculate the surface tension by dividing the measured force by the circumference of the tube or needle.2. Capillary rise method: This method involves measuring the height a liquid rises in a small diameter tube due to surface tension. The surface tension is calculated using the Jurin's law formula, which relates the capillary rise height to the surface tension, liquid density, tube diameter, and gravitational acceleration.To perform the capillary rise method, follow these steps:a. Clean and dry the capillary tube to ensure no impurities are present.b. Immerse the capillary tube vertically into the liquid to be tested.c. Measure the height the liquid rises in the tube.d. Calculate the surface tension using Jurin's law formula: Surface tension = height * liquid density * tube radius * gravitational acceleration / 2Both methods can provide accurate measurements of surface tension when performed correctly. However, it is essential to ensure that the experimental setup is clean and free of impurities, as they can significantly affect the surface tension measurements. Additionally, it is crucial to consider the temperature at which the measurements are taken, as surface tension is temperature-dependent.