1st (autumn) semester2014
Block 6
(Questions on surface tension measurements B1.15, B1.16 and B1.17)
1. What is the thermodynamic definition of surface tension? What is the unit of surface tension?
2. How high should be the flowing rate (bubbling or dropping frequency) during the surface tension
measurements carried out by the bubbling pressure and stalagmometer apparatuses? How many parallel data are
recommended to be recorded during these measurements to do a statistical analysis?
3. How does the Mohr-Westphal balance operate (principle, scheme, explanation)? At which type of surface
tension measurement do you have to use the Mohr-Westphal balance to determine the density of a liquid sample?
4. How many parallel data should be determined during all three types of surface tension measurements? Which
are the quantities at all three types of surface tension measurements that should be directly measured for the
determination of surface tension?
5. The isobutanol is adsorbed in water at the air-water surface. What is the sign of d /dc in this case? ( and c
denote surface tension and concentration, respectively.)
6. Why does not appear the normal air pressure as a quantity in the equation expressing the bubble pressure?
7. Review the Traube rule! At which type of surface tension measurement do you have to study the validity of the
Traube rule?
8. Is the sign of the d/dT differential ratio positive or negative? What does the sign of d/dT mean theoretically?
9. Express the instrumental (calibration) constant from the equation concerning the determination of surface
tension for all the three surface tension measuring methods! How can the instrumental constant be determined?
At which method should the student determine the instrumental constant and at which method is it given by the
professor?
10. Diagram the surface tension () as a function of concentration (c) if  > 0 ! Does  also depend on c?
11. At one of the three methods measuring the surface tension a uniform diameter of capillary should be used.
Which is this method? Identify it! What is the reason of the uniformity?
12. How is the Mohr-Westphal balance calibrated? Which liquid’s density should be measured by using the
Mohr-Westphal balance during the experiment? Why?
13. How is the surface excess () defined? What is the unit of ? Can the value of  be negative? Convert a
mol/m2 to mol/cm2!
14. Could we use mercury as manometer liquid instead of water in the U-manometer of the bubble pressure
measuring apparatus?
15. What is the instrumental constant at the method based on the differential capillary action? Please explain it
using the appropriate equation! How can the instrumental constant be determined?
16. Which manometer liquid (water, ethanol or mercury) is the most proper to determine the bubble pressure with
the highest accuracy? Explain your answer!
1
17. What kind of quantity is measured directly by using a cathetometer? What are the main parts of a
cathetometer? A special attention should be paid during the measurement with a cathetometer at certain periods
(phases) of the measurement. Which are these phases? Why are they important?
18. Draw the stalagmometer schematically! How many drops should be collected according to the description
(instructions) of the measurement? How many parallel runs should be performed? What is the recommended
accuracy of the measurement of mass during weighing the drops?
2