Chemistry Lecture Demonstration Facility - Demos

Polarity - Bending of a Liquid Stream

A demonstration involving polar and non polar solvents and charged rubber and glass rods.

The video starts with a demonstration of static electricity generation by fur rubbed against a hand held solid rubber rod as well as a rubber rod suspended by string on a ring stand.  Negatively charged electrons are accumulated on both rods.  When the hand held rod approaches the suspended rod, the suspended rod is repelled by the like charges.  Next, a glass rod is rubbed with flannel cloth – negative charges are removed from the glass rod, and as the glass rod is approached near the rubber rod, the rubber rod is attracted to the glass rod.  Next, a burette filled with water is used to dispense a stream when the stopcock is opened.  The negatively charged rubber rod is held close to the stream of water, diverting its direction toward the rod.  The question is posed – what would happen to the stream of water using the glass rod rubbed with flannel cloth?  The stream diverts toward the rod in the same manner as the rubber rod.  Why?  Like charges repel, but opposites attract.  Water itself is uncharged, but it has an uneven distribution of charge, which is why it is called polar – the water molecule has a positive end and a negative end – it has a dipole.  The water molecules align such that when approached by the rubber rod (negatively charged) the positive ends of the molecule are attracted to the rod resulting in the bending of the stream toward the rod.  When the glass rod is used (positively charged), the negative ends of the water molecules molecule are attracted to the rod, again resulting in the bending of the stream toward the rod.  This is an equivalent analogy to ion-dipole attraction.  Next, the negatively charged rubber rod is placed next to a stream of liquid generated from another adjacent burette.  The stream is not deflected.  Why?  The same phenomenon is noticed with the positively charged glass rod.  The reason is that the liquid in the second burette is not water.  What property must the liquid in the second burette have to explain the observation?  The second burette contains cyclohexane, a nonpolar liquid.  Both water and cyclohexane are neutral molecules, but water has an uneven distribution of charge in the molecule (has a positive and a negative end), while the cyclohexane is symmetrical and has an even distribution of charge in the molecule – water is polar while cyclohexane is nonpolar.  This is why ionic compounds dissolve in water – positive ions are attracted to the negative ends of water and negative ends are attracted to the positive ends of water.

 

Polarity - Bending of a Liquid Stream

Contact

Bob Porcja
porcja@chem.rutgers.edu
848-445-5157

Location

Beck Hall Auditorium
99 Avenue E
Piscataway, NJ 08854

The CLD Facility is located in the prep room adjacent to the Beck Hall Auditorium (map).

CLD Facility