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Mechanical properties of matter
However, when some detergent is added Questions
in the water, the metal object sinks. This (a) What is the shape of the thread when
is because the detergent decreases the the loop is dipped in a soap solution?
surface tension of the water. The water’s (b) What happens when the soap fi lm
ability to push upward on the object is inside the loop of the thread is
therefore lowered. The surface of the pierced?
liquid expands and the object sinks to the (c) Explain your observations.
bottom of the trough.
The thread hangs loosely even after the
Detergents as examples of surfactants
A surfactant is a substance that reduces wire loop is dipped in a soap solution.
the surface tension of a liquid. The term When pierced, the soap fi lm inside the
‘’surfactant’’ is an acronym for surface loop of the thread disappears but the
active agent. one outside the loop remains intact. The
thread is pulled outwards and forms a
Acti vity 6.4 circular loop. Before piercing, the thread
hangs loosely since there is a fi lm of soap
Aim: To demonstrate surface both within and outside the loop of the
tension in a soap fi lm. thread. The surface tension does not cause
Materials: Wire, thin thread and soap any tension in the thread. The sum of the
forces on the thread is zero. However, on
Procedure piercing the fi lm inside the thread loop,
1. Make a loop using the wire. the sum of the forces is no longer zero.
2. Make a smaller loop using the thin The surface of the soap wants to be at a
thread and suspend it in the wire minimum so the thread is pulled outwards
loop, as shown in Figure 6.12.
causing it to form a ring, as shown in
Loop of thread Figure 6.13.
Wire loop
Soap fi lm
Before piercing
Before piercing
After piercing
Figure 6.12 fi lm After piercing
fi lm
fi lm
3. Dip the wire loop in a soap solution fi lm
and let a fi lm (skin) of soap cover Figure 6.13: Wire ring with a loop of thread
the whole area of the loop. in a soap fi lm
4. Gently pierce the soap fi lm using a
hot pin inside the loop of the thread Surface tension is what causes drops
until it bursts. of water or soap bubbles to assume a
spherical shape. Recall that surface
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