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The freezing point of substances - such as naphthalene - can be determined using the cooling method. The graph of temperature/⁰C against time/s (known as the cooling curve) is plotted as part of the process to determine the freezing point.

By analysing the graph/cooling curve, (for example, like the one shown above), the following are deduced

* the freezing point of naphthalene is 80⁰C (where the line is flat / horizontal)
* the physical state of naphthalene at the section of the curve from P to Q is liquid
* the physical states of naphthalene at the section of the curve from Q to R are liquid and solid
* the physical state of naphthalene at the section of the curve from R to S is solid

There is no change in temperature from Q to R because heat is released to form new bonds (attractions) between the particles (molecules) of naphthalene as it changes from liquid to solid.

During this period of the cooling process, the particles / molecules of naphthalene move more slowly and the motion of the particles become more restrictive as they lose kinetic energy. The particles move closer to one another and are arranged in an orderly manner.

The formation of naphthalene particles in the solid form is an exothermic process that releases heat energy. This release of heat energy is balanced by the heat energy that is lost to the surroundings. As a result, there is no change in temperature during the crystallisation of naphthalene.

Lengkuk Pendinginan
(Naftalena)

Takat beku bahan - seperti naftalena dapat ditentukan menggunakan kaedah penyejukan / pendinginan.

Graf suhu/⁰C menentang masa/s (dikenali sebagai lengkuk pendinginan) dilukis untuk menentukan takat beku.

Dengan menganalisa graf seperti yang ditunjukkan di atas, kesimpulan-kesimpulan berikut dapat dibuat.

* takat beku naftalena ialah 80⁰C (bahagian graf yang mendatar)
* keadaan fizikal naftalena pada bahagian lengkuk PQ ialah cecair
* keadaan fizikal naftalena pada bahagian lengkuk QR ialah cecair dan pepejal
* keadaan fizikal naftalena pada bahagian lengkuk RS ialah pepejal

Tidak terdapat perubahan dalam suhu di antara Q dan R. Ini kerana haba dibebaskan untuk membentuk ikatan (tarikan) di antara zarah-zarah (molekul) naftalena apabila ia bertukar daripada cecair kepada pepejal.

Semasa proses pendinginan, zarah-zarah / molekul-molekul naftalena bergerak lebih perlahan. Pergerakan zarah-zarah yang terhad ini adalah disebabkan kehilangan tenaga kinetik. Zarah-zarah juga bergerak lebih dekat di antara satu sama lain dan tersusun rapat dan teratur.

Pembentukan zarah-zarah naftalena dalam keadaan pepejal merupakan proses eksotermik yang membebaskan tenaga haba. Pembebasan haba ini diseimbangi oleh tenaga haba yang hilang ke persekitaran. Dengan demikian, tiada perubahan suhu semasa penghabluran naftalena.