salt                  package:Rlab                  R Documentation

_T_h_e _e_f_f_e_c_t _o_f _s_a_l_t _o_n _i_c_e _m_e_l_t_i_n_g

_D_e_s_c_r_i_p_t_i_o_n:

     When salt comes into contact with ice, it tends to break apart
     into
 individual ions which then interact with the frozen water
     and disrupt
 hydrogen bonds that have formed between ice
     molecules.  This lowers the
 melting temperature of ice, and it
     was hypothesized that the melting
 process would be hastened.  The
     data set 'salt' was collected
 during an experiment to determine
     whether varying the type and amount
 of salt applied to a specific
     amount of ice has an effect on the
 interval required to melt that
     ice.

_U_s_a_g_e:

     data(salt)

_F_o_r_m_a_t:

     A data frame with 24 observations on the following 3 variables.

     _t_y_p_e : type of salt (rock salt or table salt)

     _a_m_o_u_n_t : amount of salt used (in teaspoons)

     _t_i_m_e : time for ice to melt (in seconds)

_D_e_t_a_i_l_s:

     Background: The Effect of Salt on the Rate at Which Ice Melts

     In those sections of the country that experience winter as a time
     of
 snow and ice, salt is often spread on roadways in an attempt
     to counter
 the hazardous consequences of accumulated ice.  Ice is
     formed when the
 relatively disordered molecules in liquid water
     reach a temperature of
 32 degrees F (0 degrees C) and begin to
     "nucleate" or form solid ice
 crystals consisting of ordered water
     molecules.  Salt, when in contact
 with ice, tends to break apart
     into individual ions (i.e. sodium and
 chloride) which then
     interact with the water and disrupt the hydrogen
 bonds that have
     formed between water molecules.  Since no covalent bonds
 are
     broken or formed, the resulting chemical "solvation" is not
     considered to be a chemical reaction.  However, the end result
     from the
 introduction of salt is that the ice crystals are
     disrupted and liquid
 water is achieved.

     The purpose of the current experiment is to study the effect of
     salt on
 the rate at which ice melts.  More specifically, the
     experiment is being
 conducted to answer the following questions:

     1. Does varying the amount of salt applied to a constant quantity
     of ice
 result in a change in the rate of melting?

     2. Does the type of salt used have an effect on the melting rate?

     The first question is of interest as it relates to issues such as
     the
 cost of salt and the potential harmful effects of its use on
     pavement.
 If increasing the amount of salt applied to a given
     quantity of ice is
 not accompanied by an increase in melting
     rate, any application of salt
 beyond minimal amounts would
     constitute a waste of public money and
 possibly cause unnecessary
     damage to public roadways.  It is
 hypothesized that the
     relationship between amount of salt used and the
 time required to
     completely melt a given quantity of ice is negative and
     significant.

     Likewise, the second question seeks to address the possibility
     that
 dissimilar forms of salt may produce different rates of
     melting.  To
 answer this question, table salt and rock salt were
     included in the
 experimental design.  Although both are
     chemically similar, rock salt
 consists of larger crystals than
     does the typical table salt bought in
 local supermarkets.  Given
     the greater density and more efficient
 packing of NaCl molecules
     within the larger rock salt crystals, a
 specified volume of rock
     salt will likely contain a greater number of
 salt molecules than
     a similar volume of the less tightly packed table
 salt crystals. 
     Therefor, it is hypothesized that rock salt will result
 in faster
     melting times than table salt.

     Materials

     Tap water
 42 - 6 ounce plastic cups (paper cups tend to break at
     the seam as the
 contents freeze)
 Morton brand table salt
 Morton
     brand rock salt
 1/2 cup measure
 Stop Watch

     Procedure

     To answer the questions posed above, a balanced 2 x 4 factorial
     design
 was employed with amount of salt identified as a factor
     consisting of
 four levels (i.e. no salt, 1/2 tsp, 1 tsp, 1 tbsp),
     and the other factor
 being type of salt with two levels (i.e.
     table salt, rock salt).  Three
 replications were conducted within
     each cell for a total of 24 runs.  A
 p-level of .05 was
     identified for statistical significance prior to the
 data
     collection phase of the project.

     Twenty-four small plastic cups were each labeled with a number
     designating type of salt, and a letter A-D indicating amount of
     salt.
 Each plastic cup was then filled with 4 ounces of tap water
     and placed
 in the freezer overnight (approximately 16 hours).

     Since salt could not be emptied into all of the ice cups
     simultaneously, the remaining 18 plastic cups were each labeled
     and then
 used to hold an amount and type of salt corresponding to
     one of the
 experimental conditions.  After the ice cups had been
     removed from the
 freezer, each salt cup was quickly emptied into
     a corresponding ice cup
 with matching identification so as to
     minimize the time interval between
 the application of salt to the
     first and last cups.

     After the last cup of salt had been emptied into the appropriate
     ice
 cup, the stopwatch was started.  Room temperature during the
     data
 collection phase was approximately 72 degrees Fahrenheit. 
     The time was
 recorded for each cup when ice was no longer visible
     in that cup.

_S_o_u_r_c_e:

     Taken from a 1999 project by Wayde D. Johnson

