salt {Rlab}

The effect of salt on ice melting

Description

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.

Usage

data(salt)

Format

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

type

: type of salt (rock salt or table salt)

amount

: amount of salt used (in teaspoons)

time

: time for ice to melt (in seconds)

Details

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.

Source

Taken from a 1999 project by Wayde D. Johnson