Salt, Ice and the Coldest Mixture

Objective

Old-time ice-cream makers used a mixture of rock salt and ice to freeze the ice-cream. The mixture got colder than the usual freezing point of water: 0°C. A seemingly opposite effect is the fact that salt is spread on the roads in winter to melt it. In this lab you will explore these phenomena and record data and observations about it. Your task is to explore three questions:

1. Does a given amount of ice melt faster in plain water or salt water? Does it matter how much salt is in the salt water? Keep the amount of water and ice constant while changing the amount of salt. Make a graph of time to melt vs. salt added: zero salt added would be the plain water.
2. How much can salt and ice lower the temperature of water? Use a constant amount of ice and water and vary the amount of salt added. Record temperature at regular time intervals while stirring constantly and completely. Plot a graph of Temperature vs. Time (put time on the x-axis).
3. What is the coldest temperature you can get with a mixture of salt and ice? Hold the amount of ice constant, add no water and measure the amounts of salt you add in different trials. It will help you to answer this question if you make a graph of the amount of salt added vs. the temperature.

Materials

Background

You will explore what happens in ice-salt mixtures at an everyday scale. But chemistry is the science in which people learn how to understand what the atoms and molecules are doing. Everything you can see, touch, taste, or smell is made of atoms and molecules. Knowing how they act at their own unimaginably small scale makes it possible to explain why things work the way they do.

Ice gets colder when you put salt on it. Ice also melts when you put salt on it. Why this should be seems like a mystery until you look at what is going on at the molecular level. Taking the second mystery first, ice melts when you put salt on it because it is melting faster than it is re-freezing. At 0°C the water molecules near the surface of the ice are turning into liquid at the same rate that they are turning into solid. When you added salt, or anything else that will dissolve in water, there are fewer molecules of liquid water hitting the surface of the solid ice because the dissolved salt ions get in the way. This makes it so that fewer liquid molecules stick and become part of the solid. In other words, the freezing rate slows down. Meanwhile, the rate of melting doesn’t change at all. So with salt dissolved in the water it freezes more slowly than it melts: so ice melts when you put salt on it.

But the mystery of why it gets colder remains. Ice gets colder when you put salt on it because as it begins to melt faster (see above) it requires energy to melt it. If nothing else can provide the energy the water and ice get colder. Ice needs energy to melt it because the molecules are bound together with chemical bonds. Energy is required to break chemical bonds and energy can be in the form of heat. When heat is taken away from something it gets colder. So, the ice gets colder when you put salt on it.

The fact that salt makes the ice get colder makes it difficult to measure how much it makes ice melt faster. The getting colder makes the ice melt more slowly. So the two effects work against each other and make an interesting thing to investigate.

Procedure

The procedure for this lab is entirely up to you. After your teacher gives you an introduction to the lab it is your decision about how to proceed. Some things to keep in mind:

• What experiments are you going to perform?
• Before you set up the experiments try to decide what you think is going to happen. Write this down and refer back to it once you are done.
• In your experiments, what are you going to hold constant?
• What are you going to change?
• Set up data tables to before doing your experiments.
• How are you going to decide whether you have answered the questions?

Tips and Pointers

Here are a few things you need to know how to do in order to be successful.

• Make a plan before you start working.
• Be methodical. Use techniques that are repeatable.
• Measure ice based not on apparent size but on mass.
• Do not spend too much time on answering only one of the questions: be efficient!
• Share labor. Delegate.
• Keep tabs on your partners’ progress.
• In each trial keep the amount of water and/or ice constant and only vary the amount of salt.
• Use small amounts! You will get good results whether you use 25 g or 1 kg of ice.
• For the first and second questions do not use more than a 25% by mass solution. For example, if using 200 mL of water (200 g) use no more than 50 g salt. Chances are that even this will be too much to dissolve.
• For the third question I suggest you use between 50 g and 100 g of ice and to add no more than 15% salt by mass. For 50 g of ice the maximum amount of salt is 7.5 g. For 100 g the maximum amount of salt is 15 g.
• For question three use no water at all: the salt will make the ice melt enough for the thermometer to be able to get a good reading.

Grading

No formal lab report is required.

Each group is responsible for writing a one-paragraph procedure for each of the three sections. Each paragraph should completely describe all steps required to replicate your work! This means that it may be a long paragraph. Along with each paragraph include a neat data table and any graphs you draw. Write a second paragraph for each question which answers the objective question using your experimental data and observations. Answer each objective question as definitively as you can and use your lab data to support your conclusions. You will be graded on how well your conclusions are supported by your data. This report must be completed during class by the last day of classes.

In addition to the brief report described above you will be graded on your participation. Participation will count for 25% of the grade for this lab. See below for details:

Group Participation: 25%

The teacher will observe your group during the lab and will classify your individual participation as follows:

• Professional—this member always contributes to the group in a positive manner, and is always polite and productive. 25 points
  
• Journeyman—this member tries very hard and is positive, polite, and productive. 20 points
  
• Amateur—this member tries hard but is sometimes impolite, nonproductive and/or negative. 15 points
  
• Hacker—this member goes their own way but gets some work done and may even help with the lab work. 0 points
  
• Wanna-be—this member tries sometimes but is mostly unco-operative, nonproductive, impolite and/or negative. –5 points
  
• Anarchist—this member doesn’t buy in to the whole “group thing”. –10 points