Salt, Ice and the Coldest Mixture

Here are some notes that I use when I introduce the lab to students. A visual aid that I find useful for this discussion can be found at General Chemistry Online by Fred Senese. They can be printed out from this page.

• The melting point of water is 0°C for pure water.
  • At 0°C the solid is in equilibrium with the liquid.
  • Solid particles that vibrate too fast break loose and join the liquid phase.
  • Liquid particles that hit the surface and lose their speed cause more vibrations for nearby solid particles but stick and join the solid phase.
  • These two types of events happen at the same rate at 0°C for water. That is the meaning of equilibrium: just as many molecules of the solid become molecules of the liquid and vice versa.
  • Above 0°C more solid molecules become liquid than vice versa.
  • Below 0°C more liquid molecules become solid than vice versa.
• For solutions (homogeneous mixtures of a solute and a solvent, like salt dissolved in water) the melting point is less than 0°C.
  • The molecules in the solid become liquid at the same rate in a solution at 0°C as they do in pure water at 0°C.
  • The molecules in the liquid become solid at a lower rate in a solution at 0°C than they do in pure water at 0°C.
    • The same number of particles hit the surface of the ice per unit time but fewer of them are particles of water.
    • Particles of the solute do not stick to the ice.
    • As a result, the freezing rate (the sticking rate of the liquid molecules) goes down.
• The temperature drops when salt is added to ice.
  • Temperature is a measure of the average speed of a collection of molecules.
  • The faster the average speed of the molecules is, the higher the temperature. The slower the average speed, the lower the temperature.
  • Heat is energy that can be moved from place to place. When heat enters a substance it makes the molecules move faster: this is why heating some increases its temperature. When heat leaves a substance it makes the molecules move more slowly. When the molecules move more slowly on average for the whole collection of molecules then the temperature decreases.
  • Making molecules in the solid vibrate fast enough to come loose requires energy (heat).
  • This heat comes from surrounding molecules of the solid and liquid. When fast-moving molecules strike the molecules in the solid they can give them enough energy to break free. A similar example is if you strike a billiard ball that is standing still with a fast-moving cue ball. The cue ball slows down and the other ball speeds away.
  • This sets up a situation in which feedback affects the process and complicates it:
    1. The solid melts more.
    2. The solid and surrounding liquid gets colder.
    3. The solid melts less.

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:

Sample Graph for Objective 3

1. Does a given amount of ice melt faster in plain water or salt water? Does ice melt faster/slower if there is more salt in the 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 (put salt added on the x-axis).
2. How does the temperature of a salt solution (or plain water) change as a function of time when ice melts in it? Can this information be used to explain the behavior observed in objective 1? Use a constant amount of ice and water and vary the amount of salt added. Record temperature at regular time intervals while collecting the total time to melt data for objective 1. Plot a graph of Temperature vs. Time (put time on the x-axis) for each different amount of salt.
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. This experiment benefits especially from the use of styrofoam cups. 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 mostly up to you. For each objective you must design and carry out experiments to answer the objective question(s). Keep the following in mind as you plan your work:

• 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?

The lab is broken up into sections on the following pages. Complete each section and check in with your teacher to get his/her initials.

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.
• Experiments are a given setup or set of conditions. Trials are when you repeat experimental setups. Perform multiple trials for each experimental setup so you can gauge how repeatable your methods are.
• Perform all experiments in styrofoam cups.
• 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.
• Make sure to dissolve salt completely before adding ice.
• Stir minimally and in a repeatable way. Fast stirring causes the ice to melt faster no matter how much or little salt you dissolve.
• Wait until everything else is ready before weighing the ice. It is melting the entire time and you want to get the mass of the ice: not ice plus some unknown amount of water!
• 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.
• The first and second objectives can be answered using the same trial set-ups. Collect your data in such a way as to be able to answer both questions.
• For the third question I suggest you use between 40 g and 50 g of ice.
• 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. Thermometers must be submerged to the line marked near the bottom to give good readings.

Grading

No formal lab report is required.

Each group is responsible for writing a brief report (separate from your lab handout) with three parts:

Part I 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.

Part II A neat data table and all graphs for each objective.

Part III 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.

• What actions you took to answer the question
• What happened (what you observed)
• A scientific explanation for what happened
• An explanation of how it relates to the chemistry (see the Background section)

Scientific explanations require that you refer to your observations as collected during class. Scientific information is best when it can be made quantitative: length, volume, time, etc. Report these data as part of your observations and use them to support your explanations.

You will be graded on the quality of your writing, the professionalism of your work’s appearance, the design and execution of your experiments and the degree of your understanding of the underlying science.

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

Objectives One and Two

Time to Melt

Use somewhere between 30 g and 50 g of ice for this experiment. Use from 100 mL of water to 200 mL of water. Keep the temperature of the water at the start of any experiment for this objective the same. Stir gently, minimally and consistently from trial to trial. In your data table record the amount of salt added to the water (0 g, 2 g, 5 g, etc.), the starting temperature of the water (20°C or lower), the temperature at regular intervals (say, every 15 s), and the amount of time it took for all of the ice to melt. The maximum amount of salt that will likely easily dissolve is about 3% by mass. In 100 mL this is 3 g and in 200 mL this is 6 g.

Be prepared to repeat trials! For example, try letting the ice melt in plain fresh water 2 or 3 times to be sure you are getting consistent results. Do multiple trials for at least two different amounts of salt. Repeating trials allows you to get an idea of how precise your methods are. Averaging the results of multiple trials can compensate for random errors that can increase or decrease individual results.

Write down what you are holding constant in the space below. Specify amounts.

Write down the steps of your experiment below and show them to your teacher before proceeding.

Check-in ______

Create a data table in the space below. Beneath your data table record your observations and notes about your procedure. If there is still room on this page, create your graph for this objective below as well. If there is not enough room then use a separate sheet of graph paper.

Check-in ______

When you have completed your work in this section check in with your teacher. This is a required part of the lab and your teacher’s initials are required before you can move on to the next objective. Initials will be given for good data, observations and procedural notes.

Objective Three

The Coldest Mixture

Use somewhere between 40 g and 50 g of ice for this experiment. A good rule of thumb is to use enough ice to cover the part of the thermometer necessary for it to give a good reading. Use no water! In your data table record the amount of salt added to the ice (0 g, 5 g, 10 g, etc.), and the temperature of the ice when it reaches its coldest point. Be prepared to repeat experiments! For example, try finding the coldest temperature of an ice/salt mixture when you add 5 g of salt several times (to new samples of ice).

Write down what you are holding constant in the space below. Specify amounts.

Write down the steps of your experiment below and show them to your teacher before proceeding.

Check-in ______

Create a data table in the space below. Beneath your data table record your observations and notes about your procedure. If there is still room on this page, create your graph for this objective below as well. If there is not enough room then use a separate sheet of graph paper.

Check-in ______

When you have completed your work in this section check in with your teacher. This is a required part of the lab and your teacher’s initials are required before you can move on to the next objective. Initials will be given for good data, observations and procedural notes.