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:
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.Note: the online version of this lab has a more detailed explanation including illustrations.
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:
Here are a few things you need to know how to do in order to be successful.
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.
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).
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.In your answer to each question include:
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.