Why does salt melt ice? (and why you’ll never ask another chemistry question)

By David Shultz
BU News Service

Last week in Atlanta, two inches of snow caused more than two inches of chaos. Thousands of cars were abandoned along the interstate. Helicopters were sent out in search teams to find stranded motorists in a setting that resembled a zombie apocalypse movie. While driving experience certainly counts for something, Atlanta also lacked another important defense against ice: salt. In places like Chicago, which got 33 inches of snow in January, salting the roads is taken for granted. Most people in snowy climates are used to scrubbing salt stains off their boots. But all that begs the essential question: why does salt melt ice?

Image Credit: CNN.com

The answer: thermodynamics. If you want to understand why entropy can be used to melt ice on the sidewalk, buckle up—it’s about to get serious.

If you’re like me, somebody lied to you about why salt melts ice. A teacher might’ve told you the answer had to do with salt particles interfering with the crystal structure of ice. A lot of chemistry books and online explanations give the same rationale, but the real explanation is both more fundamental and more complex.

The most common road salt is table salt: sodium chloride (NaCl). When sprinkled on ice, NaCl breaks into individual sodium and chloride ions in a thin layer of water on the surface. Adding the ions to the solution increases the system’s entropy. In chemistry, entropy is defined as the number of ways a situation can be arranged. Two balls on a table have more entropy than one. Three orange balls and three green balls have more entropy than six of the same color. Typically, the most complex environments offer the most possibilities.

A liquid like water has more entropy than a solid like ice. A salty liquid has more entropy than a pure one, just like a mixture of orange and green balls has more entropy than all orange or all green. The salt ions add combinatorial possibilities—they increase entropy.

The real question is why does entropy melt ice?

To say something is frozen is a statement about its entropy, not its temperature. Pure water doesn’t turn to solid because the temperature is 0ºC, it freezes because entropy has dropped low enough that a solid is possible. 0ºC just happens to be the temperature when this happens. This is why every liquid has a unique freezing point.

The bottom line is that adding salt to water increases entropy, and it takes colder temperatures to freeze a system with higher entropy. If you think about it in these terms, a salt truck is really just a 30-ton entropy spreading device.

VIDEO: State’s Snow Budget

By Ashley Davis
BU News Service

Ashley Davis looks at Boston’s dwindling snow budget as another storm is set to bear down on the city.

Stories of a Snow Day

On the first snow day of 2014, Mikaela Lefrak interviewed residents of Fenway as they made their way through the heaps of snow. (Mikaela Lefrak/BU News Service)

Brookline Food Trucks Face Challenges During Snowy Months

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BROOKLINE – Thirty inches of snow and whipping, freezing winds are not ideal weather conditions for running an open-air restaurant on wheels.  Foot traffic halts and parking spaces become snow mounds, making the food truck business a tough one during the cold winter months in Massachusetts.  However, the food trucks in Brookline aren’t ready to close their doors, or windows, just yet.

“Storms are typical here in Mass.,” said Bryan Peugh, owner of the Baja Taco Truck, “But we love our customers and love the business, so we stick it out.”

The Baja Taco Truck is one of the five trucks taking part in the Brookline Mobile Food Vendor Pilot Program.  Of the nine trucks that applied for the program, the Pennypacker’s Food Truck, the Paris Creperie, the Compliments Food Truck, Renula’s Greek Kitchen, and the Baja Taco Truck were chosen.  The program, which began on April 27, 2012, was granted an extension on October 16, when the Board of Selectmen unanimously voted to continue the pilot program for an additional six months. However, those six months are flying by for the owners who have seen drastically slowed sales during the winter months.

“Every day is different,” said Peugh, “On a typical winter day we maybe sell 60% of what we would in the fall or spring.”

The past few weeks in Brookline, however, have not been “typical winter days”.  With a 24-hour driving ban and a four-day parking ban caused by a major snowstorm, the food trucks in Brookline were at a standstill.

“When its really cold or extremely stormy, sales are down upwards of 75%, occasionally reaching nearly 100%,” said Peugh,  “Its really tricky, because at the beginning of each week we look at the forecast and try to plan orders based on what we see.  If we are off in our predictions, though, we can suffer big losses.”

On top of the storms, a large sewer separation project on the corner of Commonwealth Ave. and St. Mary’s St. has forced the trucks that are normally permitted to park there to shut down their grills and close their doors.  The project began in the beginning of January and is expected to continue through March.

“We were closed for seven weeks.  Between truck problems, then BU’s break, then the road construction which was pushed back even more because of the storm,” said Peugh.  “We lost two of our seven employees, but who can go seven weeks without a job? Not many.”

The Baja Taco Truck was able to reopen its doors on February 19 and is training new employees to help run the busy restaurant.

The Pennypacker’s Food truck, which also parks on St. Mary’s St., faced the same problem.

“The town basically told us sorry but you’re out of luck,” said Kevin McGuire, co-owner of the Pennypacker’s Truck sighing, “We were told two days before Christmas about the construction and we are still waiting for the second spot on the corner to be ready for our truck.”

Pennypacker’s, which also has a second truck that is located on Tide St. in South Boston, was able to open their doors a few days in various suburban towns in Massachusetts during the displacement. However, the revenue earned while open a few days a month is not comparable to the potential revenue of being open daily on the busy streets of the BU campus.

Many customers are also annoyed with the inability of the trucks to be at their usual spots around the BU campus.  Gemma Vardy, a Boston University student who often stops at the trucks to grab a “quick lunch”, was unhappy to learn about the displacement of the St. Mary’s St. vendors.

“Food trucks are such easy, on-the-go lunch spots,” said Vardy walking through campus, “So it’s unfortunate they haven’t been around because of the construction. I’m hoping once the warmer weather comes around so do the rest of the trucks.”

The extension of the pilot food vendor program ends in April of this year, but the food trucks are not ready to give up yet.

“There certainly are some challenges that we continue to face everyday,” said Peugh,” But its an awesome adventure and we love running the truck.”

Nemo: The Aftermath

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Photo Credit: Rishi Chakraborty BU CAS 2013

By Sandeep Chandrasekhar
BU News Service

Currently a senior at Boston University, I never complained about the winter weather in this city during my previous three years. Raised in California, I never experienced snow until I got to college, sans a few skiing vacations in Lake Tahoe.  Even when it snowed in Boston, I did not get the feeling that the weather was overly brutal.  After all, I’ve only had one snow day during my first three years here, and the school was only shut down for one afternoon.  One afternoon in three years.  That’s the only weather-related day off I had until senior year.  Then, the tide completely turned.

First, Hurricane Sandy in late October was the most severe weather I had ever experienced.  Winds were gusting at speeds up to 80 miles per hour, as the rain was swerving as fast as a NASCAR driver rides his final lap.  Nonetheless, I still was able to go outside because it really was not too cold.  In fact, I even played football with some of my friends at Nickerson Field.  Sure, the winds were high and it was pouring rain, but that was not enough to keep me from going outside.  The roads were just a little dirty but nothing that would force me to stay inside.  Being a huge outdoors person, rain, wind, or cold weather really is not enough to keep me from going outside.  In fact, I had a blast playing in the rain and had no issues walking a mile to and from the field.

I really could not envision a scenario where the weather could have been worse than Hurricane Sandy.  If I could go outdoors during Sandy, I could really go outside any time I wanted.  Or so I thought.  This past weekend completely changed my perspective on cold weather; “Blizzard Nemo” was by far one of the most daunting, frightening, and discomforting periods of my life.  The effects are still felt today, over 48 hours since the snow stopped.

Walking 500 feet on Friday night was a struggle.  Anywhere I went, thick snow came pestering onto my face.  There were 25 inches of snow during a 24-hour period.  Wherever I walked on Friday night, everything I wore was drenched in icy snow.  My legs, hands, arms, toes, and chest were freezing.  I could not even feel anything in body.  It felt like I had some major disability because it took me over ten minutes to open the front door of my apartment.  There was no time to create any walkways or even shovel snow because the snow just kept coming and coming.  Even the snow plows, which were everywhere in the city, were hapless that night.  Everywhere I walked, I had to lift my legs over two feet just to make it visible.  Also, the snow was extremely powdery so it was extremely difficult to make snowballs.

While a massive “Snowbrawl” event occurred on the Esplanade, I felt trapped and stranded at my Linden Street apartment.  It took me over 20 minutes to walk a mile to Nickerson Field.  I wanted to see how things were in such an open area and wouldn’t you know, a big snowball fight broke out over there.  I had no feelings in my body, so the last thing I was thinking about was joining that scene.

In the two days that have passed, there is still a plethora of snow scattered all over BU’s campus and the entire city.  The sidewalks, especially in the residential areas, are still unusable, while slush has built up in many of the intersections.  In previous snowstorms during my three years here, the city of Boston was able to restore everything extremely quickly, often taking no more than 24-48 hours to clear the snow and open up the roads and sidewalks.  That was anything but the case today.  The roads were icy, slippery, and massive snowbanks were all over the city.  It is still a struggle walking to and from class at Boston University.

Ultimately, this was clearly the worst weather-related event I’ve experienced in my life.  I’ve used more clothes over the past three days than I usually use in a week.  It is impossible to go outside without my socks being drenched in cold water, more than two days after the blizzard.  Wow, what a storm and what an experience.  Few things are remembered in life, but I can assure you Nemo will remain with me for the rest of my life.