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.

WEATHER ALERT: Storm Expected to Hit Boston Tonight

Cars buried in snow near Cleveland Circle. (Photo by Honah Liles/BU News Service)

BOSTON – St. Patrick’s Day has just passed, but that has not stopped the flurry of cold weather in New England. Weather forecasters have warned of a messy Tuesday morning commute as yet another winter storm looms.

Forecasters project approximately four to six inches of snow in Boston and up to eight inches in the outer suburbs. Snow is expected to move in some time between 10 p.m. and 2 a.m. and is expected to continue throughout the night into Tuesday. Winter Storm Warnings and Watches have been issued from tonight all the way through Tuesday morning.

Coastal communities are not likely to be hit hard by the upcoming storm. The storm is expected to wind down by Tuesday evening with potential precipitation to hit the city through Wednesday morning.

The city is preparing for the snowstorm, as the state has postponed the MCAS English Composition Test, taken by 4th, 7th, and 10th graders. The test was expected to be taken on Tuesday morning.

Although Wednesday marks the first official day of spring, temperatures are only expected to reach a high of 40 and potential lows in the teens. Do not throw away your winter jackets any time soon because winter weather is still far from over.

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.”

VIDEO: Doggie Tree Lighting

SOUTH END — Tree lightings aren’t just for humans anymore. Deedee Sun takes us to Boston’s South End for one get-down pet party sponsored by the Animal Rescue League.