How Salt Affects Ice — and Our City
This collection of easy science activities making the rounds pointed me to this super fun science demonstration using ice and salt. It’s the perfect thing to do with the boys now that Toronto is in the deep freeze (again).
After all, we have seen a lot of ice and a lot of salt lately.
“What do you think will happen if we add salt to ice?” I asked the kids.
“The salt will melt the ice,” six-year-old Ray said with certainty.
“Why do you think that?”
“Well why else would we put so much salt on the roads?” he replied.
We decided to turn the science demonstration into a simple experiment. We had an idea that we wanted to test – a hypothesis – that adding salt to the ice would melt it.
In an experiment, a scientist does something differently in one group of objects compared to another group, and then looks for differences caused by the different treatments. In other words, the scientist imposes a treatment on a group of objects in the interest of observing the response.
To do our simple experiment we needed two groups of objects: one to add the salt to (the treatment group) and one to leave alone (the control group).
We started with six identical plastic containers. The kids checked that they were all the same. Then we filled them with the same amount of water from the same source (the kitchen tap).
Why did we use identical containers and treat them all in the same way?
We knew that by keeping everything except our experimental treatment the same we would be able to see the effect of adding the salt (our manipulation) without being confused by differences caused by other things — like the type of container or the type of water.
We put the water outside so that all of the containers would experience the same temperature – a cold one! It was -22°C with a wind chill of -32°C. That is colder that a deep freezer!
The next day we brought the containers inside for our experimental manipulation (adding salt to ice).
We sprinkled salt on three of the containers and left the other three alone.
Then we watched.
Since we were indoors all of the ice melted a little bit (both the treatment and control groups), but we soon noticed something funny on our salted (treatment) ice blocks. The salt was making little holes in the ice. Soon the holes grew into crevices. It looked like the salt was eroding the ice.
Since the kids didn’t put exactly the same amount of salt on each piece of ice, they all looked a little different – this is why it was important to have three pieces of ice in each group. Despite small differences, it was clear that the salt had melted the ice in all three of the salted ice blocks.
There was some melting in the control ice blocks too — after all, we took the ice inside — but the surface of the ice on all three controls stayed relatively smooth.
To make the exercise much more fun, we added colour!
This made the result more obvious.
Why does salt melt ice?
When you add salt to ice, the salt lowers the freezing point of any water on the surface of the ice, keeping it from re-freezing, and helping to melt the rest of the ice around it.
Pure water freezes at 0°C. Water with salt (or any other substance in it) will freeze at some lower temperature. Table salt (sodium chloride) melts ice down to a temperature of about -20°C. Other de-icing agents melt ice down to even lower temperatures.
What does all this winter salt mean for nature in the city?
Learning how salt melts ice is fun. Having salted roads and sidewalks in winter makes getting around safer.
But the widespread use of salt during the winter has many negative effects, some immediate and others indirect and long-term. The most visible impact is on plants along the roadside where the salt causes dehydration that harms leaves, stems, root systems and seeds. But salt also leads to colonization of salt tolerant species (some invasive) — and therefore reduces species diversity.
Animals are also affected. Birds often mistake road salt crystals for seeds. Consuming salt even in small amounts can be toxic for birds.
Mammals such as deer are attracted to roads to lick the salt, which leads to more vehicle-related wildlife kills. High concentrations of salt can also be found in snowmelt, which many animals drink. And let’s not forget that all this salt in the melted ice runs eventually into water bodies from which we get our drinking water.
The bottom line is that salt is not good for our Wild City — so use it sparingly. Consider friendlier alternatives like sand or kitty litter (which won’t melt ice but will give traction). And apparently beet juice works well too!
* More Play Science: Meri and her daughters Separate the Dyes That Make Up Coloured Markers