Fun with Ice, Take 2/3: Colouring ice cubes from the inside

Hello out there!

Two and a half weeks ago, I wrote a blog post about why it makes all the difference in the world if there is a pinch of salt in a glass of water or not. So, of course, the logical consequence is to write a blog post about why it makes all the difference in the world if there is a pinch of salt in an ice cube or not! A remark before I start: The amount of salt in the ice cube is really tiny. Ocean water in the Arctic has a salinity of 30 ‰. When it freezes, most of the salt gets ejected, so that only 5-8 ‰ of the sea ice are salt. That’s only a tenth of the amount of alcohol which you have in a glass of beer!

But enough technical description for now, let’s start with the experiment. Here’s what you need (see also Figure 1):

  • 1 salt water ice cube
  • 1 fresh water ice cube
  • Coloured water (food colouring works fine)
  • pipette/syringe
  • A plate

    Figure 1: Setup for the experiment.

Now, you put the two ice cubes on the plate. Do you already see a difference between them? With the pipette, you start to drip the coloured water on the ice cube. Now, you should see the differences on the ice cubes pretty quickly. While the coloured water on the fresh water ice cube just rolls off, the coloured water on the salt water ice cubes stays on top and eventually starts to trickle down into the ice cube, as you see in Figure 2. If you look at it from above as in Figure 3, you can see that the salt water ice cube is coloured completely, while the fresh water ice cube stays like it is. Looking from the side as in Figure 4, you can see that the coloured water really penetrates into the ice cube. Now why is this?

Figure 2: Salt water ice cube (left) and fresh water ice cube (right) after you put some coloured water on them.

Of course, it’s again the salt that makes the difference… never underestimate a pinch of salt! There are two processes happening here: First, the surface of the salt water ice cube melts faster. This is because salt lowers the freezing point, just like the salt you put on the streets in winter. Therefore, the surface is liquid  and takes up the coloured water, while the surface of the fresh water ice cube is still solid. Then, there is a second effect: As I wrote, most of the salt in the salt water gets ejected when it freezes.

Figure 3: The ice cubes seen from above.

Therefore, you have parts within the salt ice cube with a lot of salt, all the salt which didn’t stay in the ice when it froze. There, you have only a very small amount of liquid water and a lot of salt, so the water has a very high salinity. This water is also called brine. More salt in the water means that it melts the ice around it (remember that salt lowers the freezing point). So, the brine starts to melt down its way through the ice cube, leaving something behind which we scientists call brine channels. These brine channels now take up the coloured water. You can see some of them in Figure 4 if you look carefully. This is why the salt water ice cube takes up the colour and the fresh water ice cube doesn’t.

Figure 4: The salt water ice cube after some time.

Now while it’s pretty cool that you can colour an ice cube from inside, you might still wonder what the scientific relevance of this entire hocus-pocus is. One consequence is that the brine, since it is so saline and very cold, causes a process called convection, which you hopefully remember from the last post. Another consequence is that sea ice (frozen sea water) is much more fragile than lake ice (frozen fresh water), which makes it easier to break apart and move around. Can you think of more? Let us know and if it’s something fancy and we like it, you may even get a reward!

In the next part of the series, we will explain why an iceberg is something different than sea ice, what happens to the sea level if sea ice melts and more fancy things…be prepared!

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