<p>The kind of salt used to keep roads ice-free is rock salt. Rock salt is an evaporite, which was once a salt lake like the one near Salt Lake City in Utah called Bonneville Salt Flats. In the course of millions of years, many salt evaporite deposits have become buried deep underground. A mine typically produces thousands of tons per year.</p>

<p>The reason it works is that salt water has a lower freezing point than fresh water, so fresh water precipitation such as rain or snow will easily freeze in the sort of night-time winter temperatures they have in UK, for example. As salt water freezes at lower temperatures than are usually encountered in UK, roads which have been salted usually remain clear. Icy roads are dangerous, and cause traffic accidents, so when freezing conditions are forecast the municipal authorities have vehicles which drive around spraying the road and sometimes sidewalks with rock salt. Most rock salt has natural grit mixed with it, which also helps prevent vehicles skidding.</p>

<p>The reason you might get clear roads in some places while there is ice nearby is that the spraying of the roads is not always even, and some small areas are missed. Another reason is that having been salted, the even spread of salt is spoiled by rain showers, which remove it from some places and concentrate it in others. </p>

CC BY-SA 4.0
2019-12-22T16:31:58.237Z
Thanks but here in my town in Colorado, I was driving my car behind a truck that is deployed a brown salt... what is it ?
PROBERT
CC BY-SA 4.0
2019-12-22T17:11:51.987Z
@PROBERT It's the same in Britain. Over the course of being buried in overlying sediments for millions of years, brownish impurities get into it which include some natural grit. As this improves the non-skid properties of the salt, nobody worries about it. I don't think any British salt mine produces pure white salt, but if you examine the brown stuff you  can see lots of white salt crystals in it. There are some salt mines that produce the pure white stuff, but not in Britain.
Michael Walsby
CC BY-SA 4.0
2019-12-22T23:30:36.060Z
Brown salt could be salt mixed with sand
haresfur
CC BY-SA 4.0
2019-12-22T23:35:54.353Z
Colorado road dept [info](https://www.codot.gov/travel/winter-driving/faqs.html#kindsOfRoadTreatments) confirms use of salt/sand mixtures.
justCal
CC BY-SA 4.0
2019-12-22T23:36:12.743Z
Yes, it very likely has sand in it. I haven't analysed the nature of the impurities but it isn't only sand. The brown impurities are there in the mine, there is no need to add them.
Michael Walsby
CC BY-SA 4.0
2019-12-23T21:06:36.913Z
So, when they do deployed the salt on the roads, do they do that before or after the snow ?
PROBERT
CC BY-SA 4.0
2019-12-23T21:20:46.047Z
Usually before, because they check the weather forecasts.
Michael Walsby
CC BY-SA 4.0
2019-12-29T07:59:40.220Z
While there may be impurities at the mining stage, many municipalities - including in the UK - do deliberately mix sand with the salt.
Semidiurnal Simon
CC BY-SA 4.0
2019-12-29T08:58:44.640Z
That may be so, but in the stuff we use here there is natural grit. Its colour when it leaves the mine is the same as when they scatter it on the roads.
Michael Walsby
CC BY-SA 4.0
2019-12-21T19:09:23.153Z
2019-12-21T19:09:23.153Z
Michael Walsby
<p>Sodium chloride rock salt, NaCl, is usually used for de-icing roads. As mentioned in another answer, the dissolved ions lower the freezing point of water.</p>

<p>Other kinds of salt are also used. Calcium chloride, CaCl2, is more efficient at melting ice. NaCl can not melt ice at temperatures lower than -18C (0 F). A saturated calcium chloride solution won't freeze until ~ -46 C. Calcium chloride is often applied as a solution because the solid becomes sticky as it absorbs water from the air.</p>

<p>Edit: per the <a href="https://www.codot.gov/travel/winter-driving/faqs.html#kindsOfRoadTreatments" rel="nofollow noreferrer">link to the Colorado road dept</a> from a comment on another answer, magnesium chloride is used in the liquid deicers. Here is a <a href="https://www.oxycalciumchloride.com/sidewalk-ice-melting/effective-ice-melting/how-to-melt-ice-effectively/choosing-the-right-deicer" rel="nofollow noreferrer">link to information on different deicers</a></p>

CC BY-SA 4.0
2019-12-23T23:19:49.090Z
calcium chloride is more expensive though. As a parenthetical, but geological note, we used it in drilling fluid when coring in permafrost
haresfur
CC BY-SA 4.0
2019-12-22T23:30:04.797Z
2019-12-23T23:29:00.883Z
2019-12-23T23:29:00.883Z
haresfur
<p>What kind of salt is used to keep roads from getting icy and how does it work?
Why does ice get thick in some places and other nearby places there is no ice?</p>

<snow><ice><transportation>
How does salting roads help prevent ice?
PROBERT
# How does salting roads help prevent ice?

(Asked by: username_0 on 2019-12-21)

What kind of salt is used to keep roads from getting icy and how does it work?
Why does ice get thick in some places and other nearby places there is no ice?

**Tags:** snow, ice, transportation


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## 2 answers

### Answer by username_2 on 2019-12-22. Score: 6 | Accepted answer

Sodium chloride rock salt, NaCl, is usually used for de-icing roads. As mentioned in another answer, the dissolved ions lower the freezing point of water.

Other kinds of salt are also used. Calcium chloride, CaCl2, is more efficient at melting ice. NaCl can not melt ice at temperatures lower than -18C (0 F). A saturated calcium chloride solution won't freeze until ~ -46 C. Calcium chloride is often applied as a solution because the solid becomes sticky as it absorbs water from the air.

Edit: per the [link to the Colorado road dept](https://www.codot.gov/travel/winter-driving/faqs.html#kindsOfRoadTreatments) from a comment on another answer, magnesium chloride is used in the liquid deicers. Here is a [link to information on different deicers](https://www.oxycalciumchloride.com/sidewalk-ice-melting/effective-ice-melting/how-to-melt-ice-effectively/choosing-the-right-deicer)

> **Comments:**
> **username_2**: calcium chloride is more expensive though. As a parenthetical, but geological note, we used it in drilling fluid when coring in permafrost  

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### Answer by username_1 on 2019-12-21. Score: 2

The kind of salt used to keep roads ice-free is rock salt. Rock salt is an evaporite, which was once a salt lake like the one near Salt Lake City in Utah called Bonneville Salt Flats. In the course of millions of years, many salt evaporite deposits have become buried deep underground. A mine typically produces thousands of tons per year.

The reason it works is that salt water has a lower freezing point than fresh water, so fresh water precipitation such as rain or snow will easily freeze in the sort of night-time winter temperatures they have in UK, for example. As salt water freezes at lower temperatures than are usually encountered in UK, roads which have been salted usually remain clear. Icy roads are dangerous, and cause traffic accidents, so when freezing conditions are forecast the municipal authorities have vehicles which drive around spraying the road and sometimes sidewalks with rock salt. Most rock salt has natural grit mixed with it, which also helps prevent vehicles skidding.

The reason you might get clear roads in some places while there is ice nearby is that the spraying of the roads is not always even, and some small areas are missed. Another reason is that having been salted, the even spread of salt is spoiled by rain showers, which remove it from some places and concentrate it in others.

> **Comments:**
> **username_0**: Thanks but here in my town in Colorado, I was driving my car behind a truck that is deployed a brown salt... what is it ?  
> **username_1**: @username_0 It's the same in Britain. Over the course of being buried in overlying sediments for millions of years, brownish impurities get into it which include some natural grit. As this improves the non-skid properties of the salt, nobody worries about it. I don't think any British salt mine produces pure white salt, but if you examine the brown stuff you  can see lots of white salt crystals in it. There are some salt mines that produce the pure white stuff, but not in Britain.  
> **username_2**: Brown salt could be salt mixed with sand  
> **username_3**: Colorado road dept [info](https://www.codot.gov/travel/winter-driving/faqs.html#kindsOfRoadTreatments) confirms use of salt/sand mixtures.  
> **username_1**: Yes, it very likely has sand in it. I haven't analysed the nature of the impurities but it isn't only sand. The brown impurities are there in the mine, there is no need to add them.  
> **username_0**: So, when they do deployed the salt on the roads, do they do that before or after the snow ?  
> **username_1**: Usually before, because they check the weather forecasts.  
> **username_4**: While there may be impurities at the mining stage, many municipalities - including in the UK - do deliberately mix sand with the salt.  
> **username_1**: That may be so, but in the stuff we use here there is natural grit. Its colour when it leaves the mine is the same as when they scatter it on the roads.  

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