According to city guidelines, why is calcium chloride added to rock salt?

Calcium chloride is added to rock salt primarily to enhance its effectiveness in cold temperatures. When rock salt (sodium chloride) is applied to ice and snow, it works by lowering the freezing point of water, facilitating melting. However, standard rock salt becomes less effective in very low temperatures, typically below 20°F (-6°C).

Adding calcium chloride to rock salt allows it to maintain its de-icing properties at much lower temperatures, down to approximately -20°F (-29°C). This means that the combination can effectively melt ice and snow in conditions where rock salt alone would struggle, improving safety and making winter road maintenance more efficient.

Thus, the key reason for this combination is to ensure that de-icing agents remain productive even in severe winter conditions, which is crucial for maintaining safe driving surfaces. The other options do not accurately capture the main purpose of adding calcium chloride to rock salt.