The first liquid to freeze will be the unadulterated tap water, followed by the sugar water and finally, the salted water.
The reason for this is that when you add a solute (a substance dissolved in another substance, usually the component of a solution present in the lesser amount) to water, it lowers its freezing point. Therefore in order to freeze the impure water you need to lower its temperature to a greater extent than in normal circumstances. This is called "freezing point depression". It is a "colligative property" of water.
The reason why tap water freezes first is that tap water does not contain any large amounts of minerals, chemicals or solutes, so there are less solute particles to depress the freezing points. Therefore the freezing point will be reached at a higher temperature than a water solute (this is 0° Celsius/ 32° Fahrenheit). The sugar molecules will depress the water freezing point further. If you have added 1 molecule of sugar to water, the water will freeze later (at -1.8° C/ 28.76° Fahrenheit). In the case of the salt water, one molecule contains two elements - Sodium (Na) and Chloride (Cl). When you add 1 molecule of salt into water, two ions (1 molecule of Na+ and 1 molecule of Cl-) are present in the water. This means there are more molecules of solute to depress the freezing point to an even further extent than the sugar's 1 molecule. This water solution will freeze at -13.6° Celsius (7.52° Fahrenheit).
The phenomenon is best observed in sea water, which due to its heavy salt content remains liquid at temperatures well below 0°C (32°F), the freezing point of pure water.
The reason for this is that when you add a solute (a substance dissolved in another substance, usually the component of a solution present in the lesser amount) to water, it lowers its freezing point. Therefore in order to freeze the impure water you need to lower its temperature to a greater extent than in normal circumstances. This is called "freezing point depression". It is a "colligative property" of water.
The reason why tap water freezes first is that tap water does not contain any large amounts of minerals, chemicals or solutes, so there are less solute particles to depress the freezing points. Therefore the freezing point will be reached at a higher temperature than a water solute (this is 0° Celsius/ 32° Fahrenheit). The sugar molecules will depress the water freezing point further. If you have added 1 molecule of sugar to water, the water will freeze later (at -1.8° C/ 28.76° Fahrenheit). In the case of the salt water, one molecule contains two elements - Sodium (Na) and Chloride (Cl). When you add 1 molecule of salt into water, two ions (1 molecule of Na+ and 1 molecule of Cl-) are present in the water. This means there are more molecules of solute to depress the freezing point to an even further extent than the sugar's 1 molecule. This water solution will freeze at -13.6° Celsius (7.52° Fahrenheit).
The phenomenon is best observed in sea water, which due to its heavy salt content remains liquid at temperatures well below 0°C (32°F), the freezing point of pure water.
