The third module for the Design II class

Have you ever known that sea water is not wholly frozen at 0 degree Celsius ?

We are all known that water will be changed its status from liquid to solid as ice at 0 degree Celsius but not the seawater.

Water, the most abundant molecule on Earth’s surface, constituting about 70% of the planet’s surface. In nature it exists in liquid, solid, and gaseous states. It is in dynamic equilibrium between the liquid and gas states at standard temperature and pressure. At room temperature, it is a nearly colorless with a hint of blue, tasteless, and odorless liquid. Many substances dissolve in water and it is commonly referred to as the universal solvent. Because of this, water in nature and in use is rarely pure and some of its properties may vary slightly from those of the pure substance. However, there are many compounds that are essentially, if not completely, insoluble in water. Water is the only common substance found naturally in all three common states of matter.

Normally everyone knows that water become ice at 0 degree, but for seawater, it may become just ice plate on its surface only, not all over like certain natural water resources like lakes and ponds, where sometimes freeze enough for us to go on ice-skating !  Underneath of ice plate on surface of seawater is not ice but water. This is same as what happen in Atlantic ocean. There is big ice plate on it’s surface but down deep underneath it’s seawater. It is because seawater is not pure water and the temperature is not low enough to make them all frozen.

Seawater is impure water from sea or ocean. It has sodium chloride as its major composite that is why it’s taste salty. The salinity made it’s density more than fresh water because of the added weight of the salts and electrotriction. The vast majority of seawater has a salinity average at about 3.5%. This means that every 1 kg of seawater has approximately 35 grams of dissolved salts. Seawater is not uniformly saline throughout the world. Where mixing occurs, with fresh water runoff from river mouths or near melting glazier, seawater can be substantially less saline. The most saline area is where high rate of evaporation, low precipitation and river inflow and confined circulation result in the formation of unusually salty seawater. The salinity in isolated bodies of water can be considerably greater. The density of seawater ranges, depending on the dissolved salt content as well as the temperature of the water. With the mass of salinity added, the freezing point of seawater decrease with the increasing of salinity, is about -2 degree. In addition, ocean water also undergoes more movement than the freshwater in lakes and ponds. The movement of ocean water (like from currents or wind) helps it retain more heat than smaller lakes and ponds where the water is generally rather still. 

There is an experiment which is supporting this fact. The items which are need to prepare is water, 4 tablespoon of salt, 2 plastic cups and a freezer. First, Pour the water into both plastic cups. Put 4 tablespoons of salt in one cup and stir well. Try to make the level of the water are the same in both cups and then put them into the freezer. Every hour, take the cup to examine the change of both cups until 4 hours. Take a note and photo on every change hourly. After complete 4 hours, leave the cups in the freezer over the night. Make an observeation about the different of both cups in the next morning and the result is as follows : The cup of water only became wholly ice but the cup of water with salt is not yet frozen. It’s just partially turn to ice plate on it’s surface only after time passed over the night . In addition, it is noticeable that the volume of water when turned into ice become increased. Compare to both cups, the level of the cup of water that changed status to ice is higher than the cup of water with salt

This can be summarize that dissolved salt content in water made water’s density higher than fresh water. The more concentration increasing, the lower freezing point occured.