The Properties of Water教案

3.so we know that one of water’s properties is its dissolvability(溶解性).
qs: 1) do you know what gives water such a unique property?
－the chemical structure of water.
2) what does the water molecule look like?
      －a water molecule consists of two hydrogen atoms and one oxygen atom. the two hydrogen atoms are bonded to one oxygen atom to form a 'v' shape. the two hydrogen atoms form a polar molecule, that is, one with a slightly positive end and one with a slightly negative end.
      3) can you give some other examples which are also resulted from water’s unique chemical structure?
       －plants can pull water up their roots via a capillary(毛細管) process;
       －we can get a lot of nutrients by drinking soup.
       that’s water’s another property－dissolvability.
4. since water is such a universal solvent, it can dissolve just about anything including many gases and solids. as a result, there is sea water.
qs: 1) do you know the salinity of the earth’s oceans? －about 3.5%.
2) what does it mean?
3) have you ever noticed such a phenomenon that in winter water in some containers or small rivers often freezes while that in seas not? can you explain it?
－the salinity of sea water can affect the freezing point of water. sea water has a lower a freezing point, so even when the temperature of sea water falls below 0℃, sea water is still liquid.
5. however, no matter whether it is sea water or pure water, “wood floats on water while iron sinks” always works. how do you explain that? what property of water is this?
－if a substance has a lower density than water does, it will float on water; otherwise, it will sink.
qs: 1) what is density? and what’s the density of pure water?
     －density is the relationship between mass and volume and is measured in kilos per cubic meter (kg/m3). the density of water is 1,000kg/m3.
      2) does it also go for the phenomenon that ice looks larger than water of equal mass?
     －yes. water’s density decreases when it turns to ice, so its volume increases.
      3) does all the water in the same river or ocean have equal density? tell your reasons.
     —no. changes in salinity and temperature affect water’s density. cold water of high salinity usually is most dense and will tend to sink to the bottom of the ocean.