Structure and properties of water

Take away ideas and understandings

• Water is a unique substance, its properties are different from those of molecules with similar structures.
• The key to understanding properties of water are the hydrogen bonds.
• The most important chemical and physical features of water are summaried in this Fig.

Thermal properties of water

• water freezes and boils at lower temperatures than other similar compounds (Fig)
• water is a polar molecule (Fig)
• concept of hydrogen bonds (Fig)
• heat capacity
• experiment: heating water in a paper tray. Why is the paper not burning?
• About 1 calorie (the basic unit) of heat is needed to raise the temperature of 1g of water by 1^oC. Specific heat capacity of water is larger than for most substances.
• phase changes of water as a function of added heat (Fig)
• latent heat of vaporization (539 cal/g) and fusion (80 cal/g); 1 calorie (cal) = 4.184 joule (J)

Density of water

• density of water as a function of temperature. Water has its highest density at 4^oC. Water volume increases by 9% when it changes from water to ice (Fig)
• definition of density: mass per volume
• explanation based on hydrogen bond model (Fig)(Fig)(Fig)(Fig)

Viscosity

• viscosity of water decreases by 3% per ^oC increase in temperature (Fig A2.1)
• experiment: measuring the flow velocity of cold and hot water in a capillary

Surface tension

• experiment: suspended paper clip and influence of dishwashing liquid (Fig)
• adhesion/cohesion
• capillarity: in a 1/8 mm diameter tube water would rise more than 25cm (Fig)

Absoption of radiation

• large in infrared and ultraviolet regions, less in visible regions

Compressibility

• compressibility of water is small. An increase of pressure by 1 atmosphere (= 1013mbar = 14.7 psi) causes a decrease of the water volume by 5.3*10^-5 of the original volume.

Solubility

• water is able to dissolve huge quantities of salts and is therefore an important factor in transporting substances in nature
• more than 300g of NaCl can be dissolved in a kg of water
• hydrophobic, hydrophilic substances

Summary

• Fig

Resources:

1. Manning, J.C. (1997), chapter 2.
2. Hornberger et al. (1998), Appendix 3.