American Chemical Society Wondering About Water 1

Water A water molecule (H2O), is made up of three atoms --- one oxygen and two hydrogen. H O 1

Interesting facts about Water Over 70% of the Earth’s surface is covered with this liquid (oceans, rivers, lakes, etc.) The second most common form of water on Earth is ice (if all the ice melted – the sea-level would rise by 70 meters) Water is essential for life (most animals and plants contain more than 60% water by volume). Less than 1% of all water on Earth is available or clean enough for humans to drink. The rest is salty or frozen.

Some of Water’s Physical Properties Boiling Point = 100 ⁰C (212 ⁰F) at 1 atmosphere of pressure Freezing Point = 0 ⁰C (32 ⁰F) at 1 atmosphere of pressure Density = 1 g/cc (at 4 ⁰C) Nearly Colorless Tasteless Odorless

The Structure of Water – H2O Image from: http://www.chem1.com/acad/sci/aboutwater.html

A Chemical Description of Water One atom of oxygen (O) is bound to two atoms of hydrogen (H) to form a 105 ⁰ angle “V” shape. Both H atoms are attached to one side of the O atom. This results in a molecule with a slight positive charge on one side of the molecule (H) and a slight negative charge on the other side (O) – this is called a dipole moment. Thus, water molecules tend to attract each other by a process called Hydrogen-Bonding - this gives water many unique properties.

Hydrogen Bonding – creates attraction between water molecules Image from: http://commons.wikimedia.org/wiki/File:3D_model_hydrogen_bonds_in_water.jpg

Unique Properties of Water The only natural substance found as a liquid, solid, and gas at temperatures normally found on Earth Part of every living organism It dissolves nearly everything (universal solvent) It can absorb large amounts of heat – heat exchange between the atmosphere and oceans contribute greatly to Earth’s weather It’s molecules stick together to form beads or drops (high surface tension) – for example, rain drops

Cohesion Attraction between particles of the same substance ( why water is attracted to itself) Results in Surface tension (a measure of the strength of water’s surface) Produces a surface film on water that allows insects to walk on the surface of water 1

Helps insects walk across water Cohesion … Helps insects walk across water

Adhesion Attraction between two different substances. Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and cotton. Capillary action-water molecules will “tow” each other along when in a thin glass tube. Example: transpiration process which plants and trees remove water from the soil, and paper towels soak up water. 1

Water Exists Primarily in Three States; Liquids, Solids, and Gases Image from: http://apollo.lsc.vsc.edu/.../chapter2/lat_heat3.html

Another Unique and Important Property Most liquids contract when they get colder. Water contracts until it reaches 4 ⁰C . Then it expands until it is solid. The density of ice (0.915 g/cc) is less than that of liquid water (0.9999 g/cc) at 0 ⁰C. Its density at 4⁰C is 1.000 g/cc. This is why ice floats – if it didn’t, oceans, lakes, etc. would freeze from the bottom up and remain frozen – Earth would be a completely different planet - maybe uninhabitable by humans.

Water is Less Dense as a Solid Ice is less dense as a solid than as a liquid (ice floats) Liquid water has hydrogen bonds that are constantly being broken and reformed. Frozen water forms a crystal-like lattice whereby molecules are set at fixed distances. 1

Water is Less Dense as a Solid Which is ice and which is water? 1

Water is Less Dense as a Solid Ice 1

Scarcity of Resource Each year millions of people die from diseases associated with inadequate water supply, sanitation and hygiene. Four of every ten people in the world do not have access to adequate sanitation. Two of every ten people have no source of safe drinking water. According to the United Nations World Water Development Report, by the year 2050, at least one in four people is likely to live in a country affected recurring shortages of freshwater. Source: http://www.un.org/waterforlifedecade/background.html

Exit Ticket 1.Draw a simple graph to depict the heating curve of water. Label the following parts in your graph. solid, liquid, gas, boiling point and melting point. 2. During condensation ______(ice/ water/ vapor) turns into ______ (ice/ water/ vapor) and the system _____ (gains / loses) heat energy.