The Properties of Water Water has many unique properties that allow it to support life on earth. Water is so important to life that scientists look for water on other planets to determine if that planet can sustain. Water is Polar Water is made up of two hydrogen atoms and one oxygen atom (H 2 O) held together by covalent bonds. However, oxygen attracts electrons more strongly than hydrogen, so oxygen tends to pull the shared electrons closer to its nucleus. H 2 O This gives a water molecule a slight negative charge near the oxygen atom (red) and a slight positive charge near the hydrogens (blue). This is called polarity the uneven distribution of charges across a molecule. Polarity is why oil and water don't mix. Although water is polar, oil is not. Only polar substances dissolve in polar solvents, only non polar substances dissolve in non polar solvents. 1
Strong Cohesive Properties Cohesion is the tendancy for water molecules to form weak bonds and stick to each other. This property keeps water molecules joined together as they move inside the body, as they fall as large raindrops, or as water "wicks" up a paper towel (capillary action). Surface Tension Surface tension is the force that acts on the particles at the surface of a liquid. At the surface of water, water molecules pull down and sideways, not in all directions like elsewhere in the water. This causes the molecules at the surface to be held more tightly together, forming a kind of skin on the surface of the water. It is a special example of cohesion. 2
Strong Adhesive Properties Water molecules are also attracted to other substances. A meniscus forms around a straw, or around the edges of a glass due to adhesion the tendency of water to stick to other substances. Water's adhesive property allows water to stick to the sides of vascular tubes in plants, so that water can move from a plant's roots to its leaves. Water attracts and bonds with ions (electrically charged particles) in other compounds. As water pulls the ions into solution, the other compound dissolves. Universal Solvent Remember, a solvent is a substance in which another substance dissolves. Water is called the universal solvent because it can dissolve more substances than any other known substance, due to its polarity. Remember, water will only dissolve polar (hydrophilic) substances. Non polar (hydrophobic) substances, like oil, will not dissolve in water. 3
Ice Floats on Water Density is a measure of the mass of a substance per unit of volume, but changes with temperature. The density of liquid water is 1 g/cm 3. Usually, substances become denser as they freeze as their molecules pack more closely together. Water, expands when it freezes because molecules organize into an arrangement that gives ice a greater volume and less density than liquid water. That's why ice floats! Objects Can Float on Water Buoyancy is the ability of a fluid to exert an upward force on an object immersed in the fluid. All liquids exert the buoyant force upward. If the buoyant force is greater than the downward, gravitational force, the object floats. Objects with a greater density than liquid water will sink. Boats float because of all the air trapped within its shape. This lowers the boat's density so it can float. 4
The difference between an object's weight or density and the buoyancy force determines whether the object will sink or float. Float or sink? density is less than buoyancy force density equals buoyancy force density is greater than buoyancy force sinks floats stays where it is (neutral buoyancy) 5
Hint Properties of Water Reu.notebook Complete the table by dragging in either the log or weight to determine if the listed objects will float or sink in water. Object Density (g/cm3) Sink or Float float sink Moon 3.35 penny 8.95 ice 0.92 gold medal 19.3 silver pendant 10.5 Saturn 0.69 High Specific Heat Specific heat is the amount of energy needed to raise the temperature of 1 kg of a substance by 1 degree Celsius. The specific heat of water is very high: 4,184 J/kg* o C. This means water takes a long time to heat up or cool down. High specific heat of water helps bodies of water to maintain relatively stable temperatures. It also helps your body to maintain homeostasis. 6
Characteristics of Water Characteristic Example Water is polar Strong cohesive properties Strong adhesive properties High surface tension Universal solvent Ice is less dense than water Buoyancy High specific heat Definition: Sketch: 3 Real World Examples: Sentence: Frayer Models Create a Frayer model for these terms: Polarity Cohesion Adhesion Surface Tension Density Buoyancy Universal Solvent You should define the term, draw a sketch to associate with that term, give 3 real world examples, and use the concept in a sentence. 7
Definition: The tightness across the surface of the water that is caused by the polar molecules pulling on one another. Sketch: Water striders Painful belly flops Drops of water on a penny surface tension The water striders were able to walk on the water because their weight does not have enough force to break the surface tension of water. 3 Real World Examples: Sentence: 8