Properties Of Water Lab. Name_________________ per____ date______ mailbox______

Properties of Water Lab

Water’s chemical formula is H2O. As the diagram to the left shows, that is one atom of oxygen bonded to two atoms of hydrogen. The hydrogen atoms are “attached” to one side of the oxygen atom, resulting in a water molecule having a positive charge on the side where the hydrogens reside and a negative charge on the other side, where the oxygen atom resides. This separation of charge on opposite ends of the molecule is called polarity. Since opposite

electrical charges are attracted to one another, water molecules tend to be attracted to each other. This makes water tend to be

kind of “sticky.” As the right-side diagram shows; the hydrogen end of the bottom water molecule’s (positive charge) is attracted to the oxygen end of the above water molecule’s (negative charge) and so on and so forth up the chain. This property of water is known as cohesion. All these water molecules attracting each other mean they tend to clump together. This is why water drops are, in fact drops! If it wasn’t for gravity, these drops would be ball shaped or spherical as is the case aboard the International Space Station.

Even if it doesn’t form a perfect sphere on Earth, we ought to be happy water has the properties it does. Indeed without these properties we could not exist. For instance, for many of the same reasons water is “sticky” it is also known as the “Universal Solvent” because it dissolves perhaps more substances than any other liquid on Earth. This means that wherever water goes, either through the ground, our bodies, the sky or oceans it carries with it valuable chemicals, like minerals and nutrients.

Ever wonder, how a water strider walks across the surface of a pond without sinking? The surface water bears a remarkable property. On the molecular scale, it acts like a stretched elastic membrane or “skin.” This property is referred to as surface tension and is largely due to cohesion between adjacent water molecules. It is caused by the attraction of the particles in the surface layer and the bulking up of the liquid below, tending to minimize surface area as molecules strive for the optimal pole to pole bonding arrangement. The whole effect causes tightness at the surface, which resists the step of a water strider’s foot, supports a leaf temporarily and causes water to bead up on windows, certain clothing or a duck’s back. It can slow the wetting process of certain fabrics and inhibit the cleaning process too.

Surface tension can however be reduced. Increase the water’s temperature and kinetic energy starts to move molecules about again, while also promoting evaporation. The water vapor now above can linger in transition grabbing at remaining water molecules along the surface boundary and tugging at them from above. Bring water to a boil and surface tension is all but lost.

The addition of substances such as soap or detergent (known as surfactants) can reduce surface tension by increasing the spreading and wetting properties of water. Surfactant molecules look like tadpoles. The head is hydrophilic meaning attracted to water and typically soluble. Meanwhile, the tail of

a surfactant is composed of fatty material that is insoluble in water and allows grease, oils or other fats to stick to it. This property of not dissolving in water makes it hydrophobic

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meaning to repel water. Surfactant molecules break cohesion and surface tension allowing water to contact more dirt particles, making fabrics wetter and allowing materials to be cleaned.

Water striders can walk on the surface of water despite having a density themselves greater than that of water. Meaning they ought to sink if not for water’s properties. However, surface tension and tiny hairs on their feet which are hydrophobic allow them to keep from getting wet and keeps them afloat.

1. Fill in the properties of water we investigate during this lab, from the reading above.

a. _____________ relating to the separation of charges in a molecule

b. _____________ meaning to repel water, literally “fear of water”

c. _____________ term for a chemical which can break surface tension

d. _____________ this term describes the force of attraction between adjacent water molecules.

e. _____________ meaning attracted to water or literally “water loving”

2. ______What is the property of water that describes a water molecule’s charge ?

a. polarity b. surface tension

c. cohesion

 

d. surfactant

3. ______What is the property of water that describes the force of attraction between water molecules?

a. polarity b. hydrophobic

c. cohesion

 

d. surfactant

4. ______What is the property of water allows a water strider to walk on the surface of water?

a. polarity b. surface tension

c. cohesion

 

d. adhesion

5. Definitions: use the reading.

Define: polarity- ________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Define: cohesion- ________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Define: surface tension-___________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

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Water QUESTIONS

1. Explain why water is the universal solvent using a complete sentence. Use the word solubility or dissolve.

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

2. What is a polar molecule? Explain. Use the words positive, negative and charge.

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

3. Which end of the water molecule is negative? Which end is positive?

_____________________________________________________________________________

_____________________________________________________________________________

 

Fill in the blank:

Surface tension causes water to ______________________ on surface like glass.

Water molecules are _____________________ to other water by a process called cohesion.

Surfactants are used to break cohesion between water molecules, making surfaces wetter and allowing water to _____________ away dirt.

 

STATION 1: PENNY

1. Place a penny flat on the table.

2. Hypothesize – how many drops of water do you think the penny could hold before water spills over the edge? Experiment with the head side verses the tails side of the coins.

3. Using a dropper, count how many drops the penny holds until it spills over. Draw how the water appeared on the penny right before it spilled off.

 

Hypothesis – Estimated # of drops  Heads ________ Tails ________

 

Record the actual # of drops ________

Properties Of Water Lab

 

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