01:160:159 Lecture Notes - Lecture 1: Ap Chemistry, International System Of Units, Significant Figures

Chemistry 162 Lab Activity



Introduction Water has an amazing ability to adhere (stick) toitself and to other substances. Hydrogen bonds form when hydrogenatoms covalently bonded to nitrogen (N), oxygen (O), or fluorine(F) in the form of covalent compounds such as ammonia (NH3), water(H2O) and hydrogen fluoride gas (HF). In these molecules, thehydrogen atoms do not pull as strongly on the shared electrons asthe N, O, or F atoms. Therefore, the molecules are polar; thehydrogen atoms become positively charged and are able to formhydrogen bonds to negative ions or negatively charged parts ofother molecules (such as the N, O, and F atoms that becomenegatively charged in these compounds). Hydrogen bonds are not truebonds like covalent bonds or ionic bonds. Hydrogen bonds areattractions of electrostatic force caused by the difference incharge between slightly positive hydrogen ions and other, slightlynegative ions. These attractions are much weaker than true ionic orcovalent bonds, but they are strong enough to result in someinteresting properties.

In the case of water, hydrogen bonds form between neighboringhydrogen and oxygen atoms of adjacent water molecules. Theattraction between individual water molecules creates a bond knownas a hydrogen bond (Fig. 1).
Fig. 1: Hydrogen bonds shown as the dotted lines between watermolecules.
Cohesion Molecules of pure substances are attracted to themselves.This sticking together of like substances is called cohesion.Depending on how attracted molecules of the same substance are toone another, the substance will be more or less cohesive. Hydrogenbonds cause water to be exceptionally attracted to each other.Therefore, water is very cohesive.
Adhesion Adhesion is similar to cohesion, but it involves unlike(i.e. different) substances sticking together. Water is veryadhesive; it sticks well to a variety of different substances.Water sticks to other things for the same reason it sticks toitself – because it is polar so it is attracted to substances thathave charges. In this lab activity, you will be asked to use thematerials and procedures provided to explore the properties ofwater that involve both cohesion and adhesion. Be sure to writedown your observations and results as you progress. sults as youprogress.

Fig. 1

:

Hydrogen bonds shown as the dotted

lines between water molecules.

Part 1:

Exploring Water

Procedure

A.

How many drops of water can you fit on the surface of a pennywithout the water spilling over?

1.

Predict how many drops of water will fit on a penny. Record yourprediction.

2.

Use the plastic pipette to drop water on the surface of a penny.Drop car

efully and slowly to fit as

many drops as possible on the penny. Count each drop until thewater spills over the side of the

penny and record your results.

3.

Compare your results to those of your classmates. Record therange and calculate the average

number

of drops that fit on a penny.

4.

Add more drops to the penny, without letting the water spillover. Carefully observe and draw the

water on the penny.

5.

Record

a hypothesis for the behavior of water that you observed, usingthe language of cohesion

and adhesion

in relation to the water and the penny.

A.

Record your results and

thinking in your notebook.

B.

Can you

disturb

the water on the penny without spilling it?

1.

A

dd drops of water to the penny, until the water is almost readyto spill over.

2.

Predict what will

happen when you insert a toothpick into the surface of the waterpiled on the

penny. Record your prediction.

3.

Insert the

toothpick

and carefully observe the surface of the water.

4.

Try inserting the

toothpick

in various ways. Observe the surface of the water.

Pay special attention to

the water surface where the skewer enters the water.

5.

Record your observations and draw what you see.

6.

Repeat

these

procedures using a paper clip and the metal clip part of abinder clip. Try to indent the

pile of water as much as p

ossible without spilling it.

7.

Record your observations and dra

w what you see, paying careful

attention to the surface of the

water.

8.

Record

a hypothesis for the behavior of water that you observed usingthe language of cohesion

and adhesion in relation to the

toothpick

, the paperclip, the binder clip, and the water.

B.

Record your results and

thinking in your notebook.would you please answer the questionfor me . Thanks