The oxygen side of the molecule has a partially negative charge? As a result, water molecules form hydrogen bonds, interacting as shown below. One part of isopropyl alcohol is slightly polar. Notice that in the structural formula on the left, the hydrogen H has a?
Click here to start quiz. Isopropyl alcohol: structural formula. Isopropyl alcohol. Water:highly polar. Isopropyl Alcohol: much less polar than water. Because the water molecules are polar, they grab onto one another through hydrogen bonds. The mutual attraction of water molecules is called cohesion.
Though gravity is trying to pull all the water molecules down, toward the surface of the penny, the hydrogen bonds keep the water droplet in a spherical shape. There are going to be many variables influencing the exact number of drops.
The key point is that you should have been able to pile many more drops of water on the penny than drops of alcohol. The explanation here is pretty much identical to the one above. The water molecules have much more cohesion that alcohol molecules, because they grab onto one another through hydrogen bonds. Because a sphere allows for the maximum number of connections between water molecules, the expanding drop of water continues to maintain that shape.
At a certain point, the sphere expands to where it overflows the penny, and water spills out over the side. However long you observed for, you should have noted that it takes much longer for water to evaporate than alcohol. In other words, the amount of water that evaporated was much less than the amount of alcohol that evaporated.
Because of hydrogen bonding. The water molecules, like the alcohol molecules, are vibrating and moving. But hydrogen bonds keep the water molecules from jumping away from the surface of the drop, so that the drop of water evaporates very slowly. Consequently, when alcohol molecules accelerate to a high enough speed to jump away from the surface of the liquid, they take off. But the molecules at the surface only feel attractions from the molecules next to them and beneath them. These surface molecules are pulled together and inward by these attractions.
This tight arrangement at the surface is called surface tension. The inward pull from the attractions of the molecules results in the smallest possible surface for a volume of water, which is a sphere.
This is why water forms a round drop or dome at the top of the filled test tube and on the surface of a penny. Note : Even though there are many ways to compare the surface tension of water and alcohol, the procedure written below compares each liquid on the surface of a penny.
Use a dropper to add drops of water to the surface of a penny. Count the drops until the water overflows. The water beads up on the penny and the alcohol spreads out flat. Many more drops of water can be added to the penny than drops of alcohol. Project the image Water and Alcohol. Review that water molecules are polar and that they are very attracted to each other. Point out that alcohol molecules are polar in only one area, making them somewhat attracted to each other.
They are not as attracted to other alcohol molecules as water is to other water molecules. Alcohol molecules only have 1 O—H bond and they have some C—H bonds that are pretty non polar. There is not as strong an attraction between them as there is between water molecules. The shape of the water molecule and its polarity at the top and the bottom give water molecules lots of opportunities to attract. Almost anywhere two water molecules meet they can be attracted to each other.
But alcohol has a different size and shape and has its polar part on one end. Alcohol molecules can meet at areas where they would not attract as strongly. The water is more attracted to itself than to the metal of the penny.
The alcohol is a bit less attracted to itself so it spreads more on the penny. Gently touch the water on one penny with a toothpick.
Watch the surface of the water as you touch it. Note : This activity works best if the dome of water on the pennies is pretty high. Touching the water with the toothpick causes the surface of the water to be pressed down and bend. Touching the water with the toothpick and detergent causes the water to collapse and spill off the penny. Project the image Water and Detergent.
Explain that detergent is made from molecules that have a charged end and a longer uncharged end. The detergent molecules spread out over the surface of the water with the charged end in the water and the uncharged end sticking out.
The water molecules at the surface are attracted to the charged end of the detergent molecules. As the surface water molecules are attracted outward, this acts against their inward attraction that was creating the surface tension. This reduces the surface tension, and the water does not hold its round shape and thus spills. Project the animation Water on Paper Towel. Explain that paper towel and other paper is made from cellulose. Cellulose is made from repeating molecules of glucose that are bonded together.
The glucose molecule has many O—H bonds, which are polar. Polar water molecules are attracted to polar cellulose. Add one and a half tablespoons of water to cups 1 and 3. Add one and a half tablespoons of rubbing alcohol to cups 2 and 4.
Procedure Add one teaspoon of salt to the water in cup 1. What happens to the salt? Does it dissolve in the water? Put on the lid and shake the cup for about 20 to 30 seconds. What does the mixture look like? Repeat the previous two steps using cup 2 with rubbing alcohol. What happens to the salt this time? Does the mixture look different from the water—salt mixture? Take the cap off the permanent marker and swirl its tip in the water in cup 3 for about 10 seconds.
Put the lid on the cup and shake it for five seconds. Does the ink dissolve in the water? What does the solution look like after shaking? Repeat the previous step with cup 4 rubbing alcohol. Does the resulting mixture look different? If so, what is different? Can you explain the differences? Next, pour the alcohol from cup 4 into the water in cup 3. Put the lid back on and swirl the mixture for five seconds.
Does the rubbing alcohol mix with the water? What happens to the color of the mixture? Do you see separate layers forming? Now, add one teaspoon of salt to the mixture in cup 3. Put the lid on the cup and shake it for 20 to 30 seconds. What happens when you add the salt to the mixture? Does the mixture look different before and after shaking?
If so, how does it look different? Can you explain your results? What color is the mixture? Extra: Can you separate other liquid mixtures using salt?
What about ethanol and water or acetone and water? Try different liquid mixtures to find out!
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