Water: a polar and universal solvent

Water is essential for life. It covers 2/3 of the earth’s surface and every living thing is dependent upon it. The human body is comprised of over 70% water, and it is a major component of many bodily fluids including blood, urine, and saliva. Water – with its formula H2O – is the only inorganic compound existing in its solid, liquid and gaseous physical state under natural conditions. Water serves as a medium for the transformation of highly complex organic molecules that form the basis for life processes. The solid state of water is known as ice; the gaseous state is known as water vapor (or steam). The units of temperature (formerly the degree Celsius and now the Kelvin) are defined in terms of the triple point of water, 273.16 K (0.01 °C) and 611.2 Pa, the temperature and pressure at which solid, liquid, and gaseous water coexist in equilibrium.

The above video explains the important properties of water

Two important properties of water are discussed below:

What does it mean to say the water molecules is polar and is a “universal solvent?”

  1. Water is a polar molecule and attracts ions and other polar molecules.
  2. Water can form hydrogen bonds, which make it a powerful solvent.
  3. It is water’s chemical composition and physical attributes that make it such an excellent solvent.
  4. Water molecules have a polar arrangement of the oxygen and hydrogen atoms—one side (hydrogen) has a positive electrical charge and the other side (oxygen) had a negative charge. This allows the water molecule to become attracted to many other different types of molecules.

Presentation1

1

Water has high surface tension, what does this mean?

At an interface between air and water, a water molecule on the surface forms hydrogen bonds with other molecules around and below it, but not with air molecules above it. The unequal distribution of bonds produces a force called surface tension; this causes the water surface to contract and form a surprisingly tough film or ‘skin’.

surfacetension-diagram

https://water.usgs.gov/edu/surface-tension.html

Examples of surface tension:

Walking on water: Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

Floating a needle: A carefully placed small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.

Surface tension disinfectants: Disinfectants are usually solutions of low surface tension. This allow them to spread out on the cell walls of bacteria and disrupt them.

Soaps and detergents: These help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

Washing with cold water: The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unneccessary.

Why bubbles are round: The surface tension of water provides the necessary wall tension for the formation of bubbles with water. The tendency to minimize that wall tension pulls the bubbles into spherical shapes.

Surface Tension and Droplets: Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer.
Let us explore some properties of water at surfaces with 2-3 activities. The materials needed are:

  • Water
  • Dish soap
  • Plastic cup
  • Plastic wrap
  • Salt, pepper
  • 50-60 pennies
  • Drinking straw
  • Sugar
  • Talcum powder

Activity 1

Fill a cup all the way to the top with water.

What do you think would happen if you were to add pennies to it?

Try adding pennies one at a time. What happens to the water in the cup?

How many pennies can you add without causing the water to overflow?

The video below demonstrates cohesion and surface tension in water. We add pennies into a full glass of water to show how surface tension will create a convex dome of water that rises above the rim of the glass. Surface tension is generated by cohesion and keeps an elastic tension between all the molecules in a liquid. The water has one of the highest surface tension.

Activity 2

Take some water with a straw and put a few drops on plastic wrap.

(a) What is the shape of the drop?
(b) Move a drop around with your straw. Does the drop change?

  1. Move one of the drops close to another one with your straw. What happens when 2 drops meet?
  2. Put a small amount of one of the solids (salt, pepper, sugar, talc) on one of the drops. Does the shape change?
  3. Try this again with another of the solids.

Activity 3

  • Put some water in your cup.
  • Sprinkle black pepper all over the surface. What does the pepper do?
  • Add a drop of dish soap to the water. What does the pepper do?
  1. Why does water form drops rather than spreading out?
    Why is it better for water to have the smallest possible surface area?
  2. What affects the surface of water: salt, sugar, pepper, talcum powder, dish soap? Why?

“Scientific inquiry starts with observation. The more one can see, the more one can investigate.” What are you waiting for, share with us your observations through comments.

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