Absorption

What is absorption? You have almost certainly seen materials like paper towels and clothing (made of materials like cotton, wool, and denim) absorb water. This means that molecules of water move between the molecules that make up these materials and go into the materials. Your hair also absorbs water. This means that molecules of water move between the molecules that make up strands of your hair so that the water molecules are inside each strand of your hair.

But other materials, like metals (shown below) and wax-coated bird feathers, do not absorb water or other liquids.

Why don't some materials (like the metals above) absorb liquids? The molecules that make up these non-absorbent materials tend to be much closer together (or denser), so H₂O molecules are not able to move between the molecules of the materials to penetrate into the materials.

Even though the metal surfaces of cars don't absorb water, people often put a layer of wax on their cars. This wax layer prevents water from directly touching the metal. As a result, water on the surface of the car can't bond with the iron (Fe) of the car bodies to form rust (Fe₃O₄).

Electric charges also help absorption. To further understand why certain materials absorb and "hold" liquids, we have to remember that water molecules have ends with opposite electric charges. As shown below, the hydrogen (H) side of water molecules is positively charged and the oxygen (O) side of the molecule is negatively charged. Because of this, they are attracted to other water molecules and any other molecules or atoms that have areas that are charged. These include salt molecules (such as NaCl) and sugar molecules (such as C₁₂H₂₂O₁₁).

Above are two ways to represent a water (H₂O) molecule, which has positively and negatively charged sides.

Absorption of water by different materials.

Absorption of water by paper towels. Water molecules also "stick" to the molecules that make up absorbent materials because of electric forces (see the  Atoms: Basics or electric force unit for more information on electric forces). Paper towels are mostly made from trees. And guess what? Trees contain a lot of sugar molecules. (They produce sugars during photosynthesis.)

We know that sugar molecules have charged areas, which attract oppositely-charged areas of water molecules. This is why sugar molecules dissolve in water. Water is absorbed by paper towels through the same process of attraction by electric forces.

The paper towel below absorbs water because of the electric force.

Absorption by hair, wool, yarn, etc.  When you take a shower, your hair gets wet because it also absorbs water. Your hair and similar materials like wool and yarn (which are made of animal hair, which is similar to human hair) are made of molecules called proteins. Like water, protein molecules also have areas that are positively and negatively charged. This is what allows water molecules to "stick" to hair, wool, and yarn molecules, resulting in absorption. 

Water temperature and absorption. As you might expect, there is a relationship between water temperature and how much water is absorbed by a material (such as paper towels or hair) in a given time. As water temperature increases, the amount of water absorbed by a material increases.

Why is this? As water temperature increases, the average kinetic energy (energy due to motion) of water molecules increases. So, the water molecules will be able to move farther into the materials (rather than stop because they are "caught" by the electric force of the molecules that make up the material). So more water is absorbed when the water temperature is higher.

When water is cooler, the molecules on average have less kinetic energy. Because they're moving slower, they are more easily "caught" and stopped by the electric forces of the materials. As a result, less water will be absorbed by the material (Source: Songok et al., 2014).

Q1: You have two glasses, each filled with one cup of water. In Glass A, the water is hot (80 degrees C). In Glass B, the water is room temperature (22 degrees C). You hang two identical pieces of yarn vertically in the water so that the exact same amount of yarn is in the water.

In which glass will the section of the yarn that is ABOVE THE WATER LINE absorb more water?

a) Glass A

b) Glass B

c) They would absorb the same amount.

d) There is just no way to tell.

Submit

You're right! The yarn in Glass A with the hot water will absorb more water. You will see that the part of the yarn that is above the water line will absorb MORE water in the hot water than in the cold water. This is because the hot water will move farther upward in the yarn than the cold water.

Actually, the yarn in Glass A with the hot water will absorb more water. You will see that the part of the yarn that is above the water line will absorb MORE water in the hot water than in the cold water. This is because the hot water will move farther upward in the yarn than the cold water.

In the scenario for Q1 above, the water absorbed by the yarn that is above the water level will evaporate into the air. This evaporated water will be replaced by more water from the glass that is absorbed by the yarn. This water will also evaporate. Water will continue to be absorbed and evaporate in a continuous cycle (until there's no more water).

The photo below shows salt (NaCl) growth on a plain piece of yarn after a few weeks of sitting in salt water. Most of the water has evaporated during this time. You may notice that most of the growth is toward the top of the string (the part that was just above the initial water level, represented by the blue line). This is because once Na+ and Cl- atoms start to build up on the yarn, they continue to attract the oppositely-charged atoms. Na+ atoms attract Cl- atoms and Cl- atoms attract Na+ atoms due to the electric force. 

The electric forces between Na+ and Cl- atoms are stronger than the electric forces of the protein molecules that make up the yarn. This is why the Na+ and Cl- atoms build onto each other more than they build onto the yarn to form the salt crystals you can see below.