An Introduction
There are some questions that we must have come across in our day-to-day lives. Have you ever encountered a raindrop that does not adhere to any glass panel in a uniform order? What is the speciality of detergent or soap that can wash our clothes much better than normal water? Well, in this subject, we are going to study and analyse all kinds of behaviours regarding surface tension and detergents.
What is Surface Tension?
All the substances that we find in our surroundings consist of molecules which in turn are formed by the combination of one or more atoms. The substances are usually found in three states such as solid-liquid and gas. solution rigid bodies that do not change their shape and size. This is because the molecules of a solid are bound to each other by strong chemical forces. The chemical bonds between the molecules in liquids are relatively weaker and they can move around each other. all the molecules in the gaseous state are free to move randomly with respect to each other.
Thus surface tension is defined as the attractive force which is applied to the molecules available on the surface of the liquid by the molecules underneath. This inclines to attract into the bulk of the liquid and influences the liquid to imagine the shape consisting of the lowest surface area. Another special characteristic of the molecules in a liquid is their interaction with other molecules nearby them. They can be attracted to the molecules of subsistence which is known as cohesion or can be attracted more strongly to the molecules of other substances which is known as adhesion. If you observe a drop of water you can notice the cohesive force acting strongly to keep it in a round shape. This force of attraction applied by the molecules on the surface is known as surface tension.
Surface Tension of Soap Solution
Soap molecules consist of a long chain of hydrogen and carbon atoms. This is the infrastructure of atoms that enjoy inside water. The other terminal is easily connected with the soap molecule. The molecules of soap surround the water particle. The surface tension of water breaks with the help of the detergent molecules attached to the water molecule at both its ends.
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The grease particle is moved apart and connected with soap molecules, to be accomplished by a water jet spray. The two terminals of detergent molecules help to break the surface tension of water. This is called hydrophobic. The hydrophobic terminals of the detergent particle are being pushed up to the water surface in an attempt to draw out from the water molecule. This phenomenon helps to weaken the hydrogen bonding of water molecules available at the surface. This destroys the surface tension of water. The soap molecule does not desire to convert into liquid form in a soap and water solution. The molecules which have found their path to the surface destroy their path between the molecules of surface water by raising the hydrophobic ends on the water surface. This phenomenon breaks the water molecules apart from each other.
Detergents
The long chain of hydrocarbons having sodium (Na) or potassium (K) atoms at one end are known to be a special type of sodium salts or potassium salts respectively. The Hydrocarbon part is usually the nonpolar portion that is hydrophobic in nature. and sodium or potassium atoms attached at one end are polar and hydrophilic in nature. The combination of two such distinct characteristics gives a unique property to the molecules of detergents. As when detergents mix with water it decreases the surface tension of water molecules so they are known as surfactants. When a hydrophobic substance such as oil or grease come in contact with detergent molecules then the nonpolar end gas is attached to the grease or oil molecule. This explains why grease or oil molecules get dissolved and removed in detergent or soap water instead of in pure water.
The effect of detergent on surface tension brings a non-adhesive nature to water molecules. It does not stick to the surface of the glass. It happens because of the cohesive force in the water-detergent molecule. The exact thing appears when it comes to washing clothes with water. Water can’t help much to clean the dirt. This is where we need detergent powder. The mixture of detergent and water decreases water’s surface tension. We call detergents and shops surfactants. The surfactants decrease the surface tension and increase the sticking capacity of the things. Water in combination with detergent has the capability of strain through thin fibres of dirty clothes. The detergent solution spreads throughout clothes more easily than water. This is the only reason why water can’t help along with the washing of clothes.
The Surface Tension of Water on Adding Detergent to It.
When detergent is combined with water, it reduces its surface tension. The compounds which help to reduce water’s surface tension are known as surfactants. In this whole phenomenon, the lowered surface tension area can be seen where the surfactant is combined with water. These surfactants can be illustrated as detergents, soaps, emulsifiers, foaming agents, and dispersants. The word “surfactant” is the composite of a surface-active agent.
How Detergent Reduces the Surface Tension of Water?
As we have seen how detergent is acting in the water molecule, we can easily determine that it lowers the surface tension in the water. This phenomenon is caused because of surfactants that reduce the surface tension.
What are the Factors Affecting Surface Tension?
These are the following reasons affecting surface tension
Surfactant- It is the prime factor that helps to reduce surface tension. The surfactants combine with a water molecule and lower its original surface tension.
Chemical additions- It changes the characteristics of surface tension. When some random chemical is mixed with a substance, it changes the surface tension. A surfactant in water is the best example of chemical addition.
Oxidation- It has a direct effect on surface tension. Oxidation is a natural phenomenon that appears when oxygen available in the atmosphere helps to lower the surface tension. The impurities that exist on the surface directly influence the surface tension.
Temperature- This temperature factor initiates a decrease in surface tension. Though surface tension decreases, the molecules inside the solution have become operative, with the hike of temperature turning into zero.
Uses of Surface Tension in Real Life
Some real-life applications of the surface tension are:
It generates a high surface tension in water. The molecules are combined to create a film on the water surface.
This film, or the thin layer, is strong enough to embrace light objects like a blade or a pin made of tin.
Surface tension helps to form water drops without making it into a thin layer.
It also permits water to pass through roots, stems, thermometers, and in the blood vessels of our body. The pulling effect comes into action when one molecule moves up; it attracts the nearest one.
FAQs on Detergents and Surface Tension
1. What is surface tension and why does it occur in liquids?
Surface tension is a property of a liquid's surface that allows it to resist an external force. It is caused by the strong cohesive forces (attraction between similar molecules) present in liquids. While molecules inside the liquid are pulled equally in all directions by neighbouring molecules, those at the surface are pulled primarily inwards towards the bulk of the liquid. This inward pull creates a tight, elastic-like film on the surface and causes the liquid to minimise its surface area, which is why small water droplets are spherical.
2. How do detergents reduce the surface tension of water?
Detergents act as surfactants, or surface-active agents. Detergent molecules are unique because they have two distinct ends: a hydrophilic (water-attracting) head and a long hydrophobic (water-repelling) tail. When mixed with water, these molecules move to the surface with their hydrophobic tails trying to escape the water. This arrangement pushes the water molecules apart, disrupting the strong cohesive forces between them and effectively lowering the surface tension of the water.
3. What is the importance of reducing surface tension for cleaning clothes?
Water's naturally high surface tension prevents it from properly “wetting” fabrics, meaning it tends to bead up rather than soak in. For effective cleaning, water must penetrate the tiny fibres of a cloth to reach trapped dirt. By reducing surface tension, detergents make the water “wetter,” allowing it to spread out and seep deep into the fabric. This ensures that the detergent’s cleaning components can surround and lift away grease and dirt particles that plain water would not be able to reach.
4. What are surfactants and how are they related to detergents?
A surfactant is any substance that reduces the surface tension between two different phases, such as a liquid and a gas or two liquids. Detergents are a common and powerful example of surfactants. Their primary function is to act as surfactants in water, enabling the water to mix with and wash away oily or greasy substances. Other examples of surfactants include soaps, emulsifiers, and foaming agents.
5. Why can't plain water effectively wash away oily or greasy stains?
Plain water is ineffective on oily stains for two main reasons. Firstly, water is a polar molecule, while oil and grease are non-polar. These two types of molecules do not attract each other and therefore do not mix. Secondly, the high surface tension of water, caused by its strong internal cohesive forces, makes it bead up on oily surfaces rather than spreading out to lift the stain. The water molecules are more attracted to each other than to the oil molecules.
6. How does temperature affect both the surface tension of water and the effectiveness of detergents?
Temperature has a significant impact on cleaning. As the temperature of water increases, the kinetic energy of its molecules rises, which weakens the cohesive forces between them. This directly decreases the surface tension of the water. This effect, combined with the fact that higher temperatures also increase the solubility and chemical activity of detergent molecules, makes washing in warm or hot water much more effective at removing stubborn stains.
7. Can adding a substance to water ever *increase* its surface tension?
Yes. While substances like detergents are known for decreasing surface tension, adding highly soluble inorganic impurities can increase it. For example, dissolving common salt (NaCl) in water increases its surface tension. The strong electrostatic forces of attraction between the dissolved salt ions and the polar water molecules are greater than the original cohesive forces of the water. This strengthens the net inward pull on the surface molecules, resulting in higher surface tension.
8. Besides detergents, what other factors can alter a liquid's surface tension?
Several factors can change a liquid's surface tension. These include:
- Temperature: As discussed, increasing the temperature weakens cohesive forces and decreases surface tension.
- Soluble Impurities: Highly soluble substances like salt can increase surface tension, while sparingly soluble ones like soap decrease it.
- Surface Contamination: Dust, oil, or other contaminants on the liquid's surface disrupt the molecular cohesion and typically lower the surface tension.
9. What are some real-world consequences if water had no surface tension at all?
If water had zero surface tension, it would mean the cohesive forces between its molecules were non-existent. This would have drastic effects:
- It could not form droplets, and rain would be a fine mist.
- It would instantly spread out thinly over any surface it touched.
- Phenomena like capillary action, which depends on surface tension to help water climb up narrow tubes (like in plant stems), would cease to exist.
- Insects like water striders could no longer walk on its surface.
- The liquid would evaporate much more quickly, as molecules would not be held together at the surface.
