1. Why Bubbles Made of Pure Water Do Not Last
The wall of a bubble is simply a thin film of water trapping air inside it. Gravity pulls the water in this film downward, and the water also evaporates into the air. Because of this, a film made of pure water becomes very thin quickly and breaks apart. Even if a bubble forms, a stable film like a soap bubble cannot be created with pure water [3].
However, when soap is added, the film lasts much longer. Research from Tokyo University of Science states that bubbles become stable because surfactants (the active ingredients in soap) attach themselves to the boundary between the air and the liquid, known as the gas-liquid interface [2]. This means that keeping a bubble alive involves not just water, but also soap molecules lining up at that boundary. The exact way this works can change depending on the type of surfactant and the conditions.
So, how do these soap molecules arrange themselves at the boundary between water and air?
2. How Soap Molecules Line Up at the Water-Air Boundary
Surfactants found in soap and face wash have two distinct parts within one molecule: a part that loves water (hydrophilic group) and a part that loves oil but hates water (lipophilic or hydrophobic group) [1]. You can imagine them like tadpoles, where the head likes water and the tail avoids it. However, real molecules are not tadpoles; they just have two parts with these opposite properties.
When dissolved in water, the water-loving heads stay inside the water, while the oil-loving tails point away from the water surface, toward the air [1]. Because the tails try to escape the water, the molecules gather at the boundary between the water and the air.
A bubble film has two surfaces: an outer side and an inner side. When molecules line up on both of these sides, forces such as repulsion between the molecules help slow down the thinning of the film [3]. Additionally, if the molecules are packed more tightly in one area than another, a force called the Marangoni effect pushes them from the crowded area to the sparse area. This gives the film an elasticity that helps it resist changes in thickness [3]. However, how much each of these mechanisms contributes depends on the type of surfactant and the specific conditions.
This is the basic mechanism for why bubbles last. But do all molecules create foam in the same way?
3. How Molecular Shape and Other Ingredients Change Foam
The ability to make foam also depends on the shape of the molecules. The *World Encyclopedia* explains that good foaming agents tend to have an oil-loving part that is straight, with a long chain of carbon atoms. In contrast, molecules with short chains or branched shapes often do not create foam; they may actually act to destroy bubbles [1].
In experiments at Tokyo University of Science, researchers combined anionic surfactants with higher alcohols. The results showed that surface tension decreased and surface viscosity (the stickiness of the surface) increased. This created a dense film at the gas-liquid interface, which improved bubble stability. In these specific experimental conditions, bubbles became more stable as the carbon chains of the higher alcohols got longer. However, if the chains were too long, precipitation occurred [2].
The same study also showed that when there is a lot of ethanol present, it becomes harder for surfactants to attach to the boundary, making stable bubbles difficult to form [2]. It seems that foaming depends on both the shape of the molecules and what else is in the mixture.
So, what happens if the "water" itself is different?
4. Why the Same Soap Foams Differently Depending on Water Hardness
The "hardness" of water is a number that represents the amount of calcium and magnesium dissolved in it, converted into an equivalent amount of calcium carbonate. According to explanations from Kyoto Prefecture, soap foams well in soft water but has difficulty foaming in hard water [4].
Why does it become harder to foam? Apart from regular soaps like sodium salts of fatty acids, there are substances called "metallic soaps." These form when calcium or magnesium metals bind with fatty acids. These metallic soaps are described as being difficult to dissolve in water, or insoluble [5]. In hard water, soap molecules bind with the calcium and other minerals to become these insoluble substances. This reduces the number of soap molecules available to create bubbles, which is thought to be why foaming is reduced. However, since no single source presents this entire chain of reasoning as a complete fact, we keep this explanation as a possibility ("it is thought that").
The way hardness is measured varies by country. In the German scale used in some contexts, water with 10 degrees or less is soft, 10 to 20 degrees is intermediate, and 20 degrees or more is hard [4]. Japan’s tap water quality standard sets a limit of 300 milligrams per liter for hardness. The water from the Chiba Prefectural Waterworks is about 80 milligrams per liter, which is described as moderately soft water [6]. Note that German degrees and milligrams per liter are different units, so you cannot compare the numbers directly.
5. A Kitchen Experiment: Comparing Water and Soapy Water in Bottles
You can try comparing water and soapy water at home. Prepare two clean plastic bottles and fill each one halfway with water. Dissolve a very small amount of soap in one of them. Close the lids tightly, shake them the same number of times, and compare how long it takes for the bubbles to disappear. If soap molecules slow down the thinning of the film, the soapy water should keep its bubbles longer.
You can also look at how tap water foams. If you have your home’s tap water and a store-bought water with hardness listed on the label, make soap solutions using the same type and amount of soap for both. Shake them the same number of times and compare the foam. The results are only rough guides. Because differences in water are not caused by hardness alone, you cannot say that all the difference in bubbles is due to hardness.
If you want to read more, the explanations from Kyoto Prefecture [4] and the Chiba Prefectural Waterworks page [6] allow you to check water hardness standards in Japanese.