The surface tension of pure water is 72.8 millinewtons per meter (mN/m) at 20 °C, a relatively high value compared to most other liquids due to the strong hydrogen bonds between water molecules. However, the surface tension changes when substances are dissolved in water, thus forming water solutions.
Understanding Surface Tension
Surface tension is the tendency of a liquid's surface to shrink into the minimum surface area possible. It's like an elastic skin covering the liquid, caused by the cohesive forces between the liquid's molecules. Water molecules have strong attractions to each other through hydrogen bonds, creating a high surface tension.
Surface Tension of Water Solutions
When a substance is dissolved in water, it can either increase or decrease the surface tension.
Substances That Increase Surface Tension
- Salts: Dissolved salts generally increase the surface tension of water, though the increase is usually small.
- Inorganic compounds: Some inorganic compounds dissolved in water can also slightly increase the surface tension.
Substances That Decrease Surface Tension
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Surfactants: These substances, like soaps and detergents, dramatically decrease surface tension. They have molecules with both water-attracting (hydrophilic) and water-repelling (hydrophobic) parts. These molecules tend to arrange themselves at the surface of the water, disrupting the water's hydrogen bonds.
- This is why soapy water easily forms bubbles.
- It also helps water to spread and wet surfaces better, like in washing clothes.
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Organic molecules: Many organic molecules, particularly those with long hydrocarbon chains, tend to lower the surface tension of water.
Examples
| Substance | Effect on Surface Tension |
|---|---|
| Pure Water | 72.8 mN/m at 20 °C |
| Water with Salt | Slightly Increased |
| Soapy Water | Significantly Decreased |
| Detergent Solution | Significantly Decreased |
Practical Implications of Surface Tension in Solutions
- Capillary Action: Surface tension is crucial in capillary action, which allows water to climb up narrow tubes or spaces. This is essential in plants for transporting water from roots to leaves. The reference notes that surface tension is an important factor in capillarity.
- Cleaning: Surfactants reduce the surface tension of water, enabling it to penetrate into crevices and lift away dirt.
- Bubble Formation: Soaps and detergents decrease the surface tension of water, making it possible to create stable bubbles.
- Wetting of Surfaces: By lowering the surface tension, water can better spread over and wet surfaces, like in painting or printing processes.
