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Can anyone help me with the theory of surface tension? My chemistry experiment is related to the surface tension and concentration of NaCl. I am confused about my hypothesis. On the internet, I found that increasing salt concentration also increases the surface tension of the water. But in my experiment, surface tension is decreasing with the increase of NaCl. So can anyone help me with the theory behind this?

With the help of needle and water drops, I was able to calculate the value of surface tension of water for each concentration of NaCl.

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    $\begingroup$ Suspect contamination of your equipment or the salt. Even a tiny amount of detergent, for example can greatly decrease surface tension. youtube.com/watch?v=jHWuJXYsYag $\endgroup$
    – DrMoishe Pippik
    Commented Aug 30, 2020 at 1:16

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The surface excess of a salt (NaCl) is negative because salts prefer to be hydrated. So there is less salt at the inferface than in the bulk phase. The Gibbs adsorption equation says the change is surface tension is the opposite of the surface excess - so for a salt solution, the change is positive and the surface tension increases.

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Since adding NaCl should increase the boiling point it should increase the surface tension.

This has been explained previously by others here:

How does NaCl (or any inorganic salt) increase surface tension?

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