The exams of all subjects are coming in December. Today, we got a chemistry class, and one of the questions in the test confuses me a lot. The question asks: Why do people add Chlorine in drinking water?

Add Chlorine to drinking water?
Is water with Chlorine drinkable?

My first uncertainty to this question is that: Is water with Chlorine drinkable? From my understanding of the element Chlorine, it is toxic. Chlorine in gas form was used as a chemical weapon during WWII, causing thousands of deaths. How could chlorine be added into drinking water?

I did some research online. There was an experiment in the 1980s about the effects of chlorine on human bodies. Both the animal experiments and human experiments performed directly with chlorine-treated water in this experiment showed high safety. No adverse effects were observed at a dose of 0.34 mg of chlorine per kg of body weight (This dose was the highest used in the experiment, so it was not clear that what dosage would be high).

As long as not too much Chlorine is added, it is safe for human body

But this doesn’t mean that Chlorine is safe for human body. After Chlorine enters a human body, a large amount of hypochlorous acid will be produced after a chemical reaction: Cl2 + H2O –> HCl + HClO), which causes the oxidation of tissues. Hydrochloric acid stimulates inflammatory swelling of the mucous membranes, making the mucous membranes of the respiratory tract swell and secretes a large amount of mucus, causing breathing difficulties. So therefore, the obvious symptom of chlorine poisoning is a severe cough. When the symptoms are severe, pulmonary edema can occur, making circulation difficult and possibly causing death.

Recently, I read an article from Singapore Eye: 罕见!新加坡5岁女孩游泳后全身红肿,刺痛无比……. The little girl in the article, after swimming, got a lot of bumps on her body of different shapes and sizes. They looked like mosquito bites, but they ranges from her upper body to the thigh, looking scary.
What caused such scary symptoms? Yes, it was Chlorine.
I still feel a bit scared after reading this. It was very lucky that the girl did not drink water from the pool. From the symptoms described in the article, her skin was stimulated by the Hydrochloric acid after the reaction of chlorine, but fortunate what swelled was her skin and not her throat, or else the consequences would be deadly.
Why do people add Chlorine in drinking water?

Simple answer: Sterilization

Bacteria is affected by hypochlorous acid in the same way (not sure if bacteria will be affected by hydrochloric acid, maybe through osmosis, but absolutely not because of a swelling throat). Since hypochlorous acid molecules are small in volume and do not have charges, it is very easy from them to go through the cell membranes. Hypochlorous acid is also a strong oxidant that can harm cell membranes, allowing protein and nuclei to fall out from bacteria (sounds disgusting), causing its death.

All in all, the amount of chlorine added to the water must not be too much, otherwise, let alone drinking the water, even pool water will cause great harm to people. However, as long as not too much Chlorine is added, human bodies will not be harmed.

Chemistry test – correct answer – sterilization.