Why is hot water sometimes called 开水? What is the meaning of "open" in this context? How is this different than saying 热水 or 烧水?
Is it something to do with how the water was made hot, kind of like how there are sometimes different words for frying something (炸，炒，etc.)?
One of the meanings of 開 is 沸騰 (boiling) in 《漢語大辭典》開32.
It's nothing to do with "open" or "how".
開水 boiled water
熱水 hot water
燒水 to boil water
When the water is still very hot, we can use 開水 or 熱開水 as hot water.
When it is warm, we use 溫水 or 溫開水.
When it's at room temperature, we use 冷水 or 冷開水.
When it's below, say, 10℃, we use 冰水 or 冰開水.
開 can indicate that the water has been boiled. Without 開, it may or may not have been boiled.
The above Chinese words are used in Taiwan. They are probably different in the mainland.
开水 is the general term of boiled water, it's usually fresh boiled. 开 (boiled, not opened) indicates the temperature of the water and it doesn't mean water cooking.
热水 means hot or warm water, it must not been boiled before.
烧水 means water cooking. 烧 is the verb and 水 is the object.
There are many food cooking terms associated to water or steam. For example steaming 蒸， you can see the 4 dots there, which symbolize the fire。包子 is one of the most steamed Chinese food, it's steamed stuffed bun.
Another example is poaching 煮， which contains very similar structure. One famous food is called 水煮鱼 poached sliced fish in hot chili oil. Have a try ;-)
I would like to argue against some of the answers above. 开水 means water that have been boiled, but not necessarily still boiling/hot. Actually, people use the expressions 凉开水 (cold, boiled water) or 凉白开 (where 水 is omitted, meaning cold, boiled tap water) frequently.
So, 开水 and hot water have significant overlaps but are not the same thing:
Regarding the etymology of "开" as boiling, I have no definitive answer. My guess is that boiling water in a Chinese tripod (鼎) often results in the steam pushing the lid open, hence "开".
To elaborate on @Janus Bahs Jacquet's important comment here:
In Mandarin, for something to be "open" (开) more generally refers to it being in an active state, or in a state where it is ready for use. E.g. to "turn on the computer" (iirc) is "开电脑" - bring it into the state where it can be used. To understand how this applies to the case of water, you have to remember that Chinese always boil their water prior to use - which makes sense if you think about sanitation and germs. Water that has not been boiled is thus not "ready for use", while that which has, is now ready for use: it is 开, open, in the active state, the state where it is ready for use, i.e. for drinking, since the germs that would have otherwise made it hazardous to drink have been destroyed.
Note, by the way, that this phrase specifically refers to boiled, not simply "hot", water. Hot water really is just 热水, as you would expect.
To answer this question, you have to understand that Chinese traditional characters are mostly came from pictographic scripts.
開 is the tractional style of 开
Then we focus on this character
開
the outside frame 門 is symbolize the door, (you can notice this character is really like an opening door)
the character inside 开 is like a boiler in the campfire
It is a intersting problem and I want to answer why Chinese use "开" to draw the boiled water. People use teaketettle to hot up water and you can image that when the water is boiled, the pot lid will be flushed and open with the vapor. So Chinese describe the boiled water as open water which is "开水" in Chinese. It is really funny!
a question on Baidu
把水烧热，沸腾了为什么叫开水，而不叫沸水呢是什么时候，哪个人最先发明这个名词的啊...
1 Answer:
应该是来自“开花”吧，水沸腾的时候像花开一样;白开水就是水白开了，因为又凉了，也叫凉白开。
Certainly, the old character 开 is said to show 2 hands opening a door bolt.
Maybe it's the bubbles in the water? They 'opened' the water? Maybe it is just easier than saying: 沸腾水？？
No. Hot water is 热水, not 开水.
开水 means boiling/boiled water.
"kāi shuǐ 开水" means boiled water or boiling water. "kāi 开" literally means open, but here with "shuǐ 水" refers to boiled or boiling. "shuǐ 水" means water. For example: tā yòng kāi shuǐ pào chá 她 用 开 水 泡 茶 she makes tea with boiling water
"rè shuǐ 热水" means hot water. "rè 热" means hot。For example: wǒ xǐ huān yòng rè shuǐ xǐ zǎo 我 喜 欢 用 热 水 洗 澡 I like taking a bath with hot water
"shāo shuǐ 烧水" means to boil the water. "烧 shāo" is the verb, means to boil。 For example: wǒ yào shāo shuǐ zhǔ miàn 我 要 烧 水 煮 面 I want to boil the water for cooking noodles.
If you wanna describe the different temperature of the water, you can say: wēn shuǐ 温水 warm water
rè shuǐ 热水 hot water
bīnɡ shuǐ 冰水 ice water
lěnɡ shuǐ 冷水 cold water
