Sodium hydroxide點解變藍?

A kilo sodium hydroxide 100% will sell HK$6-9 per kilo so a 80% liquid will cost you 4.8-7.2 per litre . If they sell you the product exactly like concentrated caustic soda 80% liquid , you can easily calculate the cost , now the manufacturer add some alkaline resistant dye to it & say this is a specially made product for drain opener(it is not good for most oilly blocking ) . Then most customers will buy the product . It is a way of marketing to most customers who donot know about the chemistry of the product !!

Sodium Hydroxide solution used in clearing drains is a very strong alkaline and is very corrosive.
It is dyed into blue in order to let people to identify it for safety reasons.
People can see it's blue colour clearly thus can handle it safely.

I think the blue colour is due to the dye, CI Acid Blue. It is found in the product "Vanish" – toilet bowl cleaner, just an example.

There are many types of dye. For example, CI Acid Blue 80 Dye is a colorant. Dyes are added to products for a variety of reasons including helping you see where you applied the product, when a product is used up, or for aesthetic reasons.

氫氧化鈉（NaOH，俗稱燒鹼、火鹼、苛性鈉，因另一名稱caustic soda而在香港稱為哥士的）常溫下是一種白色晶體，具有強腐蝕性。易溶於水，其水溶液呈強鹼性，能使酚酞變紅，使石蕊溶液變藍。2004年全球產量約為6000萬公噸，而需求量是51萬公噸。 純氫氧化鈉是白色固體可用顆粒並具有吸濕性，因此它應存放在一個密封的容器中。氫氧化鈉溶於乙醇和甲醇。熔融氫氧化鈉需要很高的溫度。氫氧化鈉溶液會在布料和紙張留下黃色污漬。氫氧化鈉是一種極常用的鹼，是化學實驗室的必備藥品之一。它的溶液可以用作洗滌液。
溶液為無色透明的鈉鹼液體，是強鹼之一，易在水中溶解，能與許多有機、無機化合物起化學反應，腐蝕性很強，能灼傷人體皮膚等。氫氧化鈉在水中完全電離出鈉離子和氫氧根離子，可與任何質子酸進行中和反應。
氫氧化鈉還是許多有機反應的良好催化劑
反應進行的既完全又迅速。這就是氫氧化鈉能灼傷皮膚的原因。氫氧化鈉是製造肥皂的重要原料之一。氫氧化鈉溶液加油，比例合適會反應混合，成為固體肥皂。這一反應也是利用了水解的原理，而這一類在NaOH催化下的酯水解稱為皂化反應。氫氧化鈉在潮濕的空氣中會潮解。
氫氧化鈉在空氣中易變質，其原因是遇到空氣中的CO2
如果繼續通入過量的CO2甚至可以生成NaHCO3