Onsen & Kósen

Onsens (including Kósens) are classified to 4 types according to the temperature at the gushing point of the spring. 

~ 25°C (77°F)

Cool spring (冷鉱泉 in Japanese). 
Precisely this is classified as Kósen, while the others are classified as Onsen.

25°C (77°F) ~ 34°C (93.2°F)

Warm spring (低温泉 in Japanese). 

34°C (93.2°F) ~ 42°C (107.6°F)

Hot spring (温泉泉 in Japanese).

42°C (107.6°F) ~

Extra hot spring (高温泉 in Japanese).

Conventionally, many onsen providers heat up the water or add cold water to adjust the temperature to approx. 41°C (105.8°F) which is the average temperature of Japanese bath.

Types

Onsens are classified to 10 types depending on dissolved substances (mineral depositions), the types are defined by the Japanese onsen law. See the following descriptions for details:

Simple Thermal Spring (単純温泉 in Japanese)

Does not contain enough amount of minerals to be classified as one of the others.

Carbon Dioxide Spring (二酸化炭素泉 in Japanese)

The main component is carbon dioxide (CO2).

Sodium Bicarbonate Saline Spring (炭酸水素塩泉 in Japanese)

The main component is bicarbonate (HCO3).

Chloride Spring (塩化物泉 in Japanese)

The main component is chloride (Cl).

Iodine Spring (含よう素泉 in Japanese)

Contains iodine. This type is newly defined in 2014.

Sulphate Spring (硫酸塩泉 in Japanese)

The main component is sulphate (SO4).

Iron (Ferruginous) Spring (含鉄泉 in Japanese)

The main component is iron (Fe).

Sulphur Spring (硫黄泉 in Japanese)

The main component is sulphur (S).

Acidic Spring (酸性泉 in Japanese)

Acidic water. See the "pH" section in this page for details.

Radioactive Spring (放射能泉 in Japanese)

The main component is radon (Rn).

pH 

Onsens are also classified to 6 types according to the pH value.

 ~ pH 2

Strong acidic 

pH 2 ~ pH 3

Acidic

pH 3 ~ pH 6

Weak acidic

pH 6 ~ pH 7.5

Neutral

pH 7.5 ~ pH 8.5

Weak alkaline

pH 8.5 ~

Alkaline