The solubility curve of the oxygen in the water has an asymptotic behaviour. It means that when increased the water temperature beyond 40ºC (104ºF), the curve will tend to be almost constant, so the variation of oxygen dissolved in water will be insignificant.The chemical explanation of this process of loss of oxygen in the water is attributed to the fact that the oxygen molecules (O2) are non-polar, while those of the water are polar. As the O2 and the H2O have different polarity, the O2 has low solubility in water. However, it is sometimes desirable to bring water to the boiling point in order to eliminate any bacteria or imperfections that may harm the flavour of the tea - for example, when using hard water. In chemistry, hard water (also called calcareous) is water with a level of hardness superior to 120 mg CaCO3/1. That is to say it contains a high level of minerals.These cause the water’s hardness, and the degree of hardness is directly proportional to the concentration of metallic salts. Many cities have hard water. In these places the tea liquor is very clear, and of a very bland flavour. Due to the hardness, the hot water cannot extract all the flavour, colour and fragrance from the tea. In such cases, some part of the hardness (calcium) in the water is precipitated by boiling, and the water will be slightly better for preparing the tea. But hard water is best avoided for tea brewing whenever possible. When there is no other option except to use hard water, it is highly recommended that we filter it and then boil the water and leave it to rest for a few minutes to allow the minerals to precipitate. In this way, many of the components that make the water hard can be eliminated or reduced in quantity, thus improving the tea infusion. When purchasing bottled water look at the back of the label. Mineral contents should be clearly expressed, and you will be able to pick the brand with less Calcium and Magnesium. Whether the water is boiled or not, the most important thing is that when it is used to brew tea, it must be at the right temperature. When pouring the water onto the leaves, it must never actually be boiling, but if the water boils and is then left to cool until it reaches the right temperature, the tea will be better.
Many times I have heard people saying that the water must not reach its boiling point to make tea. It is said that if it does, all the oxygen contained in the water will be lost, and the tea will be dull and flat. This is one of the biggest myths about tea. One of the habits I inherited from my career as an Engineer is questioning everything, and this is one of those myths that needed research. So I did my research and I found out not only that boiling the water to make tea does not act in detriment of the infusion, but also that, in some cases, it improves the quality of the water that is going to be used to brew tea. To support this conclusion I organized numerous blind tastings of different teas. We made the teas with the same water temperature and same steeping time, and the only difference was whether the water had been boiled or not. After several tests performed by properly trained tea cuppers and also people without any previous knowledge about tea, I found that most of the tasters chose the infusion made with boiled water. These tests demonstrate how water affects the flavour of tea, but I still wanted to explain it scientifically, to destroy the myth. What happens is that oxygen dissolved in water is released at temperatures much lower than the boiling point. As you can see in the oxygen solubility in water curve below, water looses oxygen in small amounts as you raise the temperature. At 40°C (104ºF) the presence of oxygen dissolved in the water is already so low that there is almost no difference between oxygen levels at that temperature and at the boiling point. Therefore, the difference in oxygen levels at the average temperatures used to infuse tea, between 70°C (158ºF) and 95°C (203ºF), and the boiling point, is insignificant. Solubility curve of the oxygen in the water: