Water is a strong solvent. Therefor it is difficult to get pure H2O. Something is always disolved. Some molecules makes a big difference to the taste, others play a minor role. But it all depends on levels.
For coffee brewing: some Calcium is good. But too much or too little is bad.
Here in Denmark we have hard water (= high in Calcium carbonate). When you boil it in a kettle, quickly scale builds up on the bottom. But apart from that it is very clean water because it is filtered through many layers of chalk in the underground.
Other places in the world surface water is used. It has less calcium carbonate, but the it has polutions and is added chlorid to kill off microbiology.
Therefor often filtered water is used for coffee brewing. This is a common type of filter:
But filteret water is not just one thing. There are many types of filters. And a certain type of filter don’t give the same water everywhere. The filter only adjust; so the outgoing water depends on the composition of the ingoing water.
To describe what is going on in the water, you need a lot of chemistry terminology. It is complex. And when you heat up the water, it also changes. I know many coffee people who wanted to get hold of water chemistry but have given up on understanding it.
Here I won’t go into explaining details. But instead tell about ways to explore and get hold of what is right for you.
Taste different water
Best way to get the feeling of how the water impacts the taste, is to brew the same coffee with different kinds of water:
Tap water – on its own and with a little citric acid added.
Bottled water – different kinds has different specifications
Filter jug – you can get them with the classic filter (Calcium swapped with Sodium) or the newer who adds Magnesium.
Filter installation – You can ask at a coffee bar if you may fill a container with from their filter. Maybe you can find a cafe that got remineralized RO water (RO=reverse osmosis).
Best to brew it all at once in a cupping. For this you need equipment to heat up the water at the same time.
Be aware: Water directly from the tap or a filtersystem will contain more bicarbonate (the buffer). When the waters sits still for a while CO2 will slowly leave the water and that will diminish the bicarbonate:
H++ HCO3− ⇋ H2CO3
which immediately decomposes to
H2CO3 ⇌ H2O + CO2(gas)
Same proces as when you see bubbles leave a carbonated soft drink in your glass.
Your tap water
You can get specifications on the water from your local water supplier. In most countries it is law that these data should be public.
Example from one of the water supplies in Copenhagen:
The Calcium content is 120 mg/l. This is hard water (hardness 22ºdH) but other parts of Copenhagen got as high as 165 mg/l (31ºdH). Water in Norway is much softer: 3-7 mg/l Calcium.
Types of filters
There are different types af water filters (purifiers). Very complex to get hold of exactly how it all works.
Activated carbon filter
Captures large molecules by absorption, like chloride and pesticides. But it does not take things like: Sodium ions, Nitrate, Iron and strong acids.
Traditionally Calcium ions in the water have been exchanged with Sodium ions, but it can be other ions as well. So if you got high Calcium in the ingoing water, you will get high Sodium (=salty taste) in the outgoing water.
Reverse osmosis (RO)
RO is a membrane that holds back a lot of the molecules. The point is to get pure H2O on the other side, but in practice you will get 1-2% of the ingoing water composition in the outgoing.
The pure RO water does not taste good in coffee brewing. So minerals are added. The big advantage is that here you got much more control of the composition.
The first two, activated carbon and ion exchange is typically used together. You can have a stream of water going through it for use. Reverse osmosis is more expensive. It is made under high pressure and takes longer time to produce.