Coolant versus water in a car's cooling system

Which should be used in the car in hot climate? Coolant or water?

Many vehicle users do not see the need to use coolant in their vehicles simply because they put in water and their vehicle runs fine simply because they are unaware of the long term negative effect that it will have on their vehicle's cooling system and the engine itself.

Let's see how coolant performs when used and how water performs when used in the cooling system.

Water - This could be either tap, well or borehole water. These types of water contain minerals that cause corrosion to a car's engine block, piping and the core of a radiator. Those minerals can also cause scale formation in the cooling system.
An example of where one can notice scale formation is in the home electric kettle, you will notice a brown scale inside the kettle after some use. That is scale! When water that contains these salts is heated and then evaporates, it leaves behind the salts that forms scale. It is this same phenomenon that occurs in the cooling system of a car.
Thought the use of water is not encouraged, the impurities help to raise the boiling point of the water. It is not good to be used as coolant in the engine because of the damage caused by those minerals as the minerals will eventually block passages, affect heat transfer from the engine block to the coolant. Scales and those mineral buildup can block passages in the radiator specifically since the radiator has narrow passages. It can also make the Thermostat to become stuck closed or stuck open or stock partially closed. This can have minor to severe effect on the engine as a result of overheat. 

Demineralized water/distilled water - This does not contain those minerals that cause corrosion and scale. A cheap source is water that comes out of air conditioning system.
One advantage of using demineralized water over ordinary water is; there won't be scale formation with its use and the water will boil at normal boiling temperature of pure water. Using distilled or demineralized water is not also very good as it is bad for overheat protection because once the temperature reaches about 100 degrees celcius, it begins to boil especially when the cooling system has lost pressure due to faulty radiator cap or leakage somewhere. Some vehicle's normal operating coolant temperature are as high as 105 degrees celcius.

Coolant solution- The major content of coolant solution is mostly ethylene glycol. Its boiling point is high than  that of water and its freezing point is low. This helps to give the engine high temperature protection, easy start and protection in freezing temperatures. Coolant solution also contains corrosion inhibitors that prevents corrosion and it is also used as lubricant for the thermostat and water pump.
As you can see, coolant solution is the best choice among ordinary water and demineralized water.

Again, water has better specific heat capacity than coolant hence, it conducts heat away from your engine better than coolant but coolant has higher boiling point which is desirable. There is a compromise and a balance should be achieved.
Common mixture ratio is 50% coolant and 50% demineralized water.

In a warm/hot climates, it is counter productive to have more coolant than water just because you want to raise boiling point to prevent overheat. Because of relatively high ambient temperature,  a medium for fast heat transfer is needed. It is not good to exceed 70% water, 30% coolant in order not to loose protection of the engine.
Coolant concentrate shouldbe ddiluted with demineralized water only. If common water like tap water is used, the result is a coolant that is already degraded.

Also, that there is coolant concentrate which will have to be diluted and there is pre-mixed which is mostly 50/50 mixture ratio of concentrated coolant and water. Always refer to the container of the coolant you buy for information.

Warning! Never mix coolant of different types or composition and color. If you have to change the coolant that you are using, you must do a thorough radiator flush.

Good quality coolants should also be used to get the best performance and longevity of the cooling system.

When the correct fluid is not properly used, it can shorten the lifespan of the cooling system, the engine and sometimes the transmission. How it can affect the transmission can be seen in this How Poorly Maintained Cooling System Can Damage Your Transmission.