this is a nice pictoral, now all you who have graduated SSC with science usually had access to the physics lab can relate to what I write.
there is a phenomenon which happens in liquids which is called cavitation, its an imploding bubble which has enough energy to physically erode metal. Engine coolants have additives in it to prevent this very exact thing, If you take out your water pump and have a look you may find little round craters in the backplate - this is caused by cavitation. You might consider "mehhh - 1500 rupay ka to hay sirf".
This statement can be ok if you own a POS mehran which was design and built by lazy stoned overworked labour, But if you have an engine like saad mentions backing a factory run - cavitation in that engine usually costs about Rs ten million and above, if the cylinder liners also give in. It can in a diesel engine - specially near the cylinder head/block junction. If you ever wonder why your mehran engine block coolant holes are corroded to hell and widened up - it was cavitation foaming at work.
coolants are of many types, e.g. GM use the dexcool coolant (like caltex kinoo coolant) which actually does a darn good job of scrubbing out rust and depositing it neatly in the expansion tank. Nice for systems which are 100% liquid. take MB, BMW, etc. they are EU cars and EU has water so damn hard that you need to chew it before drinking, soap forgets how to soap too. They use BASF glysantin for their applications. It is a very good HOAT coolant that does a very good job against cavitation protection, lubrication of pump bearings (its actually slick) and keeping off rust - It cannot remove rust though, for that you need to acid wash the system. It also contains some silicates (needed in diesels) that is why those manufacturers have a packet of some fumed silica in their built in surge tanks to keep adding the required dose.
Your average andaa omellete japanese car uses HOAT coolants with phosphated salts - they are quite picky on this - mostly it has to do with the plastics and metals in the unit.
SO - please everyone who lives in Pakistan and reads this - do not run your car on plain water cooling system, if you do then dont complain that you are paying heavy for maintenance etc. You yourself are inflicting this pain. Use 50% coolant conc. mixed with distilled water only. Make sure the system is top notch.
If your mechanic is so poor he cannot afford a drain tub and a gallon bottle, get your own drain tub and bottle, if he has to drain coolant - make sure he puts it back in the engine, Not let it trickle out onto the ground then proceed to fill with sea water.