🔍 Key Concepts

• waterside scale vs sludge in boilers • difference between hardness salts (calcium/magnesium) and treatment chemicals (sodium compounds) • purpose of phosphate treatment in boiler water chemistry

💭 Think About

• Which of these chemicals is normally present in raw hard water and tends to form hard, adherent deposits when heated? • Which of the options are actually added intentionally to boiler water to prevent hard scale and convert it to a soft, removable sludge? • Think about which ion (calcium, magnesium, or sodium) is the classic source of "hard" scale inside tubes.

✅ Before You Answer

• Identify which compound represents permanent hardness that produces a hard, adherent scale on heating. • Identify which compounds are commonly used as boiler water treatment chemicals rather than scale causes. • Confirm which choice is associated with soft, easily removable sludge instead of hard, tenacious waterside scale.