🔍 Key Concepts

• Boiler water scale vs. corrosion – know the difference in cause and appearance • Common hardness compounds in water treatment and how they behave when heated in boilers • How boiler water treatment chemicals (like phosphates) are supposed to work

💭 Think About

• Ask yourself: which substance is most directly responsible for hard, insulating deposits when boiler water is heated and evaporated? • Think about which option describes something that normally prevents scale rather than causing it. • Consider the typical minerals in ‘hard’ feed water and what they do inside a high‑pressure boiler when temperature and concentration increase.

✅ Before You Answer

• Identify which choice is associated with water hardness (a classic source of scale). • Separate in your mind: which options are mainly linked to corrosion (pitting, rusting) rather than solid scale buildup? • Recall that some chemicals are added specifically to control hardness and keep it in suspension, not to form hard scale on tubes.