What happens
Quicklime is calcium oxide. When water is added it reacts to form calcium hydroxide, called slaked lime, and a large amount of energy is released.
CaO(s) + H2O(l) → Ca(OH)2(aq) + heat
NCERT writes calcium hydroxide as (aq). Strictly it is only slightly soluble, so most of it is a white solid suspended in water.
Observations to write: hissing and cracking, the lumps swell and crumble to a white paste, steam forms, the beaker becomes hot, the liquid turns milky.
A combination reaction
In a combination reaction two or more substances join to form a single product. Calcium oxide and water give one product, calcium hydroxide.
C(s) + O2(g) → CO2(g)
2H2(g) + O2(g) → 2H2O(l)
2Mg(s) + O2(g) → 2MgO(s)
More combination reactions, including magnesium burning in NCERT activity 1.1.
An exothermic reaction
Energy is transferred to the surroundings, so the temperature rises. This reaction releases about 64 kJ for every mole of calcium oxide, which is about 1.1 kJ for every gram.
- 20 g of quicklime releases about 23 kJ.
- Heating 25 mL of water from 28 °C to 100 °C needs only about 7.5 kJ.
- So with a small volume of water the mixture boils, steam forms and some water is lost.
- More quicklime in the same water gives a larger temperature rise. More water gives a smaller one.
Why the lumps crack and crumble
Water reacts at the surface of each lump and forms calcium hydroxide. Calcium hydroxide takes up about twice the volume of the calcium oxide it came from, so the lump swells, cracks and breaks into a powder or paste. This process is called slaking.
Slaked lime, limewater and milk of lime
- Calcium hydroxide dissolves only slightly in water, about 1.7 g in a litre at 20 °C.
- The clear saturated solution is limewater. It turns milky when carbon dioxide is bubbled through it, because calcium carbonate forms.
- Undissolved calcium hydroxide in water makes a white suspension called milk of lime.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
The solution is alkaline
Calcium hydroxide releases hydroxide ions, OH−, so the solution is alkaline with a pH of about 12.
- Red litmus turns blue.
- Phenolphthalein turns pink.
Uses of calcium hydroxide
- Whitewashing walls. It slowly reacts with carbon dioxide in the air and leaves a shiny layer of calcium carbonate after two to three days.
- Mortar and plaster in building.
- Treating acidic soil, because it neutralises acid.
- Neutralising acidic waste water and flue gases.
Safety
- Quicklime is caustic. It reacts with moisture on skin and in eyes. Wear goggles and gloves.
- Add water slowly and in small amounts. The mixture can boil and spit hot liquid.
- Do not touch a hot beaker. Read the thermometer instead.
- Never seal a container of quicklime and water.
Write it like the mark scheme
- Say “exothermic” and link it to the temperature rise.
- Name the product: calcium hydroxide. Give the formula Ca(OH)2.
- Name the reaction type: combination.
- Describe the observations in order: hissing, lumps crack, steam, white suspension, beaker hot.
Syllabus links
CBSE class 10 science, chapter 1, activity 1.4 and the section on combination reactions, then the whitewashing example. IGCSE 0620: types of reaction, exothermic and endothermic reactions, acids, bases and salts, and uses of lime.