Lab Manual: Types of Reactions | Lab Manuals for Class 10 PDF Download

Objective

Materials Required
Calcium oxide (quicklime), distilled water, borosil beaker, test tube, glass rod, dropper, red and blue litmus paper strips.

Theory

When calcium oxide (quicklime) is dissolved in water, it forms calcium hydroxide (slaked lime). The reaction is highly exothermic, i.e. a lot of heat is produced during the reaction.
This reaction may be represented in the form of a chemical reaction as follows:

Note Calcium hydroxide is basic in nature.
Therefore, it turns moist red litmus paper blue. If we pass CO2 through clear calcium hydroxide solution (lime water), it turns milky due to the formation of a white precipitate (insoluble calcium carbonate).In reaction (i), two compounds-quicklime and water combine to give a single product slaked lime. So, this is an example of combination reaction. Hence, it may be stated that when two or more substances react together to form a single product, is called a combination reaction.
Also, it has been observed that a large amount of heat is evolved alongwith the formation of products. Such type of reactions which are accompanied by the evolution of heat, are called exothermic reactions.

Procedure

• Take a small amount of quicklime in a borosil beaker or hard glass beaker. Slowly add water to it as shown in Fig. 1 (a).
• Stir it well with a clean glass rod as shown in Fig. 1(b).
  
• Observe what happens. Touch the beaker carefully.
• Classify the reaction.
• Using a clean dropper, take a few drops of the solution formed in the beaker and place them on red and blue litmus paper strip (as shown in Fig. 2). Make your observation.

Observation
The hissing sound is produced and solution becomes hot. The heat is evolved during the reaction which raises the temperature of the reaction mixture. On putting a few drops of the solution on red and blue litmus paper, it is observed that only red litmus paper turns blue, no changes occur in blue litmus paper.

Inference
In this reaction, quicklime (CaO) and water (H₂O) combine to give a single product slaked lime [Ca(OH)₂ ].
CaO(s) + H₂O (l) → Ca(OH)₂ (aq) + Heat
It is a combination reaction and calcium hydroxide [Ca(OH)₂ ] is basic in nature.

As heat is evolved, the reaction is exothermic.

Result
The reaction between quicklime and water to form slaked lime is a combination reaction as well as exothermic reaction.

Precautions

• Always take small amount of quicklime.
• The reaction is highly exothermic, so do not touch the reaction mixture directly.
• Water is added slowly to a beaker containing quicklime.
• Quicklime can cause severe burns, therefore, it should be handled with spatula.
• Use good quality glassware, while adding water to quicklime. This is because, the reaction is highly exothermic and the beaker if made of ordinary glass, can crack.