Types of Chemical Change or Chemical Reaction - Direct Combination (or Synthesis) Reaction

A combination reaction, also known as a synthesis reaction, occurs when two or more substances chemically combine to form a single new compound. This type of reaction is the opposite of a decomposition reaction, where a compound breaks down into simpler substances.

$$A+B\to AB$$

For example, when magnesium (Mg) burns in oxygen (O₂), it forms magnesium oxide (MgO) as a new product:

Mg + O₂ → MgO

1. Aim: To observe and understand combination reactions, where two or more substances combine chemically to form a single product.

2. Requirements

• Apparatus: test tube, beaker, glass rod, tongs, burner.
• Chemicals: Hydrochloric acid (HCl), ammonia solution (NH₃), magnesium strip (Mg), calcium oxide (CaO), water (H₂O).

3. Procedure

Activity 1: Reaction of Ammonia and Hydrogen Chloride

1. Take a small amount of hydrochloric acid (HCl) in a test tube and heat it.
2. Dip a glass rod in ammonia solution (NH₃) and hold it near the mouth of the test tube.
3. Observe the formation of white smoke due to the reaction of NH₃ and HCl, forming ammonium chloride (NH₄Cl).

NH₃(g) + HCl(g) → NH₄Cl(s)

Activity 2: Burning of Magnesium in Air

1. Hold a magnesium strip with tongs and ignite it.
2. Observe the formation of white ash (magnesium oxide, MgO) due to the reaction between magnesium and oxygen.

2Mg + O₂ → 2MgO

Activity 3: Reaction of Calcium Oxide with Water

1. Take water in a beaker up to half its capacity.
2. Add a few pieces of calcium oxide (CaO) to the water.
3. Observe the formation of calcium hydroxide (Ca(OH)₂) with the release of heat.

CaO + H₂O → Ca(OH)₂ + Heat

4. Conclusion: A combination reaction occurs when two or more reactants chemically combine to form a single product. These reactions can release heat (exothermic) and are commonly observed in various chemical and industrial processes.