1. Combustion Reactions

A combustion reaction occurs when a substance reacts with oxygen to produce heat and light energy. This is commonly seen in burning fuel.

General form: $\text{Fuel} + \text{O}_2(g) \rightarrow \text{CO}_2(g) + \text{H}_2\text{O}(g) + \text{Energy}$

Burning of Fuel (Charcoal/Coal)

In Tanzania, burning charcoal for cooking is a common example of a combustion reaction. When carbon in charcoal burns:

$\text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g) + \text{Energy}$

For methane (natural gas used in gas stoves):

$\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l) + \text{Energy}$

The energy released is used for cooking food.

2. Decomposition Reactions

A decomposition reaction occurs when a compound breaks down into simpler substances. This is the opposite of combination.

General form: $\text{AB} \rightarrow \text{A} + \text{B}$

Digestion in Living Organisms

Digestion is a biological decomposition reaction where complex food molecules are broken down into simpler substances by enzymes. This process occurs in the stomach and intestines.

Example: Breakdown of starch

$(\text{C}_6\text{H}_{10}\text{O}_5)_n + n\text{H}_2\text{O} \xrightarrow{\text{amylase}} n(\text{C}_6\text{H}_{12}\text{O}_6)$

Starch (complex sugar) + Water → Glucose (simple sugar)

The glucose produced provides energy for the body through cellular respiration:

$\text{C}_6\text{H}_{12}\text{O}_6(aq) + 6\text{O}_2(g) \rightarrow 6\text{CO}_2(g) + 6\text{H}_2\text{O}(l) + \text{Energy}$

This is also a combustion reaction (slow combustion) that happens inside our bodies.

3. Other Common Reactions

Classifying Reactions in Daily Life

Question: A student observes that bread becomes softer when chewed in the mouth for a long time. Explain the type of chemical reaction involved.

Answer:

• Saliva contains the enzyme amylase, which breaks down starch (a complex carbohydrate) in bread into simpler sugars (maltose/glucose).
• This is a decomposition reaction because one compound (starch) breaks down into simpler substances (sugars).
• The overall process of extracting energy from food also involves combustion reactions when glucose reacts with oxygen in cellular respiration.

• Combustion reactions involve burning fuel with oxygen to produce carbon dioxide, water, and energy. This is seen in cooking with charcoal, gas stoves, and candles.
• Decomposition reactions involve breaking down complex substances into simpler ones. This is seen in food digestion, where enzymes break down carbohydrates, proteins, and fats.
• Cellular respiration is a slow combustion reaction that occurs in living organisms, converting glucose and oxygen into carbon dioxide, water, and energy.
• Different types of chemical reactions play important roles in everyday life processes.

In Tanzanian daily life, these chemical reactions are especially important for cooking and health. When you light a charcoal jiko for cooking nyama choma, the carbon in charcoal undergoes a combustion reaction with oxygen from the air, releasing heat that cooks the food. After eating, your body digests food through decomposition reactions where enzymes break down complex food molecules into simpler substances that your body can absorb and use for energy. Understanding these reactions helps explain why proper cooking and balanced diet are essential for daily life.