The products of a chemical reaction
A chemical change is the formation of new substances as a result of a reaction taking place with chemical bonds being broken and formed. There are numerous signs that indicate a chemical change taking place.
- Production of bubbles: Indicates that a gas has been formed (new product)
- Change in temperature: Due to absorption or release of heat (endothermic or exothermic reactions)
- Formation of precipitate: two solutions containing ions are mixed
- Production of an odour: indicates formation of a new gas
- Change in colour
Types of chemical change:
- Synthesis: A Synthesis reaction involves the reaction between two or more substances to form a new substance. A + B AB
- Formation of Water: 2H2 (g) + O2 (g) 2H2O (l)
- Formation of Magnesium oxide: 2Mg (s) + O2 (g) 2MgO (s)
- Decomposition: A Decomposition reaction is where one reactant is broken down into two or more integral substances. This reaction occurs due to an input of energy which can be in the form of light, electricity or heat. There are three main types of decomposition reactions which are thermal decomposition, electrolysis and photo decomposition. AB A + B
- Decomposition of distilled water through electrolysis: 2H2O (l) 2H2 (g) O2 (g)
- Decomposition of Calcium Carbonate through thermal decomposition: CaCO3 (s) CaO2 (g) O2 (g)
- Combustion: A Combustion reaction occurs when a substance burns in oxygen at a high temperature, resulting in the formation of new products. There are two types of combustion reactions that take place. These are “Complete Combustion” and “Incomplete Combustion”. Complete combustion occurs when there is a large supply of air. Incomplete Combustion occurs when there is a lack of oxygen present. Hydrocarbon + oxygen carbon dioxide + water C2H5OH (l) 3O2 (g) 2CO2 (g) + 3H2O (l) CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l)
- Precipitation: A precipitation reaction occurs when two solutions react together, resulting in the formation of an insoluble solid, referred to as a precipitate. AB(aq) + CD (aq) AD (s) + CB (aq) BaCl2 (aq) + NaSO4 (aq) BaSO4 (s) +2NaCl (aq) Pb(NO3)2 (aq) 2KI (aq) PbI2 (s) + 2KNO2
- Acid/base reaction: A reaction between an acid and a base is known as neutralisation. A neutralisation reaction is exothermic and results in the formation of salt (ionic compound) and water Acid + Base Salt + Water (H2O) NaOH (aq) + HCl (aq) NaCl (aq) + H2O (l) 2HNO3 (aq) + Na2O (aq) H2O (l) + 2NaNO3 (aq)
- Acid/metal and carbonate reaction: A reaction between acid/metal and carbonate is a special type of neutralisation reaction. In this reaction only, carbon dioxide gas is produced in addition to salt and water. This is due to the carbonate ion breaking down. Acid + Carbonate Salt + Water + Carbon Dioxide 2HCl (aq) + Na2CO3 (s) 2NaCl (aq) + H2O (l) + CO2 (g) 2H+ (aq) + Na2CO3 (s) 2NaCl (aq) + H2O (l) + CO2 (g)
Aboriginal and Torres Strait Islander People have eaten poisonous food items for many years. For example, one such item they eat Is cycad. Cycad is a type of palm that grows in northern Australia. The fruit from this palm is poisonous, yet the indigenous people have been harvesting it over years. The methods they use consists of heating the fruit which results in thermal decomposition process of the toxins. After, they place the fruit in mesh bags and then soak it in water for a few days allowing the toxins to leach out. The mesh bag acts as a sieve.
Similarly, the indigenous people ate Semecarpus australiensis (native cashew). This fruit requires care in harvesting and handling as it is derived from poison ivy. They use heat treatment to detoxify the fruit, followed by roasting the fruit in fire and then cover it in sand to inactivate the toxin. However, the aborigines avoided inhaling the fumes as they were toxic.
The aborigines also eat the seeds of the black bean tree which were toxic but however could be safe to eat after substantial processing. They cracked the seeds, soaked them in water, ground them into a meal, and then made it into a cake which was then roasted. Washing the seeds in water removed the soluble toxins while roasting destroyed the remaining.