NCERT Solution for Class 10 Science Chapter 1
Chemical Reactions and Equations
Exercises Questions
Page No-16
Questions No-20
(a) Corrosion
(b) Rancidity
Explain the following terms with one example each.
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NCERT Solution for Class 10 Science Chapter 1
Chemical Reactions and Equations
Exercises Questions
Page No-16
Questions No-20
(a) Corrosion
(b) Rancidity
(a) Corrosion: Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.
• Iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide (a brown flaky substance) called rust.
• Silver articles become black after some time when exposed to air. This is because it reacts with sulphur in the air to form a coating of silver sulphide.
• Copper reacts with moist carbon dioxide in the air and slowly loses its shiny brown surface and gains a green coat. This green substance is copper carbonate.
(b) Rancidity: The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.
For example, the taste and smell of butter changes when kept for long. Rancidity can be avoided by:
• Storing food in air tight containers
• Adding antioxidants
• Storing food in an environment of nitrogen
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Corrosion: Metal are reacted with atmosphere oxygen, carbon dioxide and moisture and converted into metal oxides and carbonate. This phenomenon is called corrosion.
Eg.:
(i) Black coating on silver.
4Ag + 2H2S + O2 ———–> 2Ag2S + 2H2O
(ii) Green coating on copper.
Cu + CO2 + H2O ———–> CuCO3. Cu (OH)2
(b) Rancidity: Oil and fat containing items when comes in contact with atmosphere oxygen they get oxidised and start smelling and tasting bad. The food item have gone rancid. This phenomeon is called rancidity.
Eg.:
(i) Smell and taste on food items get change after some times.
(a) Corrosion – Metals react with oxygen which is present in the atmospheric moisture. This leads to the formation of metal oxides. In due course of time, the metal keeps on changing into its oxide and finally the whole metal is lost due to oxidation. This process is called corrosion.
Example: Iron articles; like iron gates or bridges tend to rust because of oxidation by atmospheric moisture. We know that rust is nothing but iron oxide. Conversion of iron into rust leads to corrosion of the iron articles. Due to this, the iron articles weaken and finally wither away.
(b) Rancidity – When fats and oils are oxidised, their smell and taste change. This process is called rancidity. Oily food often become rancid and start giving out obnoxious smell. The taste also becomes bad. Such oily food is not fit for eating.
Example: When packets of potato chips or other oily snacks are kept open for a long time; their taste and smell become bad. The oily food is no longer safe to eat.
Certainly, I’ll explain each term with an example:
Oxidation:
Definition: Oxidation is a chemical reaction in which a substance loses electrons, typically to oxygen, resulting in an increase in the substance’s oxidation state.
Example: The rusting of iron is a common example of oxidation. When iron (Fe) reacts with oxygen (O2) in the presence of water (H2O), it forms iron oxide (Fe2O3), commonly known as rust:
4Fe(s) + 3O2(g) + 6H2O(l) → 4Fe2O3·H2O(s)
Reduction:
Definition: Reduction is the opposite of oxidation; it involves a chemical reaction in which a substance gains electrons, typically from another substance, leading to a decrease in its oxidation state.
Example: The electrolysis of water to produce hydrogen gas (H2) and oxygen gas (O2) is an example of reduction. In this process, water (H2O) is reduced to form hydrogen gas and oxygen gas by the addition of electrons:
2H2O(l) → 2H2(g) + O2(g)
Catalyst:
Definition: A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the reaction itself. It provides an alternative pathway with lower activation energy for the reaction to occur.
Example: Enzymes in biological systems are excellent examples of catalysts. For instance, the enzyme catalase accelerates the decomposition of hydrogen peroxide (H2O2) into water (H2O) and oxygen (O2) without being consumed in the process:
2H2O2(aq) → 2H2O(l) + O2(g)
Sublimation:
Definition: Sublimation is the transition of a substance from the solid phase directly to the gas phase without passing through the liquid phase. It occurs when the substance’s vapor pressure exceeds its solid phase’s pressure.
Example: An example of sublimation is the transition of dry ice (solid carbon dioxide, CO2) into gaseous carbon dioxide without melting. Dry ice goes from a solid directly to a gas when it is heated, and this process is used for creating special effects in the entertainment industry and for refrigeration.
Precipitation:
Definition: Precipitation refers to the formation of a solid (precipitate) from a solution as a result of a chemical reaction, typically a double displacement reaction, where two soluble compounds react to form an insoluble product.
Example: The mixing of silver nitrate (AgNO3) and sodium chloride (NaCl) solutions, as mentioned earlier, leads to the precipitation of silver chloride (AgCl) as a solid in a double displacement reaction:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)