FRANK Solutions for Class 9 Chemistry Chapter 5 - Physical and chemical changes
Chapter 5 - Physical and chemical changes Exercise 74
(i) The substance to be burnt must be combustible.
(ii) A supporter of combustion such as air or oxygen must be present.
(iii) A combustible substance must be heated to its ignition temperature.
(i) One or more new substance is formed during reaction.
(ii) The change occurring during the reaction is permanent.
(iii) The mass of the substance undergoing a chemical change is generally altered.
(iv) Chemical change involves making and breaking of bond.
Decomposition of silver nitrate takes place in the presence of light.
Experiment - A crucible is weighed containing about 0.5 gm of magnesium.Now crucible is heated.When magnesium begins to burn,the lid is put back on the crucible and the lid is occasionally raised to allow air to enter and burn the magnesium such that no product is lost..When,all the magnesium has been burnt up, the crucible is allowed to cool and then on weighing it we observe that there is gain in weight.
(b) Endothermic reaction
Chapter - Exercise
Chapter 5 - Physical and chemical changes Exercise 75
(b) Silver in silver oxide is reduced to silver while oxygen in hydrogen peroxide is oxidised to molecular oxygen.
(i) Burning of wood-carbon get oxidized and oxygen gets reduced.
(ii) Rusting-In it iron is oxidized.
(b) Hydrogen peroxide
(c) Barium carbonate.
(d) Silver nitrate.
(e) Manganese dioxide.
A combustible substance must be heated to its ignition temperature for burning.
(i) The change is temporary and reversible.
(ii) No new substance is formed and the chemical composition of the original substance remains the same.
(iii) Mass of the substance remains unchanged
(iv) The amount of energy required to bring about a physical change is generally equal to the amount of energy required to reverse the change. Hence,there is no net energy change involved.
Non-combustible substances-Substance which cannot burn in air or oxygen are called as non- combustible substances. Ex-Nitrogen gas, carbon dioxide etc.
(b) Two substances other than oxygen that support combustion are-
(ii) Respiration releases carbon dioxide and water vapours.
(b) (i) Photosynthesis removes CO2 from the atmosphere. Plants take carbon dioxide from the atmosphere in the presence of sunlight and use it to synthesise glucose with the liberation of oxygen.
(ii) Some man made chemical activities such as setting of mortar also use atmospheric carbon dioxide and helps in removing carbon dioxide.
If some powdered sulphur is heated gently in a glass test tube, it melts to a pale yellow liquid. Flame is removed to stop heating, it is quickly changed back to solid sulphur.
Chapter 5 - Physical and chemical changes Exercise 76
In displacement reactions, a more reactive element (metal or non-metal) displaces a lesser reactive element from its compound. With the help of the activity series, it is possible to predict which metals will displace other metals from their solutions.
Carbon dioxide returns to the atmosphere by respiration, combustion of fossil fuels like coal, wood, petroleum etc., weathering of rocks, volcanic eruptions etc.
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