FRANK Solutions for Class 9 Chemistry Chapter 5 - Physical and chemical changes
Chapter 5 - Physical and chemical changes Exercise 74
(a) A change, whether physical or chemical is always permanent.
(b) Photosynthesis in plants is a physical change.
(c) The mass of a substance does not change on burning.
(d) Carbohydrate burn in air to give carbon and water.
(e) A chemical change is always accompanied by the evolution or absorption of heat.
(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.
(a) Air is passed over white hot coke.
(b) Steam is passed over white hot coke.
(b) Endothermic reaction
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.
(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.
Chapter - Exercise
Chapter 5 - Physical and chemical changes Exercise 75
(a) A reaction which gives out heat.
(b) A reversible reaction.
(c) A reaction with a solid and a gas which produces heat and light.
(d) A reaction which takes place with the help of sunlight.
(e) A reaction which is brought about by an electric current.
Silver nitrate, Copper (II)nitrate, Aluminium hydroxide, Silver carbonate and Potassium nitrate
(b) Silver in silver oxide is reduced to silver while oxygen in hydrogen peroxide is oxidised to molecular oxygen.
(a) Gold is found free(uncombined )in nature.
(b) Compounds of sodium do not decompose on heating.
(c) Silver articles turn black on prolonged exposure to air.
(d) When ammonium nitrate is dissolved in water contained in a beaker, the beaker becomes cold.
(e) When magnesium is dissolved in hydrochloric acid contained in a beaker ,the beaker becomes hot.
(a) Dissolving common salt in water is an example of ______ change.
(b) Dissolving zinc in dil. H2SO4 is an example of ______ change.
(c) A ______ change is one in which one or more different substances are formed.
(d) A ______ change can easily be reversed.
(i) Burning of wood-carbon get oxidized and oxygen gets reduced.
(ii) Rusting-In it iron is oxidized.
(a) An oxidizing agent not containing oxygen.
(b) A substance which acts as both oxidizing and reducing agent.
(c) A carbonate stable towards heat.
(d) A compound decomposed by light.
(e) A positive catalyst.
(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.
(b) Name two substances other than oxygen that support combustion.
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-
(a) release CO2 into the atmosphere.
(b) remove CO2 from the atmosphere.
(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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