State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which include
(a) Colour change
(b) Effervescence or gas evolved
(c) Evolution or absorption of heat
(d) Formation of a precipitate
With reference to each of the above indications, state the external indication seen during
(i) Addition of dilute acid to an active metal
(ii) Addition of dilute hydrochloric acid to silver nitrate
(iii) Addition of water to quicklime
(iv) Thermal decomposition of mercury (II) oxide

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Solution

Chemical reaction
A chemical reaction is a process in which one or more substances, also called reactants, are converted to one or more different substances, known as products.
(i) Addition of dilute acid to an active metal
When dilute acid is added to an active metal, hydrogen gas is evolved and the test tube or flask becomes hot and heat is produced.
$2 Na (s) + 2 HCl (aq) \to 2 NaCl (s) + H_{2} (g)$
(ii) Addition of dilute hydrochloric acid to silver nitrate
When dilute hydrochloric acid is added to silver nitrate, it produces silver chloride a white precipitate and nitric acid.
$HCl (aq) + {AgNO}_{3} (s) \to AgCl (s) + {HNO}_{3} (aq)$
(iii) Addition of water to quicklime
When quicklime is added to water, it forms slaked lime along with the evolution of heat. There will be a rise in the temperature and a hissing sound accompanied by a lot of heat.
$CaO (s) + H_{2} O (l) \to Ca {(OH)}_{2} (s)$ + Heat
(iv) Thermal decomposition of mercury (II) oxide
When mercury(II) oxide is heated, it decomposes into mercury metal and oxygen gas. The red colour of mercury(II) oxide changes to silvery.
$2 HgO (s) \overset{∆}{\to} 2 Hg (l) + O_{2} (g)$

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(a) State the various characteristics of chemical reactions.

(b) State one characteristic each of the chemical reaction which takes place when :
(i) dilute hydrochloric acid is added to sodium carbonate

(ii) lemon juice is added gradually to potassium permanganate solution

(iii) dilute sulphuric acid is added to barium chlorid solution

(iv) quicklime is treated with water

(v) wax is burned in the form of a candle

Q. Question 3
Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

Q. When metal X is treated with a dilute acid Y, then a gas Z is evolved which burns readily by making a little explosion.
(a) Name any two metals which can behave like metal X.
(b) Name any two acids which can behave like acid Y.
(c) Name the gas Z.
(d) Is the gas Z lighter than or heavier than air?
(e) Is the reaction between metal X and dilute acid Y, exothermic or endothermic?
(f) By taking a specific example of metal X and dilute acid Y, write a balanced chemical equation for the reaction which takes place. Also indicate physical states of all the reactants and products.

State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which includes 1.colour change 2.effervesence or gas evolved 3. Evolution or absorption of heat 4. Formation of precipitate. With reference to each of the above indications, State the external indications seen during - 1. Addition of dilute acid to an active metal 2. Addition of dilute hydrochloric acid to silver nitrate. 3. Addition of water to quicklime. 4. Thermal decomposition of mercury(ll) oxide

State why addition of sodium chloride to water is considered a physical change,while addition of dilute sulphuric acid to zinc is considered as chemical change.

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