cbse science

SCIENCE CLASS10 TEST L – 1 [chemical reactions &equations] Q1.What do u observe when Lead nitrate &Potassium iodide are mixed in a test tube? . Ans- When Pb[NO3]2 IS MIXED WITH KI ,a yellow colour precipitate of Pb I2 [Lead iodide is formed.This reaction is an example of double displacement reaction. What happens when Ferrous sulphate is heated?Do u find any change in colour of ferrous sulphate after heating? Ans—WHEN FeSO4 is heated, its green colour changes.It decompose to ferric oxide .sulphur dioxide,& sulphur tri oxide . Q3.Explain your observation in following chemical a.When Silver chloride is exposed to sunlight? Ans –Silver chloride decompose to give silver[Ag] &chlorine[Cl]. b.When water is added to quick lime? Quicklime reacts with water to produce slakedlime [Ca [OH]2.In this reaction large amount of heat is realeased. It is an example of combination of reaction. .Q4.Write an example of double displacement reaction. When sodium sulphate [Na2SO4] IS mixed with bariun chloride[BaCl2] a white substance Soluble in water is formed.White precipitate is of barium sulphate.this reaction is an Example of double displacement. Q5.Write observations which help us to know that chemical reaction has taken place. Ans Change in state,change in colour, evolution of a gas ,change in temp help us to know Whether chemical reaction has taken place. Q6.Why decomposition of vegetable matter is an exothermic reaction? Ans –During decomposition of vegetable matter heat is released in the form of CO2 & methane,so it is an exothermic reaction. Q7.Why the iron nail become brownish when dipped in copper sulphate solution? Ans- Iron nail turns brown due to coating of copper on it which is displaced by iron. CuSO4 +Fe =Feso4+Cu Q8.Why copper powder become black when heated in the presence of air?Write chemical equation. Ans –The surface of copper powder becomes black due to formation of copperoxide. Cu+O2 = CuO. Q9.Why copper gets green coating when exposed to air? Ans- Copper when exposed to air reacts with carbondioxide to form basic copper Carbonate , which is green in colour. Q10.What are anti oxidants. Ans- In food industry a special type of substances are added to prevent the process of oxidation in food containing oil &fat . These substances are called antioxidants. Q11.Why is double amount of gas collected in on of the test tube on electrolysis of water? Ans-On electrolysis of water it decompose to give & oxygen in the ratio of 2:1 by volume.The double volume of gas collected is hydrogen. Q11.Identify the substance oxidised & reduced in the following reaction. 4Na+O2 = 2Na2O. Ans-In this reaction Na has gained oxygen so oxidized to Na 2O.Oxygen is redused. Q12.Why do we paint iron articles? Ans-Iron articles are protected from rusting by applying paints,because paint does not allow moist air to come in contact with iron. Q13.Hydrogen per oxide is kept in coloured bottles .Why?x ANS-This done to prevent decomposition of H2O2.It decomposes in the presence of sunlight in hydrogen & oxygen. Q14.What is an oxdising & reducing agent? Ans-A substance which gives oxygen is called oxidizing agent& a substance which gains oxygen is called reducing agent. Q15.What type of reaction is digestion of food in our body? Ans-It is an example of decomposition reaction.Starch decompose in to simple sugars & proteins into amino acids in the presense of an enzymes. Q16.Write two examples of redox reactions from our daily life. Ans-a.Rusting of iron where iron is oxdised to iron oxide b.Silver jewellary loose its shine due to oxidation of silver in to silver oxide. Q17.Why is hydrogen placed in activity series of metals. Ans-This is because hydrogen loose electrons like metals &form positive ions. Q18.Why does zinc react with dil sulphuric acid to give hydrogen but copper does not? Ans-Zinc is more reactive than hydrogen where as copper is less reactive than hydrogen . Q19.Write chemical formula of rust. Ans-Rust is hydrated ferric oxide.Fe2o3.XH2O. Q20.Why a stale food gives a bad smell& taste bad? Ans-This is due tooxidation of fats present in the foods forming compounds which have bad smell & taste.