CBSE Class 10 Science CHEMICAL REACTIONS AND EQUATIONS - Board Questions (2008-2014) and Answers

CHEMICAL REACTIONS AND EQUATIONS

Board Questions (2008-2014) and Answers

Q1(2008): What happens chemically when quick lime is added to water?

Answer: Quick lime is CaO which reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

                           CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Q2: A student burns magnesium ribbon in the lab and observes a white powder falling on the table. What type of reaction has taken place and name the white powder?

Answer: It is a chemical reaction. The white powder formed is magnesium oxide (MgO).

Q3: What are the essentials of a chemical reaction?

Answer:

• It should be balanced and confirm to the law of conservation of matter
• It should be molecular
• It must refer to an actual chemical equation

Q4: How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal?

Answer: When a burning match stick is brought near the mouth of the test tube in which reaction is taking place, it will burn with popping sound. It detects the presence of hydrogen (H₂) gas.

Q5: What change in colour is observed when white silver chloride is left exposed to sunlight? What type of chemical reaction is this?

Answer: White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.It is a photo-chemical decomposition reaction.

                2AgCl(s) —^Sunlight—→ 2Ag(s) + Cl₂(g)

Q6: Why is respiration considered as exothermic process?

Answer: In respiration heat is released so it is termed as an exothermic reaction.

                   C₆H₁₂O₆ + 6O₂ —→ 6CO₂ + 6H₂O + energy

Q7(2010):What is meant by skeletal equation?

Answer: The equation which is unbalanced because the mass is not the same on both sides of the equation, is a skeletal chemical equation for a reaction.

During exam, try to write chemical equations and chemical formulae in your answers wherever possible. It enhances visibility and gives you more scores.

Q8: Identify the type of reaction in the following example:

Na₂SO₄(aq) + BaCl₂(aq) —→ BaSO₄(s) + 2NaCl(aq)

Answer: A double displacement reaction.

Q9: Why are bags of chips flushed with nitrogen gas?

Answer: Chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidised.

Q10 : What is precipitation reaction?

Answer: When two ionic compounds in aqueous form react to form two different compounds, such that one of the products formed is insoluble in water (called precipitate). Such reaction is called precipitation reaction.

CuSO₄(aq) + Na₂CO₃(aq) ? Na₂SO₄ + CuCO₃ (ppt)

Q11: Name a reducing agent that may be used to obtain manganese from manganese dioxide.

Answer: Coke or carbon is used as reducing agent.

Q12(2010): What is thermite reaction?

Answer: A thermite reaction is a highly exothermic reaction. e.g. When iron oxide burns with aluminium, it gives iron in molten state and generates great deal of heat.

Q13: When zinc metal reacts with dilute hydrochloric acid then zinc chloride is formed and a gas is released. Can you identify the gas? The conical flask becomes hot, it shows which type of reaction it is? Write the balanced chemical equation.

Answer: Hydrogen gas is released. It is an exothermic reaction.
                  Zn(s) + 2HCl —→ ZnCl₂(aq) + H₂(g) + Δ(Heat)

Be crisp and to the point in very short questions.

Q14: What is the brown coloured gas evolved when lead nitrate crystals are heated in a dry test tube?

Answer: Nitrogen Oxide (NO₂)

Q15: Why do we apply paint on iron articles?

Answer: Iron articles when kept in open air, they get corroded in the contact of oxygen and moisture. It is called rusting. Applying paint prevents iron articles from rusting, because the coated layer surface does not come in contact with oxygen and moisture.

Q16(2008): Identify the type of reaction in the following example:

Fe(s) + CuSO₄(aq) —→ FeSO₄(aq) + Cu(s)

Answer: It is displacement reaction. Here Fe (Iron) displaces Cu (Copper) from CuSO₄.

Q17(2014): A metal 'X ' left in moist air for a longer time, loses its shiny brown surface and gained a green coat. Why has this happened? Name and give chemical formula of this given coloured compound and identify the metal. List two ways to prevent this process.

Answer: The metal 'X' is Copper. It is due to the corrosion of copper with carbon dioxide and moisture present in air. It form a green coating of basic copper carbonate (CuCO3.Cu(OH)₂). It can be prevented by 

   (a) applying paint on copper surface or 

   (b) alloying it with other metals like zinc, tin etc.

Q18: Marble statues often slowly get corroded when kept in open for a long time. Why doe this happen?

Answer: Marble (CaCO₃) reacts with oxides of nitrogen and sulphur present in air. It forms Calcium Sulphate and Calcium nitrate which gives it dull and corroded appearance.

SO₃ + H₂O —→ H₂SO₄

2NO₂ + H₂O —→ 2HNO₃

CaCO₃ + H₂SO₄ —→ CaSO₄ + H₂O + CO₂

CaCO₃ + HNO₃ —→ Ca(NO₃)₂ + H₂O + CO₂

Q19: What is the significance of a chemical equation?
                         OR
  
What information does a chemical equation convey? 

Answer: A chemical equation has both qualitative and quantitative significance.

1. It represents the names of products and  reactants (qualitative importance)
2. It tells relative number of molecules (or atoms or moles or masses) of products and reactants.
3. It represents relative volumes of gaseous products and reactants.

_☛ Ch 1 - Chemical Reactions & Equations  (Q &A)

☛ Ch 1 - Chemical Reactions & Equations (MCQs)