Expected short questions of Chemistry 9th Class in 2020 CHAPTER 1

Question#1: Differentiate homogeneous and heterogeneous mixture?

Homogeneous mixture:

The mixture having uniform composition throughout is called homogeneous mixture.

For Example:  Ice cream, gasoline and air.

Heterogeneous mixture: 

The mixture which does not have uniform composition throughout is called heterogeneous mixture.

For example: Rock, soil and wood.

Question#2:  What is relative atomic mass?  How it is related to gram?

 The relative atomic mass of an element is the average of mass of an atom of element as compared to 1/12th the mass of an atom of carbon 12 isotope.  The symbol of atomic mass unit is ‘amu’. When this atomic mass unit is expressed in grams it is,

1amu= 1.66×10−24g.

Question#3:  Define empirical formula with an example?

Empirical formula is the formula that represents the simplest whole number ratio of atoms present in a compound. 

Example: Formula of glucose is C6H12O6.  Simplest ratio of carbon, hydrogen and oxygen is 1:2:1.  Hence empirical formula of benzene is CH2O.

Question#4:  State three reasons why do you think that air is a mixture and water is compound?

  • Air is formed by simple mixing of nitrogen, oxygen, carbon dioxide and noble gases.  Whereas water is formed by the chemical combination of hydrogen and oxygen.
  • Air has no practical chemical formula whereas water has a chemical formula
  • In air the components retain their chemical properties whereas in water components lose their properties. 

Question#5:  Explain why hydrogen and oxygen considered elements while water as a compound?

Hydrogen and oxygen are considered elements because:

  • Each one consists of same type of atoms.
  • Each one has same atomic number
  • Each one cannot split into two or more simpler substances by simple chemical methods

Water is a compound because

  • It consists of different types of atoms combined together in a fixed proportion by mass
  • Properties of water are entirely different from its constituent components.  For example water exists as liquid while hydrogen and oxygen exist as gases at room temperature
  • Water can be split up into its constituent components by passing electric current through it

Question#6: State reasons soft drink is a mixture and water is a compound?

Soft drink is a mixture because:

  • Its components are not bounded chemically
  • The components do not have fixed ratio by mass
  • The components retain their physical and chemical properties

Water is a compound because:

  • Its components are bounded chemically
  • The components have fixed ratio by mass
  • The components lose their chemical and physical properties

Question#7:  Define atomic mass unit?

1/12th of the mass of one atom of carbon 12 is called atomic mass unit.  The symbol of atomic mass unit is ‘amu’.  It is the unit used to compare masses of atoms.

Question#8: Differentiate molecular mass and formula mass?

Molecular mass:

“The sum of atomic masses of all atoms present in one molecule of a molecular substance is called molecular mass”

Formula mass:

The sum of atomic masses of all the atoms present in one formula unit is called formula unit

Question#9:  In which of the following number of hydrogen atom is more 2 moles of HCl or 1 mole of NH3?

1 mole of HCl contains one mole hydrogen atoms.  2 mole of HCl contains 2 moles of hydrogen atoms, while one mole of NH3 contains 3 moles of hydrogen atom.  Hence 1 mole of NH3 contains more number of hydrogen atoms.

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