UNIT-9

                            Unit-9 [MATTER]

                                                   Exercise

Q.1 Fill in the blanks.

a. Density of a substance depends on mass and volume.
b. The substance which has less density than water floats in water.
c. A sinking body has more density than water.
d. The mass of an electron is 1/1837 amu.
e. Potassium is represented by symbol K.
f. Rusting of iron is chemical change.

Q.2 Match the following.

Name of Atom                    Atomic number

Potassium                             19
Helium                                   2
Boron                                     5
Sulphur                                16
Silicon                                 14
Oxygen                                  8



4. Differentiate:

a. Chemical Change and Physical Change

Chemical Change Physical Change
Produces a new substance with different properties. No new substance is formed, only changes in form.
Irreversible under normal conditions.Usually reversible.
Example: Rusting of iron. Example: Melting of ice.

b. Element and Compound

ElementCompound
A pure substance made up of only one type of atom.A pure substance made up of two or more elements chemically combined.
Example: Oxygen (O).Example: Water (H₂O).


c. Density and Relative Density

DensityRelative Density
Mass per unit volume of a substance (kg/m³).Ratio of density of a substance to the density of water.
Has units (e.g., kg/m³).Dimensionless (no units).

5. Give Reasons:

a. Egg sinks in pure water but floats in salty water.
The density of pure water is lower than that of the egg, causing the egg to sink. However, salty water has a higher density because of the dissolved salt, making the density of the water greater than that of the egg. As a result, the egg floats in salty water.

b. An atom is electrically neutral.
An atom is electrically neutral because the number of protons (positively charged particles) in the nucleus is equal to the number of electrons (negatively charged particles) surrounding the nucleus. Their charges balance each other, resulting in no net charge.

c. Breaking down of a lump of salt into fine powder is a physical change.
Breaking a lump of salt into fine powder is a physical change because the chemical composition of salt remains the same. Only the size and shape of the salt particles change, and the process is reversible.

d. The process of formation of water from hydrogen and oxygen is a chemical change.
When hydrogen and oxygen combine to form water, a chemical reaction occurs, creating a new substance (H₂O) with properties different from the original gases. This process is irreversible and involves bond formation, making it a chemical change.

e. Regular painting is necessary in iron frames where it is not necessary for aluminum frames.
Iron easily reacts with moisture and oxygen in the air, leading to rusting (a chemical change). Painting acts as a protective layer, preventing exposure to air and water. Aluminum, on the other hand, forms a natural oxide layer that protects it from further corrosion, making regular painting unnecessary.


6. Answer the Following Questions:

a. Draw a structure of an atom showing proton, electron, and neutron:
An atom consists of:

  • Protons (positively charged particles) located in the nucleus.
  • Neutrons (neutral particles) also in the nucleus.
  • Electrons (negatively charged particles) revolving around the nucleus in shells or orbitals.



b. What is density? What is the mass of aluminum with volume 2 m³? (Density of aluminum = 2700 kg/m³):

  • Density is the mass per unit volume of a substance. It is expressed as:

    Density(ρ)=Mass(m)Volume(V)\text{Density} (\rho) = \frac{\text{Mass} (m)}{\text{Volume} (V)}

  • To find the mass:

    m=ρ×Vm = \rho \times V

    Substituting the given values:

    m=2700 kg/m³×2 m³m = 2700 \, \text{kg/m³} \times 2 \, \text{m³} m=5400 kgm = 5400 \, \text{kg}

    Mass of aluminum = 5400 kg.

c. What is relative density? If the densities of pure milk and water at 4°C are 1030 kg/m³ and 1000 kg/m³ respectively, what is the relative density of pure milk?

  • Relative Density is the ratio of the density of a substance to the density of water. It is dimensionless.

    Relative Density=Density of SubstanceDensity of Water\text{Relative Density} = \frac{\text{Density of Substance}}{\text{Density of Water}}

  • Substituting the given values:

    Relative Density of Pure Milk=1030 kg/m³1000 kg/m³\text{Relative Density of Pure Milk} = \frac{1030 \, \text{kg/m³}}{1000 \, \text{kg/m³}} Relative Density of Pure Milk=1.03\text{Relative Density of Pure Milk} = 1.03

    Relative density of pure milk = 1.03.

e. If relative density of an object is 0.92, identify whether the body sinks or floats in water. Analyze with suitable reason:

  • If the relative density of an object is less than 1, the object will float in water.
  • If the relative density is greater than 1, the object will sink in water.

Since the relative density of the object is 0.92, it is less than 1. Therefore, the body will float in water.
Reason: The object’s density is lower than the density of water, so the upward buoyant force exerted by the water is greater than the weight of the object, causing it to float.

f. What is an element? Write the name and symbol of elements with atomic number 7, 13, and 17.

  • Element: An element is a pure substance made up of only one type of atom and cannot be broken down into simpler substances by chemical means.
  • Elements with given atomic numbers:
    • Atomic Number 7: Nitrogen (Symbol: N)
    • Atomic Number 13: Aluminum (Symbol: Al)
    • Atomic Number 17: Chlorine (Symbol: Cl)

h. Which atoms are present in a molecule of water? Also, state their numbers.

  • A molecule of water (H₂O) is composed of:
    • Hydrogen (H): 2 atoms
    • Oxygen (O): 1 atom

Thus, the water molecule consists of 2 hydrogen atoms and 1 oxygen atom.

i. What are sub-atomic particles? Draw an atomic structure and label the sub-atomic particles.

  • Sub-atomic particles are the fundamental particles that make up an atom. They are:
    • Protons: Positively charged particles found in the nucleus.
    • Neutrons: Neutral particles found in the nucleus.
    • Electrons: Negatively charged particles revolving around the nucleus in shells.

(You will need to create a drawing showing a nucleus with protons and neutrons, surrounded by circular shells containing electrons. Label each particle appropriately.)

k. What type of changes are called physical changes? Write with example.

  • Physical Changes: These are changes that affect the physical properties of a substance, such as shape, size, state, or appearance, without changing its chemical composition.
  • Example:
    • Melting of ice into water.
    • Breaking a glass.

l. What is a chemical change? Mention any three examples of the change that occur in our daily life.

  • Chemical Change: A chemical change occurs when a substance undergoes a transformation that alters its chemical composition, resulting in the formation of a new substance.
  • Examples of chemical changes in daily life:
    1. Burning of paper.
    2. Cooking of food.
    3. Rusting of iron.

m. Classify the following examples into physical and chemical changes:

  1. Evaporation of water: Physical change
  2. Boiling of egg: Chemical change
  3. Condensation of vapor: Physical change
  4. Burning of fossil fuel: Chemical change
  5. Dissolving of sugar in water: Physical change

n. What is the name of the element having atomic number 18? Prepare a model of its atomic structure.

  • Name of the Element: Argon (Symbol: Ar)
  • Atomic Structure of Argon:
    • Protons: 18
    • Neutrons: 22 (based on its most common isotope, Argon-40)
    • Electrons: 18, arranged as:
      • 2 electrons in the first shell.
      • 8 electrons in the second shell.
      • 8 electrons in the third shell.



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