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SCIENCE (Class-7)
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Nutrition in Living Organisms-Plants
Nutrition in Living Organisms-Animals and Man
Chemical Substances and Processes
DAV (Science) Class-7

    Chemical Substances and Processes

    A. Fill in the blanks:

    1. Materials made up of same kind of particles are called pure substances.

    2. The type and number of particles of each kind present in a substance is given by its chemical formula.

    3. The chemical formula of water is H₂O.

    4. Iron gets rusted on coming in contact with oxygen and moisture.

    5. The process that leads to a chemical change is called a chemical reaction.

    6. In a neutralisation reaction, salt and water are formed.

    B. Write True or False for the following statements:

    1. All matter is made up of atoms. — True

    2. Compounds are substances consisting of two or more elements chemically combined in a fixed ratio. — True

    3. The symbol of element copper is Cu. — True

    4. Formula of sodium chloride is written as ClNa. — False (Correct: NaCl)

    5. The chemical formula of aluminium sulphate Al₂SO₄ is. — False (Correct: Al₂(SO₄)₃)

    6. New substance formed in a chemical reaction is called product. — True

    7. Magnesium hydroxide is an acid. — False (It is a base)

    C. Tick (✓) the correct option:

    1. The chemical symbol Ag represents the element—

    ☐ sodium

    ☐ aluminium

    ☐ sulphur

    ✅ silver

    1. One molecule of nitric acid is made up of—

    ☐ two atoms of hydrogen, two atoms of nitrogen and two atoms of oxygen

    ✅ one atom of hydrogen, one atom of nitrogen and three atoms of oxygen

    ☐ one atom of hydrogen, one atom of nitrogen and two atoms of oxygen

    ☐ one atom of hydrogen, two atoms of nitrogen and three atoms of oxygen

    1. The chemical formula of magnesium phosphate is—

    ☐ Mg(PO₄)

    ☐ Mg₂(PO₄)₃

    ✅ Mg₃(PO₄)₂

    ☐ Mg(PO₄)₃

    1. The following reaction is an example of a— Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

    ☐ combination reaction

    ✅ displacement reaction

    ☐ decomposition reaction

    ☐ neutralisation reaction

    1. The chemical formula of quicklime is—

    ✅ CaO

    ☐ CaCO₃

    ☐ Ca(OH)₂

    ☐ CaCl₂

    D. Answer the following questions in brief:

    1.What are elements?

    Ans.
    Elements are pure substances made up of only one kind of atom. Example: Oxygen (O), Iron (Fe).

    2.Give the chemical symbol of iron and chlorine.

    Ans.
    Iron – Fe
    Chlorine – Cl

    3.Write the chemical formulae of the following compounds:

    (a) Aluminium oxide (b) Zinc acetate

    Ans.
    (a) Aluminium oxide – Al₂O₃
    (b) Zinc acetate – Zn(CH₃COO)₂

    4.Balance the following equation:
    Ca(OH)₂ + HCl → CaCl₂ + H₂O

    Ans.
    Balanced Equation:
    Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

    5.What does a chemical equation represent?

    Ans.
    A chemical equation shows what happens during a chemical reaction using symbols and formulas.

    6.What are reactants and products in a chemical equation?

    Ans.

    • Reactants are substances that react (left side of the equation).

    • Products are new substances formed (right side of the equation).

    7.Give one example of a combination reaction.

    Ans.
    Example:
    H₂ + Cl₂ → 2HCl
    (Hydrogen and chlorine combine to form hydrogen chloride)

    E. Answer the following questions:

    1.Write the steps involved in writing the chemical formula of calcium phosphate.

    Ans.

    To write the formula of calcium phosphate, we first write the symbols for calcium and phosphate ions along with their charges. 

    Ca²⁺ (calcium) and PO₄³⁻ (phosphate)

    There are no common factors in the number corresponding to their charges. We now crisscross their charges and write the crisscrossed numbers as subscripts against the two charges.

    Ca²⁺ + PO₄³⁻  →  Ca₃(PO₄)₂

    2.How is a chemical change different from a physical change?

    Ans.

    • A chemical change forms a new substance and is mostly irreversible (e.g. rusting of iron).

    • A physical change does not form a new substance and is usually reversible (e.g. melting of ice).

    3.‘Neutralisation reaction is a chemical change.’ Justify this statement with the help of an example.

    Ans.
    In a neutralisation reaction, an acid reacts with a base to form salt and water.
    Example:
    HCl + NaOH → NaCl + H₂O
    A new substance (salt) is formed, so it is a chemical change.

    4.Define a decomposition reaction and give an example.

    Ans.
    A decomposition reaction is one in which a single compound breaks down into two or more simpler substances.
    Example:
    2H₂O → 2H₂ + O₂ (Water breaks into hydrogen and oxygen)

    5.Classify the following reactions into different types, giving reason:

    Ans.

    (a) CaO + SiO₂ → CaSiO₃
    Type: Combination reaction (two substances combine)

    (b) KOH + HCl → KCl + H₂O
    Type: Neutralisation reaction (acid + base)

    (c) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
    Type: Displacement reaction (copper displaces silver)

    (d) BaCO₃ → BaO + CO₂
    Type: Decomposition reaction (compound breaks into two substances)

     

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