Chemical Substances and Processes Class 7 DAV Chapter 3 Solutions, Notes, Questions & Answers

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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 air 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.

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B. Write True or False

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
5. The chemical formula of aluminium sulphate Al₂SO₄ is. — False
   (Correct formula: Al₂(SO₄)₃)
6. New substance formed in a chemical reaction is called product. — True
7. Magnesium hydroxide is an acid. — False

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C. Tick (✓) the correct option

1. The chemical symbol Ag represents the element — silver ✓
2. One molecule of nitric acid is made up of —
   one atom of hydrogen, one atom of nitrogen and three atoms of oxygen ✓
3. The chemical formula of magnesium phosphate is —
   Mg₃(PO₄)₂ ✓
4. The following reaction is an example of a —
   Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) displacement reaction ✓
5. The chemical formula of quicklime is —
   CaO ✓

D. Answer the following questions in brief

1. What are elements?

Elements are pure substances made up of only one kind of atom. They cannot be broken down into simpler substances by ordinary chemical methods.
For example, oxygen, iron, gold and hydrogen are elements.

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2. Give the chemical symbol of iron and chlorine.

• Symbol of iron — Fe
• Symbol of chlorine — Cl

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3. Write the chemical formulae of the following compounds:

(a) Aluminium oxide

Chemical formula: Al₂O₃

(b) Zinc acetate

Chemical formula: Zn(CH₃COO)₂

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4. Balance the following equation:

Ca(OH)₂ + HCl ⟶ CaCl₂ + H₂O

Balanced equation:

$Ca(OH)_2 + 2HCl \rightarrow CaCl_2 + 2H_2O$Ca(OH)2​+2HCl→CaCl2​+2H2​O

Explanation:
To balance the equation, we make the number of atoms of each element equal on both sides. Here, 2 molecules of HCl and 2 molecules of water are needed to balance hydrogen and chlorine atoms.

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5. What does a chemical equation represent?

A chemical equation represents a chemical reaction using symbols and formulae of substances. It shows:

• the reactants taking part in the reaction,
• the products formed, and
• their relative quantities.

For example:

$2H_2 + O_2 \rightarrow 2H_2O$2H2​+O2​→2H2​O

This equation shows that hydrogen reacts with oxygen to form water.

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

• Reactants are the substances that take part in a chemical reaction. They are written on the left-hand side of the equation.
• Products are the new substances formed after the reaction. They are written on the right-hand side of the equation.

Example:

$Mg + O_2 \rightarrow MgO$Mg+O2​→MgO

Here:

• Magnesium (Mg) and oxygen (O₂) are the reactants.
• Magnesium oxide (MgO) is the product.

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7. Give one example of a combination reaction.

A combination reaction is a reaction in which two or more substances combine to form a single product.

Example:

$2Mg + O_2 \rightarrow 2MgO$2Mg+O2​→2MgO

In this reaction, magnesium combines with oxygen to form magnesium oxide.

Step 4: Use brackets for polyatomic ions

Since phosphate (PO₄) is a group of atoms acting as one ion and its number is more than one, brackets are used around it.

Therefore, the chemical formula of calcium phosphate is:

Ca₃(PO₄)₂

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

A physical change is a change in which no new substance is formed, while a chemical change is a change in which one or more new substances are formed.

Physical Change

• Only the shape, size or state changes.
• No new substance is produced.
• Usually temporary and reversible.

Examples:

• Melting of ice
• Breaking of glass
• Boiling of water

Chemical Change

• One or more new substances are formed.
• Properties of substances change completely.
• Usually permanent and irreversible.

Examples:

• Rusting of iron
• Burning of paper
• Cooking food

Difference in simple words

When ice melts into water, it is still water, so it is a physical change. But when iron rusts, a new substance called rust is formed, so it is a chemical change.

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

A neutralisation reaction is considered a chemical change because new substances are formed during the reaction.

When an acid reacts with a base, they form salt and water. The properties of the original substances change completely.

Example

When hydrochloric acid reacts with sodium hydroxide, sodium chloride and water are formed.

$HCl + NaOH \rightarrow NaCl + H_2O$HCl+NaOH→NaCl+H2​O

Explanation

• Hydrochloric acid is acidic in nature.
• Sodium hydroxide is basic in nature.
• After the reaction, common salt and water are produced, which are different substances.

Since new substances are formed and the change cannot be reversed easily, neutralisation is a chemical change.

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4. Define a decomposition reaction and give an example of the same.

A decomposition reaction is a chemical reaction in which a single compound breaks down into two or more simpler substances.

This usually happens due to heat, electricity or light.

Example

When calcium carbonate is heated, it breaks into calcium oxide and carbon dioxide.

$CaCO_3 \xrightarrow{heat} CaO + CO_2$CaCO3​heat​CaO+CO2​

Explanation

• Calcium carbonate is one compound.
• On heating, it decomposes into two simpler substances:
  • Calcium oxide
  • Carbon dioxide

Hence, it is called a decomposition reaction.

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

(a)

$CaO + SiO_2 \rightarrow CaSiO_3$CaO+SiO2​→CaSiO3​

Type of reaction:

Combination reaction

Reason:

Two substances, calcium oxide and silicon dioxide, combine to form a single product, calcium silicate.

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(b)

$KOH + HCl \rightarrow KCl + H_2O$KOH+HCl→KCl+H2​O

Type of reaction:

Neutralisation reaction

Reason:

An acid (HCl) reacts with a base (KOH) to form salt (KCl) and water.

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(c)

$Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag$Cu+2AgNO3​→Cu(NO3​)2​+2Ag

Type of reaction:

Displacement reaction

Reason:

Copper displaces silver from silver nitrate solution and forms copper nitrate and silver.

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(d)

$BaCO_3 \xrightarrow{heat} BaO + CO_2$BaCO3​heat​BaO+CO2​

Type of reaction:

Decomposition reaction

Reason:

Barium carbonate breaks down into barium oxide and carbon dioxide on heating.