YASRAJ STUDIES
DAV Class 7 Science Chapter 3: Chemical Substances and Processes
Complete Question Answers
Introduction
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Matter around us is made up of different substances. Some substances are pure, while others contain more than one type of substance.
In this chapter, we learn about:
- Elements
- Compounds
- Chemical formulae
- Chemical equations
- Chemical reactions
- Physical changes
- Chemical changes
- Combination reactions
- Decomposition reactions
- Displacement reactions
- Neutralisation reactions
- Rusting of iron
Below are the complete solved questions and answers for DAV Class 7 Science Chapter 3 – Chemical Substances and Processes.
A. Fill in the Blanks
1. Materials made up of same kind of particles are called __________.
Answer: Pure substance
2. The type and number of particles of each kind present in a substance is given by its __________.
Answer: Chemical formula
3. The chemical formula of water is __________.
Answer: H₂O
4. Iron gets rusted on coming in contact with __________ and __________.
Answer: Moisture and air
Remember:
Iron + Oxygen + Water → Rust
5. The process that leads to a chemical change is called a __________.
Answer: Chemical reaction
6. In a neutralization reaction, __________ and __________ are formed.
Answer: Salt and water
General form:
Acid + Base → Salt + Water
B. Write True or False for the Following Statements
1. All matter is made up of atoms.
Answer: True
2. Compounds are substances consisting of two or more elements chemically combined in a fixed ratio.
Answer: True
3. The symbol of element copper is Cu.
Answer: True
4. The formula of sodium chloride is written as ClNa.
Answer: False
Correct Answer: NaCl
5. The chemical formula of aluminium sulphate is Al₂SO₄.
Answer: False
Correct Answer: Al₂(SO₄)₃
6. A new substance formed in a chemical reaction is called a product.
Answer: True
7. Magnesium hydroxide is an acid.
Answer: False
Correct Answer: Magnesium hydroxide is a base.
C. Tick (✓) the Correct Option
1. The chemical symbol Ag represents the element—
Answer: (c) Silver
2. One molecule of nitric acid is made up of—
Answer: (b) One atom of hydrogen, one atom of nitrogen and three atoms of oxygen.
Therefore, one molecule of nitric acid contains:
- 1 hydrogen atom
- 1 nitrogen atom
- 3 oxygen atoms
3. The chemical formula of magnesium phosphate is—
Answer: (c) Mg₃(PO₄)₂
4. The following reaction is an example of a—
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Answer: (b) Displacement reaction
5. The chemical formula of quicklime is—
Answer: (a) CaO
D. Answer the Following Questions in Brief
1. What are elements?
Remember:
Element → One kind of atom
2. Give the chemical symbols of iron and chlorine.
Answer:
- Iron — Fe
- Chlorine — Cl
3. Write the chemical formulae of the following compounds.
(a) Aluminium oxide
Answer: Al₂O₃
(b) Zinc acetate
Answer: Zn(CH₃COO)₂
4. Balance the following equation:
Ca(OH)₂ + HCl → CaCl₂ + H₂O
Answer:
Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
Explanation:
A chemical equation is balanced when the number of atoms of each element is equal on both sides of the equation.
Check:
Left side:
- Ca = 1
- O = 2
- H = 4
- Cl = 2
Right side:
- Ca = 1
- O = 2
- H = 4
- Cl = 2
Therefore, the equation is balanced.
5. What does a chemical equation represent?
Remember:
Chemical equation → Reactants → Products
6. What are reactants and products in a chemical equation?
Answer:
The substances written on the left-hand side (LHS) of a chemical equation are called reactants.
The substances written on the right-hand side (RHS) are called products.
Example:
HCl + NaOH → NaCl + H₂O
Here:
Reactants: HCl and NaOH
Products: NaCl and H₂O
7. Give one example of a combination reaction.
Answer:
2H₂(g) + O₂(g) → 2H₂O(l)
Remember:
Two or more substances → One product = Combination reaction
E. Answer the Following Questions
1. Write the steps involved in writing the chemical formula of calcium phosphate.
Answer:
The ions present in calcium phosphate are:
- Calcium ion — Ca²⁺
- Phosphate ion — PO₄³⁻
To balance the charges, we take:
- 3 calcium ions
- 2 phosphate ions
Therefore, the chemical formula of calcium phosphate is:
Ca₃(PO₄)₂
Charge check:
3 × (+2) = +6
2 × (−3) = −6
Total charge = 0
Therefore:
Calcium phosphate = Ca₃(PO₄)₂
2. How is a chemical change different from a physical change?
Answer:
| Physical Change | Chemical Change |
|---|---|
| No new substance is formed. | A new substance is formed. |
| It is generally reversible. | It is generally not reversible by simple physical methods. |
| Only physical properties may change. | Chemical properties change. |
| Example: Melting of ice. | Example: Rusting of iron. |
Easy way to remember:
Physical change → No new substance
Chemical change → New substance formed
3. ‘Neutralisation reaction is a chemical change.’ Justify this statement with the help of an example.
Example:
HCl + NaOH → NaCl + H₂O
Hydrochloric acid reacts with sodium hydroxide to form new substances, sodium chloride and water.
Therefore, neutralisation is a chemical change.
Remember:
Acid + Base → Salt + Water
4. Define a decomposition reaction and give an example of the same.
Example:
Water decomposes into hydrogen and oxygen when electricity is passed through it.
2H₂O → 2H₂ + O₂
Remember:
One substance → Two or more substances = Decomposition reaction
5. Classify the following reactions into different types, giving reasons.
(a) CaO(s) + SiO₂(s) → CaSiO₃(s)
Answer: Combination reaction
(b) KOH(aq) + HCl(aq) → KCl(aq) + H₂O(l)
Answer: Neutralisation reaction
(c) Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
Answer: Displacement reaction
(d) 2H₂O(l) → 2H₂(g) + O₂(g)
Answer: Decomposition reaction
Value-Based Questions
The school principal told her students that she would like them to follow a special practice of Japanese schools. The students work as a team and take pride in maintaining the cleanliness of their school.
She compared replacing old habits with better habits to a displacement reaction, in which one element replaces another in a compound.
1. State two values displayed by students of Japanese schools.
Answer:
Two values displayed by the students are:
- Respect for others' work
- Self-help
2. Why did the principal tell her students that the suggested ideas were, in a way, similar to what happens in a displacement reaction?
She compared this change with a displacement reaction, in which one element replaces another element in a compound.
Remember:
Old habits → Better habits
This was compared with:
One element → Replaces another element
3. Give one example of a displacement reaction.
Answer:
Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
Something To Do
1. List any five physical and chemical changes that you see around you.
Five Physical Changes
- Melting of ice
- Melting of wax
- Making of dough
- Drying of clothes
- Folding of paper
Five Chemical Changes
- Cooking of rice
- Growth of a tree
- Germination of seeds
- Burning of a candle
- Burning of paper
2. Think of a small activity to show that rusting of iron requires both oxygen and water.
Answer:
Take three clean iron nails and three test tubes.
Step 1:
Put a nail in the first test tube containing water and leave it exposed to air.
Step 2:
Put a nail in the second test tube containing boiled water and cover the water with a layer of oil. The oil prevents air from coming into contact with the water.
Step 3:
Put a nail in the third dry test tube and keep a drying agent inside it to remove moisture. Close the test tube.
Observation:
After a few days, the nail exposed to both water and air will show rust, while the other nails will show little or no rust.
Conclusion:
This activity shows that both oxygen and water are necessary for rusting of iron.
Remember:
Rusting → Oxygen + Water
⭐ Important Points to Remember
- Matter is made up of particles.
- An element is a pure substance made up of similar atoms.
- A compound contains two or more elements chemically combined in a fixed ratio.
- A chemical formula represents the type and number of atoms present in a substance.
- Water has the chemical formula H₂O.
- Sodium chloride has the formula NaCl.
- A chemical equation represents a chemical reaction.
- The substances on the LHS of an equation are called reactants.
- The substances on the RHS are called products.
- A physical change does not produce a new substance.
- A chemical change produces one or more new substances.
- A combination reaction forms a single product from two or more substances.
- A decomposition reaction breaks one substance into two or more substances.
- A displacement reaction occurs when one element replaces another element.
- A neutralisation reaction produces salt and water.
- Rusting of iron requires oxygen and water.
🌟 Super Easy Chapter Revision
Matter
Matter → Particles → Elements / Compounds
Element
Element → Same kind of atoms
Compound
Compound → Two or more elements → Fixed ratio
Chemical Formula
Chemical formula → Type + Number of atoms
Chemical Equation
Reactants → Chemical Reaction → Products
Types of Chemical Reactions
1. Combination Reaction
Two or more substances → One product
Example:
2H₂ + O₂ → 2H₂O
2. Decomposition Reaction
One substance → Two or more simpler substances
Example:
2H₂O → 2H₂ + O₂
3. Displacement Reaction
One element → Replaces another element
Example:
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
4. Neutralisation Reaction
Acid + Base → Salt + Water
Example:
HCl + NaOH → NaCl + H₂O
Physical Change vs Chemical Change
Physical Change
No new substance is formed.
Example:
Melting of ice
Chemical Change
New substance is formed.
Example:
Rusting of iron
🌟 Quick Revision Chart
Element → Same kind of atoms
Compound → Elements chemically combined
Formula → Shows atoms present
Reactants → LHS
Products → RHS
Combination → Many → One
Decomposition → One → Many
Displacement → One replaces another
Neutralisation → Acid + Base → Salt + Water
Rusting → Oxygen + Water
Conclusion
Chemical substances and chemical processes are an important part of our everyday life. By learning about elements, compounds, chemical formulae, chemical equations and different types of chemical reactions, we can understand how substances combine, break down and change.
Physical and chemical changes occur around us every day. Understanding these changes helps us explain many common processes, such as rusting, burning, cooking and decomposition.
The most important ideas of this chapter are:
Element → Compound → Chemical Formula → Chemical Equation → Chemical Reaction
and
Physical Change → No New Substance
Chemical Change → New Substance
📚 YRS YASRAJ STUDIES
DAV Class 7 Science | Chapter 3 | Chemical Substances and Processes
Complete question answers, explanations, activities, value-based questions and revision notes for students.
