Class 10 Science Notes Chapter 1 Chemical Reactions and Equations

Chemical Reactions and Equations

What is a Chemical Reaction?

A chemical reaction is a process in which one or more substances (reactants) are changed into new substances (products) with different chemical properties.

For example: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc.

A chemical reaction is when substances react and form new substances with new properties.

The substances which take part in a chemical reaction are called reactants.

The new substances produced as a result of a chemical reaction are called products.

Signs of a Chemical Reaction:

Change in color
Formation of gas or bubbles
Change in temperature (heat produced or absorbed)
Formation of a solid (precipitate)
Change in smell or light produced

Characteristics of Chemical Reactions

✅ Formation of a New Substance
- New product(s) with different properties are formed.
- Example: Iron + Oxygen → Rust
🔥 Change in Energy
- Heat, light, or sound may be released or absorbed.
- Example: Burning of magnesium produces heat and light.
💨 Evolution of Gas
- Some reactions produce a gas.
- Example: Vinegar + Baking soda → Carbon dioxide gas
- Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) ↑
🎨 Change in Color
- The color of the reactants or solution may change.
- Example: Iron + Copper sulfate → Blue to green color change
- citric acid + purple coloured potassium permanganate solution → purple to colourless.
- sulphur dioxide gas + acidified potassium dichromate solution → orange to green.
🌫️ Formation of Precipitate
- An insoluble solid (precipitate) forms in a liquid reaction.
- Example: Mixing silver nitrate and salt solution forms white precipitate of silver chloride.
- BaCl2(aq) + H2SO4(aq) → BaSO4(s) (ppt) + 2HCl(aq)
Change in state of substance:

The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas
Change in temperature:

The chemical reaction between quick lime water to form slaked lime is characterized by a change in temperature (which is a rise in temperature).

The chemical reaction between zinc granules and dilute sulphuric acid is also characterised by a change in temperature (which is a rise in temperature).

What is a Chemical Equation?

A chemical equation is a short, symbolic way of representing a chemical reaction using chemical formulas of the substances involved.

Format of a Chemical Equation:

Reactants → Products

Reactants are the starting substances.
Products are the new substances formed.
The arrow (→) shows the direction of the reaction.

Example:

Hydrogen + Oxygen → Water

H₂ + O₂ → H₂O

(This equation needs to be balanced: 2H₂ + O₂ → 2H₂O)

Balancing Chemical Equations
A balanced chemical equation has the same number of atoms of each element on both the reactant and product sides.

This follows the Law of Conservation of Mass:

"Matter cannot be created or destroyed in a chemical reaction."

Steps to Balance a Chemical Equation:

Write the unbalanced equation
Example:
$H 2 + O 2 \to H 2 O$
Count atoms of each element on both sides
- Reactants: H = 2, O = 2
- Products: H = 2, O = 1
Balance one element at a time
- Oxygen is unbalanced. To balance it, put 2 in front of water:
  $H2 + O2 → 2H2O$
Update atom count
- Reactants: H = 2, O = 2
- Products: H = 4, O = 2
- Now, hydrogen is unbalanced.
Balance hydrogen by putting 2 in front of H₂:
$2H2 + O2 → 2H2O$
Final check – Atom count:

H = 4, O = 2 on both sides ✅

Write the physical states of reactants and products
✅ Balanced Equation:
$2H2(g) + O2(g) → H2O(l)$

example 2 - STEPWISE BALANCING (Hit and Trial)

Step 1. Write a chemical equation and draw boxes around each formula.

Fe + H₂O → Fe₃O₄ + H₂

* Do not change anything inside the box.

Step 2. Count the number of atoms of each element on both the sides of chemical

equation.

Element No. of atoms at No. of atoms at

reactant side product side

1. Fe 1 3

2. H 2 2

3. O 1 4

Step 3. Equalise the number of atoms of element which has maximum number by

putting in front of it.

Fe + 4H₂O → Fe₃O₄ + H₂

Step 4. Try to equalize all the atoms of elements on reactant and product side by

adding coefficient in front of it.

3Fe + 4H₂O → Fe₃O₄ + 4H₂

* Now all the atoms of elements are equal on both sides.

Step 5. Write the physical states of reactants and products.

3Fe _(s) + 4H₂O _(g) → Fe₃O₄ _{( s)} + 4H₂ _(g)

Types of Chemical Reactions

1. Combination Reaction

Two or more substances combine to form a single product.
🧪 Example:

CaO + H₂O → Ca(OH)₂

(Calcium oxide + Water → Slaked lime)

Mg(s) + O₂(g) → 2MgO(s)

Magnesium + Oxygen → Magnesium Oxide

C (s) + O₂(g) → CO₂(g)

Carbon + Oxygen → Carbon dioxide

2. Decomposition Reaction

A single compound breaks down into two or more simpler substances.
Needs heat, light, or electricity.
🧪 Example:

Electrolytic Decomposition : When decomposition is carried out by
passing electricity

2H₂O →2H₂+O₂ [ in presence of Electricity]

(Water decomposes into hydrogen and oxygen)

Thermal Decomposition:

The decomposition of a substance on heating is known as Thermal Decomposition.

CaCO₃(s) △→ CaO(s) + CO₂(g)

Calcium carbonate → Calcium oxide + Carbon dioxide

2Fe(OH)₃(s) △→ △→ Fe₂O₃(s) + 3H₂O(l)

ferric hydroxide is heated, it decomposes into ferric oxide and water

2FeSO₄ (s) △→ Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)

(Ferrous sulphate) (Ferric oxide)

Green colour Red-brown colour

2Pb(NO₃)₂(s) heat−→−△→ 2PbO(s) + 4NO₂(g) + O₂(g)

Photolysis or Photo Decomposition Reaction :

When decomposition is carried out in presence of sunlight

2AgCl(s) (white) _Sunlight_→ 2Ag(s) (grey) + Cl₂(g)

silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.

Photographic paper has a coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal
2AgBr (s) _Sunlight_→ 2Ag (s) + Br2 (g)

3. Displacement Reaction

A more reactive element displaces a less reactive element from a compound.
🧪 Example:

Zn + CuSO₄ → ZnSO₄+ Cu

(Zinc displaces copper from copper sulfate)

Fe + CuSO₄ → FeSO₄+ Cu

(Iron displaces copper from copper sulfate)

Pb(s) + CuCl₂(aq) ® PbCl₂(aq) + Cu(s)

(Copper chloride) (Lead chloride)

Displacement only happens if the element replacing the other is more reactive (based on the reactivity series).

Most Reactive to Least Reactive:

Potassium (K)
Sodium (Na)
Calcium (Ca)
Magnesium (Mg)
Aluminium (Al)
Zinc (Zn)
Iron (Fe)
Tin (Sn)
Lead (Pb)
(Hydrogen – non-metal, used for comparison)
Copper (Cu)
Mercury (Hg)
Silver (Ag)
Gold (Au)
Platinum (Pt)

[memory tricks -

केटरीना ने कार मांगी अल्टो जैन फ़रारी फिर भी हाई क्यू मिली सिल्वर प्लेटेट ऑडी

"Please Stop Calling Me A Zebra, I Like Her Calling Smart Guy Police"

Please Send Cats, Monkeys and Zebras in Lovely, Happy Cages Made of Silver, Gold, Platinum"

4. Double Displacement Reaction

A double displacement reaction (also called a double replacement reaction) is a chemical reaction in which two compounds exchange their ions to form two new compounds
Often forms a precipitate.

🧪 Example:

=> Silver Nitrate + Sodium Chloride

AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq)

Silver chloride (AgCl) is a white precipitate.

=> Barium chloride + Sodium sulphate

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq)

Barium sulfate (BaSO₄) is a white precipitate

=> Barium Chloride + Sulphuric Acid

BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(s)↓ + 2HCl(aq)

Barium sulfate (BaSO₄) is a white precipitate.

=> Lead Nitrate + Potassium Iodide

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s)↓ + 2KNO₃(aq)

Lead iodide (PbI₂) forms a bright yellow precipitate.

=> Calcium Chloride + Sodium Carbonate

CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s)↓ + 2NaCl(aq)

Calcium carbonate (CaCO₃) is a white precipitate.

5. Oxidation and Reduction (Redox Reaction)

Oxidation = Gain of oxygen / Loss of hydrogen
Reduction = Loss of oxygen / Gain of hydrogen
If both happen together → it’s a redox reaction.
🔁 A Redox Reaction is a chemical reaction that involves both:
- Oxidation (loss of electrons or gain of oxygen)
- Reduction (gain of electrons or loss of oxygen)
🔺 Oxidation means: 🔻 Reduction means:
- Gain of oxygen, or Loss of oxygen, or
- Loss of hydrogen, or Gain of hydrogen, or
- Loss of electrons Gain of electrons
🧪 Example:

2Cu + O₂ __Heat ¾®2CuO
[The surface of copper powder becomes coated with black copper(II) oxide

CuO+H₂→Cu+H₂O
[the black coating on the surface turns brown as the reverse reaction take place]
- CuO (copper oxide) gets reduced to Cu (loses oxygen)
- H₂ gets oxidized to H₂O (gains oxygen)
✅ This is a Redox Reaction because both oxidation and reduction happen.

🔄 Oxidation and Reduction Always Happen Together!

When one substance is oxidized, another is reduced — like a chemical teamwork!

⚡ Another Example:

Fe₂O₃+3CO→2Fe+3CO₂

Iron oxide is reduced to iron
Carbon monoxide is oxidized to carbon dioxide

Zn + CuSO₄ → ZnSO₄ + Cu

Zinc is oxidized (Zn → Zn²⁺ + 2e⁻)
Copper ions (Cu²⁺) are reduced (Cu²⁺ + 2e⁻ → Cu)

Oxidizing Agent & Reducing Agent

🔺 Oxidizing Agent

👉 A substance that causes oxidation in another substance by accepting electrons (or giving oxygen).
➡️ It gets reduced itself.

🔥 Example:

CuO+H₂→Cu+H₂O

Here, CuO is the oxidizing agent because:

It gives oxygen to hydrogen
And gets reduced to Cu

🔻 Reducing Agent

👉 A substance that causes reduction in another substance by donating electrons (or removing oxygen).
➡️ It gets oxidized itself.

🔥 Same Example:

CuO+H₂→Cu+H₂O

Here, H₂ is the reducing agent because:

It removes oxygen from CuO
And gets oxidized to H₂O

🧠 Memory Tip:

Agent Type	What it does	What happens to it
Oxidizing Agent	Oxidizes others	Gets reduced
Reducing Agent	Reduces others	Gets oxidized

Exothermic and Endothermic Reactions

🔥 Exothermic Reactions

Definition: Reactions that release heat (energy) to the surroundings.
The surrounding gets hot.
Energy is given out in the form of heat or light.

✅ Examples:

1. Combustion of fuels

CH₄ + 2O₂ → CO₂ + 2H₂O +heat

2. Respiration in cells

C₆H₁₂O₆ + O₂ → CO₂ + H₂O+energy

3. Neutralization reaction

HCl + NaOH → NaCl + H₂O + heat

❄️ Endothermic Reactions

Definition: Reactions that absorb heat (energy) from the surroundings.
The surrounding gets cold.
Energy is taken in to carry out the reaction.

✅ Examples:

1. Photosynthesis

6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂

2. Cooking of food (heat is absorbed)

3. Dissolving ammonium chloride in water (temperature drops)

🧠 Quick Summary:

Type	Heat is...	Surrounding feels	Example
Exothermic	Released	Hot	Combustion, Respiration
Endothermic	Absorbed	Cold	Photosynthesis, Cooking

Effects of Oxidation Reactions in Everyday Life

1. Rusting of Iron 🧲

When iron reacts with oxygen and moisture, it forms iron oxide (rust).
Weakens bridges, pipes, vehicles, etc.

Reaction:

4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃

✅ Prevention: Painting, oiling, galvanizing

2. Combustion of Fuels 🔥

Burning of petrol, diesel, wood, LPG is oxidation.
Produces heat and light.

Example:

CH₄ + 2O₂ → CO₂ + 2H₂O + Energy

✅ Used in cooking, transportation, electricity

3. Respiration 🧬

In our body, glucose reacts with oxygen to release energy.

Reaction:

C₆H₁₂O₆ + O₂ → CO₂+ H₂O + Energy

✅ Helps us stay alive and active!

4. Rancidity of Food 🍟

Fats and oils oxidize in air, making food smell bad or taste stale.

Prevention:

Storing in airtight containers
Using antioxidants

5. Photosynthesis (Reverse – Reduction) 🌱

While technically reduction happens in photosynthesis, the process starts with oxidation of water, showing how oxidation is part of life creation.

📌 Summary Table:

Effect	Oxidation Role	Useful/Harmful
Rusting of iron	Iron oxidizes	Harmful
Combustion of fuels	Fuel oxidizes	Useful
Respiration	Glucose oxidizes	Useful
Rancidity of food	Fats oxidize	Harmful
Photosynthesis (partly)	Water oxidized	Useful

🧲 Corrosion

Corrosion is the gradual destruction of metals due to their chemical reaction with air, water, or other substances in the environment.

🔍 Definition:

Corrosion is the process in which metals are eaten away by the action of air, moisture, water, acids, etc., forming undesirable compounds (usually oxides).

🧪 Example: Rusting of Iron

Iron reacts with oxygen and moisture from the air.

4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ → Rust (Fe₂O₃.xH₂O)

⚠️ Common Examples of Corrosion:

Metal	Corrosion Product	Appearance
Iron	Rust (Fe₂O₃·xH₂O)	Reddish-brown
Copper	Copper carbonate	Greenish coating
Silver	Silver sulphide	Black coating
Aluminium	Aluminium oxide	Thin white layer

💣 Harmful Effects:

Weakens metal structures (bridges, ships, railings)
Damages machinery and tools
Causes economic losses

🛡️ Prevention of Corrosion:

1. Painting or oiling

2. Galvanization (coating iron with zinc)

3. Electroplating

4. Alloying (e.g., stainless steel)

5. Using anti-rust solutions or coatings

Rancidity

Rancidity is a process in which oils and fats get spoiled when exposed to air, making the food smell and taste bad.

🔍 Definition:

Rancidity is the condition produced by the oxidation of fats and oils in food, leading to unpleasant taste and odor.

🧪 Why does it happen?

Fats & oils react with oxygen in the air.
This reaction is a slow oxidation, forming peroxides and other compounds that smell bad.

🍟 Examples of Rancidity:

Chips left open for days become stale.
Ghee or butter smells sour when stored too long.

💡 Types of Rancidity:

1. Oxidative Rancidity – due to oxidation by air (most common)

2. Hydrolytic Rancidity – due to moisture/water breaking down fats

🛡️ Prevention of Rancidity:

1. Storing food in airtight containers

2. Refrigeration

3. Vacuum packing

4. Adding antioxidants (like BHA, BHT)

5. Using nitrogen gas in chip packets to displace oxygen

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