Acids, Bases and Salts Class 10th Important Questions with Answers Science

Reaction of Zinc with Sodium Hydroxide:

When zinc reacts with sodium hydroxide, it forms a salt called sodium zincate and hydrogen gas. Names of the Products are -

• Sodium zincate (Na₂ZnO₂)
• Hydrogen gas (H₂)

Balanced Chemical Equation:

Zn + 2NaOH → Na₂ZnO₂ + H₂↑

Test for the Gas Evolved:

• Bring a burning matchstick near the mouth of the test tube.
• If hydrogen is present, it will burn with a ‘pop’ sound.

Differences between Acids and Bases:

• Taste:
  • Acids taste sour.
  • Bases taste bitter.
• Touch:
  • Acids are corrosive.
  • Bases feel soapy to touch.
• Effect on Litmus Paper:
  • Acids turn blue litmus red.
  • Bases turn red litmus blue.
• pH Value:
  • Acids have pH less than 7.
  • Bases have pH more than 7.
• Ions Produced in Water:
  • Acids produce hydrogen ions (H⁺).
  • Bases produce hydroxide ions (OH⁻).
• Reaction with Metals:
  • Acids react with metals to produce hydrogen gas.
  • Bases generally do not react with metals.
• Examples:
  • Acids: Hydrochloric acid (HCl), Sulfuric acid (H₂SO₄)
  • Bases: Sodium hydroxide (NaOH), Calcium hydroxide (Ca(OH)₂)

Ammonia is basic in nature because it produces hydroxide ions in water and turns red litmus blue.

Source of Litmus Solution:
Litmus solution is obtained from lichen, a natural plant-like organism.

Use of Litmus Solution:
Litmus solution is used as an acid-base indicator. It helps to check whether a substance is acidic or basic.

• Acids turn blue litmus red.
• Bases turn red litmus blue.

Distilled water is neutral.

How to verify: -  You can verify it using litmus paper:

• Dip red litmus paper into distilled water – it remains red.
• Dip blue litmus paper into distilled water – it remains blue.

Since there is no change in color, it means distilled water is neither acidic nor basic, i.e., it is neutral.

Neutralisation is a reaction in which an acid reacts with a base to form salt and water.

Example:
When hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the products are sodium chloride (NaCl) and water (H₂O).

HCl + NaOH → NaCl + H₂O

This reaction is called a neutralisation reaction because the acid and base cancel each other's effect.

Dorji can use litmus paper to test each soft drink and find out whether it is acidic, basic, or neutral.

Steps Dorji can follow:

• Take a drop of each drink and put it on blue and red litmus papers.
• Observe the color change:
  • If the blue litmus turns red, the drink is acidic.
  • If the red litmus turns blue, the drink is basic.
  • If no color change in either litmus, the drink is neutral.

(a) An antacid tablet is taken when you suffer from acidity:
When you have acidity, your stomach has excess acid, which causes discomfort. An antacid tablet is a base, and it neutralises the excess acid in your stomach. This neutralisation reaction helps to reduce the pain and discomfort caused by acidity.

(b) Calamine solution is applied on the skin when an ant bites:
Ant bites inject acid into the skin, causing pain and irritation. Calamine solution is basic in nature. When applied, it neutralises the acid from the ant bite, soothing the skin and reducing irritation.

(c) Factory waste is neutralised before disposing it into water bodies:
Factory waste may contain harmful acids or bases. If these wastes enter water bodies without treatment, they can harm aquatic life and pollute water. Neutralising the waste by adding suitable acids or bases makes it neutral and safer to dispose of in water.

Step 1: Dip turmeric paper in each liquid.

• If turmeric paper turns red or reddish-brown, the liquid is a base.
• If turmeric paper does not change color (stays yellow), the liquid is either acid or neutral.

Step 2: Identify liquids.

• Hydrochloric acid will not change the color of turmeric paper (stays yellow) because it is an acid.
• Sodium hydroxide will turn turmeric paper red or reddish-brown because it is a base.
• Sugar solution will not change the color of turmeric paper (stays yellow) because it is neutral.

If blue litmus paper is dipped in a solution and it stays blue, then the solution is either basic or neutral. Blue litmus paper turns red only in acidic solutions. Since it did not change color, the solution is not acidic.

To know if it is basic or neutral, you can test with red litmus paper:

• If red litmus paper stays red, the solution is neutral.
• If red litmus paper turns blue, the solution is basic.

You have three test tubes: one with distilled water, one with an acid, and one with a base. You have only red litmus paper to identify them.

1. Dip red litmus paper into each test tube.
2. If the red litmus paper turns blue, the solution is basic.
3. If the red litmus paper stays red, the solution is either acidic or neutral (distilled water).
4. To distinguish between acid and distilled water (both keep red litmus red), use other properties like smell or taste carefully (if safe), or by elimination after identifying the base.

Curd and sour substances should not be kept in brass or copper vessels because they contain acids. These acids react with the metal and form toxic (poisonous) substances, which are harmful to health.

So, to stay safe, curd and sour foods should be kept in glass, plastic, or stainless steel containers.

When an acid reacts with a metal, hydrogen gas is usually released.

Example:
When zinc (Zn) reacts with hydrochloric acid (HCl):
Zn + 2HCl → ZnCl₂ + H₂ (gas)

Test for hydrogen gas: -Bring a burning matchstick near the mouth of the test tube. If hydrogen gas is present, it will burn with a ‘pop’ sound.

HCl (hydrochloric acid), HNO₃ (nitric acid), etc. show acidic character in aqueous solutions because they release hydrogen ions (H⁺) when dissolved in water.

Example:
HCl → H⁺ + Cl⁻
HNO₃ → H⁺ + NO₃⁻

These H⁺ ions are responsible for the acidic nature.

On the other hand, alcohol and glucose also contain hydrogen atoms, but they do not release H⁺ ions in water. So, they do not show acidic character.

An aqueous solution of an acid conducts electricity because it contains ions.

When an acid (like HCl) is dissolved in water, it ionises and releases hydrogen ions (H⁺) and other negative ions (like Cl⁻).

Example:
HCl   --->  H⁺ + Cl⁻

These free ions help in the flow of electric current through the solution.

Sodium chloride is formed from hydrochloric acid and sodium hydroxide. Hydrochloric acid is the acid and sodium hydroxide is the base. It is a neutral salt because it is formed from a strong acid and a strong base.

Sodium chloride is called rock salt when it is found in the earth in the form of large crystals. Rock salt is usually brown or pink due to the presence of impurities.

Rock salt is formed when sea water or salt lakes dry up over a long period of time. As the water evaporates, the salt is left behind. It gets deposited in layers and becomes buried under the ground. These deposits of salt are called rock salt and are mined from the earth.

The white powder is baking powder.

Main ingredients:

• Sodium hydrogen carbonate (baking soda)
• Tartaric acid

Functions:

• Baking soda releases carbon dioxide gas on heating, making the cake soft and spongy.
• Tartaric acid neutralizes the bitter taste of sodium carbonate formed.

Chemical reaction:
2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

Carbon dioxide makes the cake rise and fluffy.

An acid-base indicator is a substance that changes its colour in acidic or basic solutions. It is used to identify whether a solution is acidic or basic.

Example of a synthetic acid-base indicator:
Phenolphthalein – It is colourless in acids and pink in bases.

If someone in the family is suffering from acidity after overeating, I would suggest using a base as a remedy. Substances like baking soda (sodium hydrogen carbonate) or milk of magnesia (magnesium hydroxide) can be used. These are mild bases that neutralize the extra acid in the stomach and give relief from acidity.

Water of crystallisation is the fixed number of water molecules that are present in the crystal structure of a salt. These water molecules are a part of the salt and give it its shape and colour.

Examples:

• Copper(II) sulphate – CuSO₄5H₂O
• Gypsum – CaSO₄2H₂O

Proof:

1. When blue copper sulphate crystals (CuSO₄·5H₂O) are heated, they lose water of crystallisation and become white anhydrous copper sulphate (CuSO₄).
2. When water is added back to the white powder, it turns blue again, showing that water of crystallisation affects the colour and state of the compound.

A student dropped a few pieces of marble in dilute hydrochloric acid. A gas was evolved which was passed through lime water. The lime water turned milky.

This happens because the gas formed is carbon dioxide, and it reacts with lime water to form calcium carbonate, which is a white insoluble substance.

Balanced chemical equations:

CaCO₃ + 2HCl   --------->  CaCl₂ + CO₂ + H₂O
Ca(OH)₂ + CO₂   ---------->  CaCO₃ + H₂O

The milkiness in lime water is due to the formation of calcium carbonate.

Anhydrous salts are salts that do not contain water molecules in their structure.

Hydrated salts are salts that contain a fixed number of water molecules called water of crystallisation as part of their structure.

Example of a hydrated salt is copper(II) sulphate pentahydrate (CuSO₄·5H₂O), which is blue in colour.

Example of an anhydrous salt is anhydrous copper(II) sulphate (CuSO₄), which is white in colour.

When hydrated copper sulphate is heated, it loses water of crystallisation and becomes anhydrous copper sulphate. This shows the difference between hydrated and anhydrous salts.

Plaster of Paris (POP) is prepared by heating gypsum (calcium sulphate dihydrate) at about 150°C. This removes part of the water of crystallisation.

The reaction is:
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½ H₂O (water vapor)

When POP is mixed with water, it absorbs water and sets into a hard mass by changing back to gypsum.

The setting reaction is:
CaSO₄·½H₂O + 1½ H₂O → CaSO₄·2H₂O

This forms a hard, solid mass because of the formation of interlocking crystals.

Chemical formulas:

• Hydrated copper sulphate: CuSO₄·5H₂O
• Anhydrous copper sulphate: CuSO₄

Activity to show interconversion:

• Take some blue hydrated copper sulphate crystals (CuSO₄·5H₂O) in a watch glass.
• Heat the crystals gently using a burner.
• The blue crystals will lose water and turn into white anhydrous copper sulphate (CuSO₄) powder.
• Now, add a few drops of water to the white powder.
• The white powder will dissolve in water and turn blue again, showing the formation of hydrated copper sulphate.

Chemical name and formula - 

Plaster of Paris

• Chemical name: Calcium sulphate hemihydrate
• Formula: CaSO₄·½H₂O

Gypsum

• Chemical name: Calcium sulphate dihydrate
• Formula: CaSO₄·2H₂O

Name of the process: Chlor-alkali process

Sodium hydroxide is produced by the electrolysis of brine, which is a concentrated solution of sodium chloride (NaCl).

During the process:

• Hydrogen gas is released at the cathode.
• Chlorine gas is released at the anode.
• Sodium hydroxide is formed in the solution.

Chemical equation:
2NaCl (aq) + 2H₂O (l) → 2NaOH (aq) + Cl₂ (g) + H₂ (g)

This process is called the Chlor-alkali process because it produces two main products: chlorine gas (chlor) and sodium hydroxide (alkali).