Questions Related to NCERT

A concave mirror can give an erect and enlarged image of an object.

This happens when the object is placed between the pole and the principal focus of the concave mirror. It is commonly used in makeup mirrors and shaving mirrors for this reason.

The radius of curvature of a spherical mirror is given as 20 cm.

We know that the focal length of a spherical mirror is half of its radius of curvature.
So,
Focal length = Radius of curvature ÷ 2
Focal length = 20 cm ÷ 2 = 10 cm

Hence the focal length of the mirror is 10 cm.

The principal focus of a concave mirror is a point on the principal axis where light rays that are parallel to the principal axis converge (meet) after reflecting from the concave mirror. Since the rays actually meet at this point, it is called a real focus.

The principal focus lies in front of the concave mirror, and the distance between the mirror's pole (the center of its reflecting surface) and the principal focus is called the focal length.

Given:
Speed1 = 40 km/h, Time1 = 15 minutes = 0.25 hours
Speed2 = 60 km/h, Time2 = 15 minutes = 0.25 hours

Distance1 = 40 × 0.25 = 10 km
Distance2 = 60 × 0.25 = 15 km

Total distance = 10 + 15 = 25 km

Hence the car covers 25 kilometers distance in total.

Given:
Time = 15 minutes = 15 × 60 = 900 seconds
Speed = 2 m/s

Distance = Speed × Time
Distance = 2 × 900 = 1800 meters

Hence the distance between Salma’s house and school is 1800 meters (or 1.8 kilometers).

Given:
Initial odometer reading = 57321.0 km
Final odometer reading = 57336.0 km

Initial time = 08:30 AM
Final time = 08:50 AM

Distance moved = 57336.0 – 57321.0 = 15.0 km
Time taken = 20 minutes

Speed in km/min = 15.0 ÷ 20 = 0.75 km/min
Speed in km/h = 0.75 × 60 = 45 km/h

Hence distance moved = 15.0 km  &  Speed = 0.75 km/min or 45 km/h

To calculate the speed of the train, use the formula:

Speed = Distance ÷ Time

Given:
Distance = 240 km
Time = 4 hours

Speed = 240 ÷ 4 = 60 km/h

Hence the speed of the train is 60 km/h.

Total time = 32 seconds
Number of oscillations = 20

Time period = Total time ÷ Number of oscillations
Time period = 32 ÷ 20 = 1.6 seconds

So, the time period of the pendulum is 1.6 seconds.

The following statements are not correct:
(ii) Every object moves with a constant speed.
(iv) The time period of a given pendulum is constant.
(v) The speed of a train is expressed in m/h.

(i) Motion of hands while running – Oscillatory
(ii) Horse pulling a cart on a straight road – Straight line
(iii) Child in a merry-go-round – Circular
(iv) Child on a see-saw – Oscillatory
(v) Hammer of an electric bell – Oscillatory
(vi) Train on a straight bridge – Straight line

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.

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.

(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.

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.

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.

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.

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.

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

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)

Colloidal particles are large enough to scatter light passing through the mixture. This scattering of light makes the path of light visible, which is known as the Tyndall effect.

Four instances of observing the Tyndall effect:

1. Sunlight passing through the canopy of a dense forest filled with mist.
2. A beam of torch light visible in a foggy room.
3. Sunlight entering a dark room through a window and becoming visible due to dust.
4. Light passing through a glass of diluted milk.