When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f ) Digestion of food

Take three test-tubes. Fill æth of each with water. Label them A, B and C. Keep a snail in test-tube A, a water plant in test-tube B and in C, keep snail and plant both. Which test-tube would have the highest concentration of CO₂?

Sketch the cross section of soil and label the various layers.

Given below is a crossword puzzle based on this lesson. Use hints to fill in the blank spaces with letters that complete the words.
Down                                               Across
(D) 1 : Thorough washing           (A) 1 : Keeps warm
      2 : Animal fibre                            2 : Its leaves are eaten by silkworms
      3 : Long thread like structure      3 : Hatches from egg of moth

Match the items in Column I with those in Column II:
Column I                                              Column II
(i) A home for living organisms         (a) Large particles
(ii) Upper layer of the soil                 (b) All kinds of soil
(iii) Sandy soil                                   (c) Dark in colour
(iv) Middle layer of the soil                (d) Small particles and packed tight                                                                                                                                               (v) Clayey soil                                  (e) Lesser amount of humus

Why does an athlete breathe faster and deeper than usual after finishing the race?

David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be

(i) 3 m        (ii) 5 m
(iii) 6 m      (iv) 8 m

Write one similarity and one difference between the nutrition in amoeba and human beings.

Stainless steel pans are usually provided with copper bottoms. The reason for this could be that

(a) copper bottom makes the pan more durable.
(b) such pans appear colourful.
(c) copper is a better conductor of heat than the stainless steel.
(d) copper is easier to clean than the stainless steel.

An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will

(a) flow from iron ball to water.
(b) not flow from iron ball to water or from water to iron ball.
(c) flow from water to iron ball.
(d) increase the temperature of both.

A wooden spoon is dipped in a cup of ice cream. Its other end

(a) becomes cold by the process of conduction.
(b) becomes cold by the process of convection.
(c) becomes cold by the process of radiation.
(d) does not become cold.

What are decomposers? Name any two of them. What do they do in the forest?

Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation.

State the main difference between asexual and sexual reproduction.

Fill in the blanks:
(a) The main steps of nutrition in humans are __________, __________, __________, _________ and __________.
(b) The largest gland in the human body is __________.
(c) The stomach releases hydrochloric acid and ___________ juices which act on food.
(d) The inner wall of the small intestine has many finger-like outgrowths called _________.
(e) Amoeba digests its food in the ____________ .

Which one of the following is not responsbile for water shortage?

(i) Rapid growth of industries
(ii) Increasing population
(iii) Heavy rainfall
(iv) Mismanagement of water resources

Fill in the blank:

(a) The insects, butterflies, honeybees and birds help flowering plants in                        .
(b) A forest is a purifier of                           and                           .
(c) Herbs form the layer in the forest                               .
(d) The decaying leaves and animal droppings in a forest enrich the                           .

11. A virtual image larger than the object can be produced by a

(i) concave lens           (ii) concave mirror
(iii) convex mirror        (iv) plane mirror

When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

Explain how forests prevent floods.