Chemicals like paints, solvents and insecticide should not be thrown down the drain as they may kill microbes that help to purify water.

A convex lens bends inwards the light falling on it. It is called a converging lens. On the other hand, concave lens bends outwards the light and is called diverging lens.

To digest their food completely.

The farmers sometimes loosen the soil. This helps the root to respire through the air present in the space between soil particles.

The substances which pollute the soil are
polythene bags, plastics, waste products, chemicals, pesticides, etc.

It is true that when a candle burns, both physical and chemical changes occur. initially, candle burns to produce heat and light. It is a chemical change because candle burns with the formation of gases. Due to the heat produced, some candles simultaneously melts. This is a physical change as the molten candle wax behaves like original candle.

Difference between an open circuit and a closed circuit

There are various reasons for which we should be vigilant about matters related to forests:
(i) A decrease in forest area would lead to an increase in carbon dioxide in air. This will lead to an increase of earth's temperature.
(ii) Soil erosion would occur if there are no forests.
(iii) Floods would occur if there are no forests
(iv) Forests provide shelter and food to the living organisms.

The heart acts as a pump for the transport of blood. The human heart is divided into four chambers. The upper two chambers are called right and left atrium and the lower two chambers are called the right and left ventricles. The right side of the heart, i.e. the right auricle and ventricle receive carbon dioxide rich blood from all parts of the body and transport it to the lungs.
Its left side, i.e. the left auricle and ventricle, receive oxygen-rich blood from the lungs which is transported it to all parts of the body.

The swallowed food moves downwards in the alimentary canal because of the alternate relaxation contraction movement of muscles in the wall of foodpipe called peristalsis.

Substances that allow heat energy to flow through them are called good conductors of heat. e.g., metals. Substances that do not allow heat energy to flow through them are insulators. e.g., wood, plastic, glass, etc.
(a) Practical applications of good conductors of heat:
(i) Cooking utensils are made of metals so that heat can easily get transferred to food.
(ii) Coils of refrigerators and air conditioners are made of copper to conduct away heat.
(iii) Mercury is used as a thermometric liquid in thermometers.
(b) Practical applications of insulators:
(i) Handle of cooking utensils are made of insulators like bakelite, wood, etc.
(ii) Woollen clothes are insulators and keep us warm in winter.
(iii) Hair and fur in animals are insulators. So they keep them warm during winter.

Take a funnel. Take a filter paper, fold and place it as shown in the figure. Weigh 50 gm of dry, powdered soil and pour it into the funnel. Measure a certain amount of water in a measuring cylinder and pour it drop by drop on the soil. Do not allow the water to fall at one spot. Pour water all over the soil.

Keep pouring water till it starts dipping. Subtract the amount of water left in the measuring cylinder from the amount you started with. This is the amount of water retained by soil.
Record it in this manner:
Weight of soil = 50 mg
Initial volume of water = U mL
Final volume of water = V mL
Volume absorbed by the soil = (U- V) mL
Weight of water absorbed by the soil = (U - V) gm
because weight of 1 mL of water = 1 gm
Percentage of water absorbed = \(\frac { U-V }{ 50 } \times 100\)
Repeat this activity with different samples of soil. You observe that different soils have different absorbing capacity of water.

The forest acts as a natural absorber of rain water and allows it to seep. It helps to maintain the water table throughout the year. Forests not only helps in controlling floods but also help to maintain the flow of water in the streams so that we get a steady supply of water. On the other hand, if trees are not present rain hits the ground directly and may flood the area around it. Heavy rain may also damage the soil. Roots of trees normally bind the soil together, but in their absence the soil is washed away or eroded.

 

(i) Plane mirror:
The image formed by a plane mirror is always erect. It is virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it. In an image formed by a mirror, the left side of the object is seen on the right side in the image, and right side of the object appears to be on the left side in the image.
(ii) Concave mirror:
A concave mirror can form a real and inverted image. When the object is placed very close to the mirror, the image formed is virtual, erect and magnified.
(iii) Convex mirror:
Image formed by a convex mirror is erect, virtual and smaller in size than the object.
(iv) Concave lens:
A concave lens always forms erect, virtual and smaller in than the object.
(v) Convex lens:
A convex lens can form real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. When used to see magnified objects, the convex lens is called a magnifying glass.
The image is virtual, erect and magnified.

(a) Respiration is the process of breakdown of food to produce energy for performing various functions. It involves two major steps:
(i) Inhalation is the process of taking in of air which is rich in oxygen, inside our body.
(ii) Exhalation is the process of giving out of air which is rich in carbon dioxide from our body to the external environment.
(b) The number of times a person breathes, in a minute is called as breathing rate. An adult human being can inhale or exhale 15-18 times in a minute. It is the average breathing rate of an adult human being. Breathing rate is not constant always.