Water, which covers around 70 percent of the Earth's surface, comes in the forms of freshwater and saltwater. The four sources of fresh water are atmospheric water (rain water), groundwater, ice water (water trapped in glaciers, ice caps, etc.), and bodies of water such as lakes, streams, rivers, and natural springs.

Ways to Prevent Soil Erosion: The four most common soil erosion prevention methods are vegetation, geotextiles, mulch, and retaining walls.
(i) Vegetation: The simplest and most natural way to prevent erosion control is through planting vegetation. Plantsestablish root systems, which in turn, stabilises the soil, preventing erosion.
(ii) Geotextiles: Using geotextiles is an effective method by itself as it stabilises soil. When used in conjunction with growing vegetation, it is even more effective. There are three main types of geotextiles: woven,nonwoven, and coir.
(iii) Mulch/Fertilizer: Applying a layer mulch to the soil top allows the soil to slowly soak up water, protects against rain impact, and restores pH levels.
(iv) Retaining Walls: Retaining walls can be built around the area of erosion to prevent water run off, which leads to further erosion. If used with other methods, this can be very effective.

The blue colour of sky is due to the scattering of light by small particles of the atmosphere (air molecules) when the light is incident on particles whose size is smaller than the wavelength of light, it is scattered.
According to Rayleigh law, the intensity of scattered light is inversely proportional to the fourth power of wavelength.
As the wavelength of blue colour is smallest and that of red light is longest, the blue light is scattered most and the red light is scattered least.
The scattered blue light reaching the eye gives the appearance of a blue sky. The sky will appear black in the absence of earth’s atmosphere because no scattering of any colour takes place in that case. While flying in an aeroplane one can observe the sky to be black at high altitudes.

Benjamin Franklin had one of the greatest scientific minds of his time. He was interested in many areas of science, made many discoveries, and invented many things, including bifocal glasses. In the mid-1700s, he became interested in electricity.
In 1752, Franklin conducted his famous kite experiment. In order to show that lightning was electricity, he flew a kite during a thunderstorm. He tied a metal key to the kite string to conduct the electricity.

An electromagnet is a magnet that is created using an electric current, created by electricity. Since electricity can be turned on and off, so can an electromagnet. It can even be weakened or strengthened by decreasing or increasing the current.
Properties of electromagnet:
1) they are very strong temporary magnets and are demagetised as soon as the current is switched off.
2) Their strength depends on three things
* Strength of the current
* Number of turns of the coil.
* Material formation its core.
3) They can be made in suitable sizes and shapes depending on the purpose.

During respiration in plants, very little amount of gas exchanges takes place within the different parts of the plants. Therefore, each part takes care of its own energy requirements. Roots, stems, and leaves of plants exchange gases for respiration separately. As we all know, leaves have tiny pores called as stomata, which is used for the exchange of gases. The oxygen, taken in through stomata is used by the cells in the leaves to break down glucose into carbon dioxide and water.
Roots also absorb air from the air spaces present between the soil particles. Thus, the oxygen absorbed through roots are used to release energy which is later utilized for the transport of minerals and salts from the soil.

Ionic Bonding : Ionic bonding occurs when one element donates an electron (or electrons) to another so that both elements will have a full outer shell.
Example: Here is an example showing lithium (which has 3 electrons and 1 in the outer shell) and fluorine (which has 9 electrons and 7 in the outer shell) donating an electron to form LiF or lithium fluoride. This is called an ionic bond.

Covalent Bonding: In covalent bonding electrons are shared between atoms rather than donated in order for the atoms of both elements to gain full outer shells. Electrons are always shared in pairs.
Example: An example of covalent bonding is the molecule of carbon dioxide. In this example carbon has 4 of 8 electrons in its outer shell and oxygen has 6 of eight electrons. By combining two oxygen atoms with one carbon atom, the atoms can share electrons such that each atom has a full outer shell.

Water cycle is the cyclic movement of water from the atmosphere to the earth and back to the atmosphere through various processes.  This constant, never ending circulation of water in nature is known as the water cycle.
The process includes:
Evaporation: Water in seas, rivers, lakes, ponds or streams evaporates because of the heat of the sun.
Transpiration: Plants give out a large amount of water through their leaves
Condensation: Water vapor rises up and condenses on dust particles to form cloud.
Precipitation: Water stored in clouds reaches the ground in the form of rain, hail or snow.

The ventricle wall is thicker as it needs to pump blood from the heart to other parts of the body (either the lungs, or other parts of the body, depending which side of the heart is concerned). its extra thickness is due to more cardiac muscle, giving a stronger force of contraction to allow the blood to pumped further, under higher pressure. Little muscle is needed to force blood from the atria into the ventricles, so the atrial walls are relatively thin.
In a human heart, the right-hand side of the heart is smaller than the left as less muscle is needed for the pulmonary cycle (blood being pumped to the lungs from the right ventricle) as the blood does not need to travel as far and under such high pressure as it does when pumped all around the body from the left ventricle.

Respiration is one of the metabolic processes which play an essential role in all living organisms. When we breathe, we inhale oxygen and exhale carbon dioxide. The process so-called exchange of gases is necessary for respiration and which can be considered as a chemical process.

Every organism, from an individual bacteria cell to a mighty blue whale, undergoes respiration.
The human respiratory system consists of the following parts:

• External nostrils – For the intake of air.
• Nasal chamber – which is lined with hair and mucus to filter the air and remove dust and dirt.
• Pharynx – It is a passage behind the nasal chamber and serves as the common passageway for both air and food.
• Larynx – Also known as the sound box as it helps in the generation of sound and thus helps us in communicating.
• Epiglottis – It is a flap-like structure that covers the glottis and prevents the entry of food into the windpipe.
• Trachea – It is a long tube passing through the mid-thoracic cavity.
• Bronchi – The trachea divides into left and right bronchi.
• Bronchioles – Each bronchus is further  divided into finer channels known as bronchioles.
• Alveoli – The bronchioles end up into the balloon-like structures known as the alveoli.
• Lungs – We have a pair of lungs, which are sac-like structures and covered by a double-layered membrane known as pleura.

Anaerobic respiration is the breakdown of glucose in the cell in the absence of oxygen. Glucose is converted into alcohol and carbon dioxide with release of little amount of energy.
Anaerobic respiration can also be seen in yeast. The breakdown of glucose in the absence of oxygen in yeast forms alcohol, energy and carbon dioxide. Yeasts and other organisms that respire in the absence of oxygen are also known as anaerobes. As yeast is a unicellular organism that respires anaerobically to yield alcohol, it is used in the production of beer and wine. Yeast is used in beverage industries to produce beer and wine by the process of fermentation. Fermentation is the process of breaking down glucose into alcohol and carbon dioxide releasing little amount of energy. Fermentation is otherwise called as anaerobic respiration.
Anaerobic respiration is observed in muscle cells. Accumulation of lactic acid inside the cells leads to muscle cramps. Supply of oxygen will break down lactic acid into carbon dioxide and water. Hot water bath relieves muscles from cramps by converting lactic acid into carbon dioxide and water.

White blood cells only live a short life: only a few days to a few weeks. A drop of blood contains an average of between 7,000 and 25,000 white blood cells at a time.
 Lists two important functions of white blood cells?
Functions of the white blood cells
White blood cells can:
(i)  ingest pathogens and destroy them.
(ii)  produce antibodies to destroy pathogens
(iii) produce antitoxins that neutralise the toxins released by pathogens

Transportation in Unicellular Organisms: In unicellular organisms, transportation of substances happens through diffusion and osmosis. Gases move in and out of the cell by diffusion. Other substances move by osmosis.
Diffusion: Random motion of particles in order to attain equilibrium of concentration is called diffusion. Diffusion can be observed in many aspects of day to day life. The aroma of food comes from the kitchen because of diffusion. A pleasant smell of flowers comes because of diffusion. Bad odour of garbage comes because of diffusion.
Osmosis: Movement of water through a semi-permeable membrane from high water concentration to low water concentration is called osmosis. Osmosis is a type of diffusion. Cell membrane is a semi-permeable membrane. Substances move across the cell membrane because of osmosis. Seeds swell up; when soaked in water; because of osmosis.