The outer layer of the earth is formed of soil. So when we fall on the ground, our clothes get dirty because of soil. Its thickness varies from few millimeters to several meters. Soil formation occurs as a result of a gradual breakdown of rocks. Rocks are broken down into finer particles through many processes such as weathering and erosion. The geological components are mixed with organic materials to form soil. This implies that it is a mixture of rocks which are broken into smaller particles and the dead and decayed organic matter along with the little microbes.

Plants do not grow in the sand because pure sand is only rock particulate matter and it does not contain the minerals required for the proper growth of plants. Sand can be converted into soil by adding organic matter to it. There are two main components of soil, the rock particles and the organic matter. Soil also has a lifecycle of its own. Formation of soil starts with the disintegration of rocks under certain environmental conditions.

Weight is the measure of the force of gravity acting on a body.

The formula for weight is given by:  w = mg

As weight is a force its SI unit is also the same as that of force, SI unit of weight is Newton (N).

The transmission of characters from parents to their offsprings is called heredity. The units of heredity are genes

Given mass of nitrogen = 56g

Molar mass of nitrogen = 14g

No. of moles of nitrogen = given mass of the element/ molar mass of the element = 56g/14g = 4 moles

Equal number of moles of all the susbtances contain equal number of molecules.

So, 4 moles of nitrogen and 4 moles of oxygen contains same no. of molecules.

Hence, mass of 4 mole of oxygen = 4 × 16g = 64 g

We know molar mass of water is 18g.

Molecules in 18g are 6.022 x 10²³
Therefore in 9 gm are

6.022 x 10²³ /2 = 3.011 x 10²³ molecules

• Cellulose is a complex substance and provides structural strength to plants.
• Cellulose provides rigidity to the plant cell and helps it to withstand in dilute medium.

Meristematic tissues have capacity to divide qhere permanent tissue do not have.

•Meristematic tissue have thin cellulose wall whereas permanent tissue has thick cellulose wall.

•Meristematic tissue do not have intercellular space whereas permanent tissue has large intercellular space.

•Meristematic tissues are responsible for the primary and secondary growth of the plants whereas permanent tissue is responsible for various growths.

•Meristematic tissue has dense cytoplasm with prominent nucleus whereas permanent tissue has thin cytoplasm with normal nucleus.

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Non-homogeneous systems in which solids are dispersed in liquids, are called suspensions. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye. Soil solution is an example of suspension.

In the vegetation area it s very hard almost to impossible to experience the soil erosion because the soils are not exposed to the agents of the soil erosion

The regions with bare vegetation,mountains and hilly regions soil erosion is very difficult to revert.

(i) We can conserve water by turning off the tap when running water is not necessary. This helps prevent water shortages and reduces the amount of contaminated water that needs treatment.

(ii)We should be careful about what we throw down our sink or toilet.

(iii)We should use environmentally household products, such as washing powder, household cleaning agents and toiletries.

(iv)We should take great care not to overuse pesticides and fertilisers. This will prevent runoffs of the material into nearby water sources.

(v)We should not throw litter into rivers, lakes or oceans. Help clean up any litter we see on beaches or in rivers and lakes, make sure it is safe to collect the litter and put it in a nearby dustbin.

The two forms of oxygen found in the atmosphere are:

(i) Elemental oxygen is normally found in the form of diatomic molecule (O₂) in the lower part of atmosphere. It is about 21% in the air and non-poisonous.

(ii) Ozone is found in the stratophere part of atmosphere. It contains three atoms of oxygen (O₃). It is the poisonous form of oxygen.

(iii) Some other forms of oxygen are also found in the combined state. In Earth's crust, it is found as the oxides of most metals and silicon and also as carbonate, sulphate, nitrate and other minerals. In other forms, it is the part of biological molecules like carbohydrates, proteins, fats and nucleic acids, etc.

The pesticides and insecticides sprayed  in the fileds are carried away by rain water. These chemicals are washed away with the rain or irrigational water into the nearby water bodies like ponds, lakes or rivers.

The main reservoir of gaseous carbon is in the form of carbon dioxide gas, CO₂. Oceans are the main depositors of biological carbon.

The Ozone Layer is a planetary sunscreen and it plays a vital part in protecting us from most of the sun 's harmful radiation.The ozone layer prevents much of the ultraviolet (UV) light from the sun from reaching the Earth. Ultraviolet light strikes oxygen molecules in the stratosphere - the layer of the atmosphere located directly above the one we breathe, which includes a thin ozone layer - and splits them into two oxygen atoms. Each of these atoms, when it encounters another oxygen molecule, combines with the molecule to make ozone. The UV light also breaks the ozone into an oxygen molecule and an oxygen atom. This is called the ozone-oxygen cycle, and it protects the Earth from UV radiation by converting the radiation to heat.

Nodules (Gall like structures) are found in the roots of legumes that contain nitrogen fixing bacteria.

Two factors that contribute to the weathering of rocks:
1) Physical factors - these include factors that contribute the mechanical breakdown of rocks. These factors include; action of frost, wind and rainfall.

2) chemical factors - this involves the chemical changing of the structure of the rocks. Chemicals such as carbonic acid formed from the reaction of CO2 and rain, dissolve or weaken the rocks to cause weathering.

Two factors that replenish water back in the environment are:
1- Rain - It helps the ground to recharge by seeping of water
2- Evaporation - By evaporation water evaporates and forms clouds

Chlorofluorocarbons have two major effects in the atmosphere.
1. Greenhouse effect - Chlorofluorocarbons act as greenhouse gases and are considered major contributors to the greenhouse effect. CFCs have created both unnatural atmospheric cooling and warming.
2. Ozone-depletion effect - Chlorofluorocarbons are not destroyed in the lower atmosphere but waft slowly upward toward the stratosphere where they finally break down. Each of the chlorine or bromine atoms released in that breakdown is capable of destroying tens of thousands of ozone (O3) molecules - thus contributing to the thinning of the protective ozone layer. The loss of ozone allowed more UV light to pass through the stratosphere at a sufficient rate to warm the lower troposphere. The UV rays also cause considerable harm to all living organisms on Earth.
Even though chlorofluorocarbons emissions are being stopped today, the destruction of the stratospheric ozone layer will still continue, because chlorofluorocarbons already released will move slowly up to the stratosphere and destroy ozone for the next 60 to 100 years.

Alternative energy encompasses all those things that do not consume fossil fuel. They are widely available and environment friendly. They cause little or almost no pollution. Examples - wind energy, hydroenergy, solar energy, geothermal energy, etc.

• Green plants convert C0₂ into glucose in the presence of sunlight by the process of photosynthesis.
• Many marine animals use carbonates dissolved in sea water to make their cells.

An increase in the carbon dioxide content in the atmosphere would cause more heat to be retained by the atmosphere and lead to global warming. The global warming is quite dangerous as it tends to melt polar ice and glaciers existing on mountains, rise in the water level of the oceans and submerge several coastal areas and islands. This could further lead to floods.