The process of the classification of objects based on some known criteria is called grouping or sorting.
We have a huge variety of objects present around us. Because of this, it is very important to classify objects into groups. If we know the properties of one member of the group, it would be very easy to predict the properties of the other members too.
Some of the properties of materials are:
Appearance
The various parameters governing the appearance of materials are colour/texture, roughness, shape, size, etc.
Lustre
Some materials shine when light falls on them. This property is generally observed in objects that are grouped as metals, e.g. gold and silver. They are hard, strong, flexible and good conductors of heat and electricity. They can be drawn into wires (ductile) and sheets (malleable).
Hardness
It is the measure of how resistant a material is to various kinds of shape changes when a force is applied. Hardness can also be measured by the ease with which a material can be compressed or stretched. Materials that can be easily stretched and compressed are called soft materials. Materials which are difficult to compress and stretch are called hard materials.
Solubility
In general, solubility is defined as the ability of a substance to dissolve in water. In the process of dissolving/the substance which is being dissolved is called a solute and the substance in which the solute is dissolved is called a solvent. A mixture of solute and solvent is called a solution, e.g. if we add sugar to water, then, sugar is the solute and water is the solvent. The mixture of sugar and water is called sugar solution. Substances which are not soluble in water are called insoluble substances, e.g., sand, wood. If two liquids mix together completely then they are called misdble liquids, e.g. water and ink. If two liquids do not mix but form separate layers, then they are called immiscible liquids, e.g., water and oil.
Relative density or Density with respect to water
\[\text{Relative density =}\frac{\text{Mass of substance}}{\text{Mass of water}},\,\,\,\text{Density=}\frac{Mass}{Volume}\]
The density of water at \[{{4}^{o}}C\] is taken as 1. Substances having relative density of more than 1 tend to sink in water and those having less than 1 tend to float on water.
Behaviour towards light
Substances can be divided into three groups depending on their behaviour towards light.
Transparent materials
They allow most of the light to pass through them, e.g., glass.
Translucent materials
They allow only some part of the light to pass through them, e.g., butter paper.
Opaque materials
They do not allow light to pass through them, e.g. iron sheet or wood.
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Changes can also be classified based on the rate at which they take place.
Fast change: The change that takes place within a very short time.
Slow change: The change that occurs over a long period of time.
Another factor for change is uniformity with respect to time.
Periodic changes: Changes that repeatedly occur at fixed intervals of time e.g., change of seasons, formation of day and night.
Non-periodic changes: Changes that do not repeatedly occur at fixed intervals of time are non-periodic e.g., occurrence of storms and rain.
Reversible change: A change in which the original substances can be retrieved. e.g., mixing of iron particles and sulphur.
Irreversible change: A change in which a new substance is formed and the original substances cannot be retrieved, e.g., heating a mixture of iron pieces and sulphur.
Exothermic reaction: A change in which heat is released e.g., burning of coal.
Endothermic reaction: A change in which heat is absorbed e.g., evaporation of water.
When substances react with each other, they require the right conditions like temperature, pressure, light, catalyst, etc. The rate and the extent of change depends on these conditions.
Unstable substances undergo change easily.
Differences between physical and chemical changes:
Physical Changes
Chemical Changes
1. Change is temporary.
1. Change is permanent.
2. It can be reversed.
2. It cannot be reversed by simple, chemical or physical means.
Water is one of the vital components required for life and is called the compound of life.
Water has many properties that makes it unique and vital.
The primary source of water on land is rain water.
Water in its various forms is very abundant. About two-thirds of the earth is covered with water but most of it is not fit for drinking.
Rain, hail and snow replenish water in rivers, lakes, ponds, wells and soil.
Clouds are formed by floating droplets of water condensed in the higher layers of the air.
The exchange of water between land, atmosphere and water bodies is called water cycle.
In the water cycle, water gets purified by the processes of evaporation and condensation.
All the forms of water undergo changes continuously owing to water cycle and become potable (water suitable for drinking).
Water (from rain and snow) gets collected in the rivers, and ground water is the main source of potable water.
Ordinary water contains many other substances (minerals and salts) in addition to the various inpurities that are essential for life. This is because a wide variety of substances have affinity for water. Rarely do we find or use pure water.
Water is not only used for drinking but also for various other purposes like in the cleaning, in the preparation of food by plants, separation or recovery of pure substances, sustaining aquatic life etc.
Salinity, pollutants, bacteria, etc. make most of the water found in nature unfit for drinking.
Plants also lose water into the air through a process called transpiration.
Excessive rains may cause floods while lack of it for long periods may cause droughts.
The process of collecting rain water and storing it for later use is called rain water harvesting.
Conservation by recycling storage water, rain water harvesting, afforestation and other methods restore the balance of water on the earth.
Deforestation, excessive use of ground water, reduction in the absorption of water by the soil and other factors cause an imbalance in rainfall and the availability of water, leading to many natural disasters.
Just like soil and water, air is also a part of the earth. Gases like hydrogen, oxygen, nitrogen, carbon dioxide and many other gases together form a mixture called air.
Air and some other components form a very thin layer over the earth's surface. This layer is called the atmosphere.
The different gases present in the air have unique properties and play a vital role in the creation and evolution of life.
Air cannot be seen because it is transparent, colourless but it occupies space. When air moves, it is called wind which can be felt.
The blanket of air surrounding the earth is the atmosphere.
As air is a mixture, all the constituent gases have their original properties and they can be recovered separately. One such process of obtaining nitrogen from air includes liquification (cooling) followed by fractional distillation.
Nitrogen makes up the major part of air. It is not required by us directly, but it is used in the form of compounds. It is essential for plant and animal growth and has various other uses.
About one-fifth of air is made up of oxygen. It is required by all living organisms for breathing.
Plants release oxygen into the air through a process called photosynthesis.
The other important constituents of air are carbon dioxide, water vapour and inert gases.
Water vapour plays an important role in the water cycle. The amount of water in the air is a measure of the humidity of that place.
Carbon dioxide is required by plants to prepare their food by the process of (photosynthesis).
Air is also present in soil and water.
Air is necessary for life as we breathe it. Organisms living on land directly consume oxygen from the air.
Oxygen in the air is continuously being used in burning and breathing (respiration), and carbon dioxide is being added to the air.
Green plants absorb carbon dioxide from the air and make food by combining it with water and energy from sunlight with the help of chlorophyll and release oxygen into the air.
Air plays a major role in sailing of boats/communication/wind mills, generation of electricity, water cycle, etc.
Earthworms come out of the soil after heavy rain as they cannot breathe in waterlogged soil.
After heavy downpour, ants also carry their eggs from their anthills to safe places.
The substances which we eat every day, containing various edible components that provide us energy and materials which help our body to grow and remain healthy are called food.
The materials that are needed to prepare a particular type of dish are called ingredients.
The substances which one can eat are called edible.
The germinating seeds, in the initial stage produce a white structure which grows out of the seed (radicle), are called sprouted seeds.
The sweet juice produced by some flowers is called nectar.
Honeybees collect nectar (sweet juices) from flowers and convert it into honey. Bees store it in beehive for future use.
The edible substances which we eat to obtain energy, materials required for growth, maintenance, development and to repair worn out tissue in our body is called food.
The sources of different components of food are listed below
Food on the basis of their role in the body, are classified into three major groups.
Energy giving foods
Body building foods
Protective foods
Cereals, sugar, potato. honey, oils and fats
Milk, pulses, cheese, eggs, fish, meat
Fruits, vegetables, milk, eggs
A diet that contains adequate amount of different components of food required for the healthy functioning of our body is called a balanced diet.
Marasmus is a disease in children caused by the deficiency of proteins, carbohydrates and fats.
Kwashiorkor is a disease caused by protein deficiency in children. The stomach becomes swollen, the legs become thin and the skin show ugly patches.
The thin strands of thread which are used to make yarns are called fibres.
Yarn is made by twisting the fibres.
Fibres are of two types - natural and synthetic.
Natural fibres are obtained either from plants or animals. e.g., cotton, jute, coconut, flax, silk, wool, etc.
Synthetic fibres are man-made fibres, which are made by using chemicals. e.g., polyester, acrylic, nylon, etc.
Separating seeds from cotton bolls is called ginning.
Yarn is spun from cotton fibre using takli or charkha.
Process of spinning yarn into a fabric is called weaving.
Jute fibre is made by immersing jute stem in water for a few days.
Woollen fabric is made by knitting. It is the criss-cross laying of yarns tightly using weaving machines or knitting rods. For example, a sweater is made of a fabric (wool) made of a single yarn (thread).
Woollen fibres are obtained from the fleece or fur of animals. Silk fibres are obtained from cocoons of silkworms.
Coconut fibre is obtained from the outer covering of the coconut fruit (coconut husk).
In ancient times people used leaves, animal skins and furs to cover themselves.
Vines, animal fleece or hair are also twisted together into long strands and were woven into fabrics.
A plant system is made up of different parts of a plant working together, each performing its own function.
Based on size, we can divide the plants into herbs, shrubs and trees.
All flowering plants have two main systems - the root system which grows underground and the shoot system which grows above the ground.
Roots fix the plant firmly in the soil and absorb water and minerals from the soil.
Root systems are of two types:
Tap root system and
Fibrous root system
The shoot system consists of the main stem, leaves, buds, branches, flowers and fruits.
The leaf is a green flattened structure, meant for manufacturing food for the plant body by the process of photosynthesis. A leaf consists of three parts, leaf base, petiole and lamina.
If the veins and veinlets form a network, it is called reticulate venation. This is common in dicots. If veins and veinlets are parallel to the midrib, then it is called parallel venation. This is common in monocots.
Flower is the reproductive part of the plant. The main parts of a flower are pedicel, thalamus, calyx, corolla, androecium and gynoecium.
Androecium represents the male reproductive organs. It consists of stamens and anther lobes. Pollen grains, the male gametes are formed in the anther lobes. Gynoecium is the female reproductive organ. It consists of style, stigma and ovary. Ovary contains ovules. Female gamete (egg) is formed in the ovule.
Roots get modified to store food, as in carrot, radish, sweet potato and dahlia.
Stems get modified for storing food and water, providing support, growing new plants and manufacturing food.
The process by which water comes out from the leaves in the form of water vapour is called transpiration.
The skeleton comprises of the skull, backbone, ribs, breastbone, shoulder, hip bones and the bones of hands and legs.
Bone and cartilage form the skeleton of the human body. It gives support and shape to the body and helps in movement. It also protects the inner organs.
Cartilage is the additional part of our skeleton.
They are not as hard as bones but strong and flexible. Cartilage is present in the outer part of ear, nose tip, etc.
A joint is the place where two or more bones meet (articulate).
There are four kinds of joints in our body. They are ball and socket joint, pivot joint, hinge joint and immovable joints.
The act of moving from place to place is known as locomotion. It involves the movement of the whole body, as in walking, running, swimming or flying.
Body movements are helped by muscular contractions and relaxations.
Birds fly with the help of wings, strong muscles and light (hollow) bones.
Fish swim by forming loops alternately on two sides of the body with the help of the tail fin.
An earthworm moves with the help of its muscles. The contraction and expansion of muscles helps the earthworm to move.
A snail has a muscular foot which helps it to move.
A cockroach has three pairs of legs which help it to walk and move. It has two pairs of wings which help it to fly.
Snakes have a long back bone and many thin muscles. While moving a snake curves into many loops. Each loop gives it a forward push by pressing against the ground.
