13.Changes- Physical and Chemical

Question 1:

 Distinguish between the following.
 (a) Physical change and chemical change
 (b) Periodic change and non-periodic change
 (c) Natural change and man-made change

ANSWER:

a. 

Physical Change	Chemical Change
The chemical composition of a substance does not change	The chemical composition of a substance does not remain same
Easily reversible	Not easily reversible
No new substance is formed	New substances are always formed
Melting of ice is an example of physical change	Burning of paper is an example of chemical change

b.

Periodic changes	Non periodic changes
1. Events that repeat themselves after a fixed period of time and whose ocurence can be predicted are called periodic changes.	The events that do not repeat themselves after regular interval of time are called non periodic changes.
2. Example- rotation of earth, revolution of earth, day and night, seasons etc.	Example- natural disasters like earthquake, cyclones etc.

c.

Natural changes	Man made changes
1. The changes which are brought about by nature itselfand are not under the control of humans are called natural changes.	The changes which are brought about by humans are called man made changes.
2. Examples- change of season, tides in sea, ageing of plants etc.	Examples- burning of fuels, cutting hair, switching on lights etc.

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Question 2:

Under which different types can the following changes be classified?
(a) Conversion of milk into yoghurt.
(b) Bursting of a cracker.
(c) Occurence of an earthquake.
(d) Revolution of the earth around the sun.
(e) Stretching of a spring.

ANSWER:

(a) Conversion of milk into yoghurt- Chemical change
(b) Bursting of a cracker- Chemical change
(c) Occurence of an earthquake- Natural change
(d) Revolution of the earth around the sun- Periodic change
(e) Stretching of a spring- Physical change

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Question 3:

Give reasons.
(a) While purchasing canned food, its expiry date should be checked.
(b) An iron article should be given a coat of paint
(c) A wooden article should be polished.
(d) Copper and brass utensils should be tinned.
(e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.

ANSWER:

a. While purchasing canned food their expiry date should be checked because an expiry date indicates that for how long a canned food is safe for consumption. Canned foods with expired dates are not considered worth consumption as they could affect your health. They might get contaminated with microorganisms like bacteria or may have no nutritional value left after its expiration.

b. Iron articles often undergo rusting resulting in the formation of a reddish brown layer on its surface. Rusting is caused by the exposure of iron to the air and water in the surroundings. An iron article which is coated with a paint prevents the interaction of iron with the surrounding air and water and thus protects it from rusting.

c. A wooden article should be polished because it protects the wooden article from sunlight and all other external conditions and also it reduces the probability of wooden mites damaging the furniture. 

d. Copper and brass materials also undergo corrosion resulting in the formation of a greenish layer on this surface. When the surface of copper/ brass comes in contact with air or water, it results in corrosion and to prevent this, the process of tining is used. In this process copper and brass articles are coated with tin.

e. A dry handkercheif gets wet at once because it quickly absorbs the water in which it is dipped. However, it takes longer for a wet handkerchief to dry because the water needs to evapourate from the handkerchief. Evaporation is a slow process which is dependant on other factors as well like relative humidity.

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Question 4:

What will you take into account while identifying the following?
(a) A physical change in a substance.
(b) A chemical change in a substance.

ANSWER:

a. Physical changes are those changes in which only the forms of substances get modified; the chemical natures and compositions of the substances involved are not altered.

Characteristics:

• No new substance is formed during a physical change.
• Most physical changes can be reversed easily.
• The chemical composition of a substance undergoing physical change remains the same.

b. Chemical changes are changes that involve reaction of substances with one another. Such reactions result in alterations in the chemical compositions of the substances involved. These changes lead to the formation of new substances.

Characteristics:

• One or more new substances are formed during a chemical change.
• Most chemical changes cannot be reversed easily.
• The chemical composition of a substance undergoing chemical change does not remain the same.
• A chemical change is always accompanied by a change in energy.

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Question 5:

Read the paragraph and note down the various types of changes it mentions.
It was nearing six o'clock in the evening. The sun was setting. A breeze was blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys. Then he felt hungry and went into the house. Mother made a dough from wheat flour and fried purees. While eating hot purees, his attention was drawn outside the window. It had started raining. There was lightning, too. Sahil was enjoying his dinner in the dim light.

ANSWER:

It was nearing six o'clock in the evening (Periodic change). The sun was setting (Periodic change). A breeze was blowing (Natural change). Leaves on the tree were shaking (Physical change). Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys (Physical change). Then he felt hungry (Chemical change) and went into the house. Mother made a dough from wheat flour and fried purees (Irreversible change). While eating hot purees, his attention was drawn outside the window. It had started raining (Natural change). There was lightning (Natural change), too. Sahil was enjoying his dinner in the dim light.

Cell structure and Micro-organisms STD. 7

Question 1:

Answer the following questions.
(a) What is a 'cell' ?
(b) Name the different organelles in a cell ?
(c) What are micro-organisms ?
(d) which are the different types of micro-organisms ?

ANSWER:

a. Cell is the smallest unit of life. They are the structural, functional and biological uits of life. The discovery of cells was first made by Robert Hooke. While examining a section of a cork under the microscope, he observed small compartment-like structures and named them cells. However, Hooke observed dead cells under the microscope as cork is made up of dead cells. Cells exhibit different types of properties like:

• It is the smallest living unit of life.

• It is so small that it is not visible to the naked eye.

• The shape of the cell varies in different organisms and within an organism.

• Size of cells also differs.

• All living cells exhibit certain basic characteristics like respiration, growth, metabolism, etc.

• Cells originate from a pre-existing cell. A mother cell divides to produce daughter cells. Hence, cells exhibit cell division.

b. The cells may conatin the following cell organelles depending upon wether it is a plant or animal cell:

Cell membrane, cell wall, vacuole, plastids, endoplasmic reticulum, chloroplast, golgi apparatus, ribosomes, mitochondria, nucleus, lysosomes, centrosome and inclusion bodies.

c. Living organisms which are not visible to the naked eye are known as micro-organisms. They are living organisms that can be seen only with a microscope or a magnifying glass. Microorganisms were observed for the first time by Anton von Leeuwenhoek in 1674, using a microscope of his own.

d. Microorganisms can be divided into 5 major groups like:

Bacteria- Bacteria are the most primitive and diverse unicellular organisms found in living world. They are prokaryotic in nature as their genetic material, i.e. DNA, is not enclosed within a nuclear membrane. Because of their unique biochemical pathways and properties, they are of high economic importance to the human beings.

Fungi- Fungi is a group of eukaryotic, unicellular as well as multicellular, non-photosynthetic organisms that includes mushrooms, moulds and yeasts. These organisms have a rigid cell wall made up of chitin and are found in diverse shapes and sizes.

Algae- Algae are a diverse group of aquatic organisms that have the ability to conduct photosynthesis.

Protozoa- Protozoa are a group of unicellular micoorganism which are non photosynthetic in nature. Many protozoa cause disease in animals and humans.

​Viruses- Viruses are ultra microscopic and non cellular organisms. They have simple structure having genetic material coated by protein coat. They are obligate parasites and highly pathogenic.

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Question 2:

Fill in the blanks with the proper word.
(a) The organelle called the ........... is present in plant cells only.
(b) Garbage is converted into .......... by micro-organisms. 
(c) In the cell, photosynthesis is carried out with the help of ............ .
(d) An electron microscope is necessary for the study of ............ .

ANSWER:

(a) The organelle called the chloroplast is present in plant cells only.
(b) Garbage is converted into compost by micro-organisms. 
(c) In the cell, photosynthesis is carried out with the help of chloroplast.
(d) An electron microscope is necessary for the study of cells .

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Question 3:

What is difference between us ?
(a) Plant cell and animal cell.
(b) Prokaryotic cell and eukaryotic cell.

ANSWER:

 a. 

	Animal cell		Plant cell
i.	Animal cells are smaller in size.	i.	Plant cells are comparatively larger.
ii.	They lack a cell wall.	ii.	Cell wall is the outermost structure in a plant cell.
iii.	They lack plastids except Euglena.	iii.	Plastids are present in all plant cells.
iv.	Many vacuoles are present and they are smaller in size.	iv.	They have a single large central vacuole.
v.	They have centrioles.	v.	They lack centrioles.

b. 

Characteristics	Prokaryotes	Eukaryotes
Size of the cell	Cells are small in size.	Cells vary in size and are generally larger than those in prokaryotes.
Nucleus	No nucleus with a nuclear membrane is present.	There is a well-defined nucleus with a nuclear membrane.
Membrane-enclosed organelles	Organelles having a membrane around them (e.g., mitochondria, plastids) are absent.	Membrane-enclosed organelles are present.
Cell wall	Cell wall is usually present. It is composed of peptidoglycan.	Cell wall is usually present in plant cells. It is composed of cellulose.
Genetic material	The genetic material is present as nucleoid, i.e., a properly defined nucleus is absent.	The genetic material is present inside the well-defined nucleus.

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Question 4:

Sketch and describe in your own words, the plant cell and animal cell.

ANSWER:

Diagram

Plant cell
 

• Cell membrane: A cell is composed of cell membrane, cytoplasm and nucleus. The cytoplasm and nucleus enclosed in the cell membrane together constitute the plasma membrane. It checks the transport of substances in the cell.

• Cell wall: In plants, an extra protective covering known as cellulose is present. It is called cell wall and protects the plant cell from environment variations.

• Cytoplasm: It is a jelly-like substance present between the cell membrane and nucleus. It contains various cell organelles such as mitochondria and Golgi body.

• Nucleus: It is a dense spherical body located at the centre of the cell. It is surrounded by a porous nuclear membrane. It contains a spherical body called nucleolus and thread-like structures called chromosomes. Chromosomes are the structures that carry genes and play an important role in inheritance. The entire living substance in a cell is known as protoplast.

• Vacuoles: Vacuoles are fluid-filled membrane-bound structures in the cell. In plant cells, a single large vacuole is present. In animal cells, numerous small vacuoles are present.

• Plastids: They are present only in plant cells. Chloroplast is a plastid containing the green pigment called chlorophyll, which is required for photosynthesis.

Mitochondria: They are involved in cellular respiration, and hence, are called ‘power houses of the cell’.

Animal cell

Diagram 


All the cell organelles in plant and animal cells are same except that animal cells have plasma membrane in place of cell wall and it does not contain chloroplasts.

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Question 5:

Explain the uses and the harmful effects of micro-organisms.

ANSWER:

Microorganisms have found widescale application in various industries. Some of their applications are:

Use of Microorganisms in Food Industry

(i) Lactobacillus bacterium promotes the conversion of milk into curd.

(ii) Yeast is used in preparation of breads, pastries, and cakes.

Use of Microorganisms in Vaccine Production

(i) Protection of the body from the attack of various disease-causing microorganisms through vaccines is known as vaccination.

(ii) Vaccine includes dead or weakened microbes that trigger the production of antibodies in the body.

Use of Microorganisms in Increasing Soil Fertility

(i) Blue green algae and Rhizobium are called biological nitrogen fixers.

(ii) They fix atmospheric free nitrogen to enhance soil fertility.

These are just few examples which highlight the commercial importance of these microorganisms. However, they are not always beneficial can lead to various kinds of damage. This damage can be in terms of health of humans and animals, crop production etc. There are various kinds of microorganisms which are known to cause disease in humans and animals. 
 

Human disease	Pathogen
Tuberculosis Measles Chicken pox Polio	Bacteria Virus Virus Virus
Cholera Typhoid	Bacteria Bacteria

Similarly there are certain species of microorganisms which cause disease in plants and affect the crop productivity. 
 

Plant disease	Pathogen
Citrus canker	Bacteria
Rust of wheat	Fungi
Yellow vein mosaic of bhindi (okra)	Virus

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Question 6:

Give reasons.
(a) Diseases spread on a large scale during periods of heavy rainfall and floods.
(b) There is a possibility of food poisoning if we eat stale food.
(c) Soil is turned over during tilling.
(d) Fungus grows quickly in moist or humid conditions.
(e) A refrigerator is used in almost every home.
(f) Bread 'rises' during baking.
(g) Fodder is soaked in water before offering to cattle.

ANSWER:

a. Diseases spread on a large scale during periods of heavy rainfall and floods because it provides the optimum conditions for the growth of vectors which transmit diseases. After heavy rainfall and floods, water gets accumulated at places and acts as breeding grounds for mosquitoes and other vectors. These vectors lead to transmission of the microorganisms to their host and result in the development of disease.
For example, malaria is caused by a virus which is transmitted by a mosquito.

b. There is a possibility of food poisoning if we eat stale food because there are chances that it can be infested by microorganisms. Bacteria and fungi can infect the foods which are stored for longer durations of time. For example, the greenish powdery substance which is seen on stale breads is an example of fungus infection.

c. Tilling is a mechanical procedure of agitating soil where the top layer is mixed with the lower layers of soil. This promotes the mixing of organic matter throughout the soil and also promotes the growth of microorganism. These microorganisms help in the conversion of organic matter.

d. Fungus grows quickly in moist or humid conditions because these are the favourable conditions for its growth. Fungus is not seen growing in areas which are well exposed to sunlight.

e. Refrigerator is used in almost every home because it prevents the spoilage of food. Refrigerator provides an optimum environment for storage of vegetable, fruits, cooked and uncooked food. If an optimum temperature is not provided, it can lead to the growth of bacteria and fungi thus leading to spoilage of food.

f. Bread rises during baking because of addition of yeast to its dough. Yeast cells respire anaerobically and liberates CO₂ in the process. When yeast cells are added to the bread dough, they utilise the nutrients (carbohydrates) found in the dough and respire anaerobically. Also, they multiply at a very fast rate under such favourable conditions. As a result of the growing bacterial population, large amount of CO₂ is released that makes the dough very soft and fluffy, and thus improves the quality of the bread. 

g. Fodder is soaked in water before offering it to cattles because it may contain seeds which have been left behind after harvesting. Soaking this fodder helps in the sprouting of these seeds/grains and thus increases the nutritional value of the fodder. 

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Question 7:

When will you use a simple microscope and when, a compound microscope? Explain in detail how you will use them.

ANSWER:

The simple microscope is generally considered to be the first microscope which was created by Antony van Leeuwenhoek in the 17th century. It was essentially a type of which was used by Leeuwenhoek to gain information about biological specimens, including the difference in shapes between red blood cells. Today, simple microscopes are not used often because the introduction of compound microscopes which have better resolution and magnifying power.

Compound microscope offers better magnification than a simple microscope. These devices provide a magnification of 1,000 times. It can be used to observe micoorganisms like bacteria, fungi, protozoa etc. It can be used in their detailed study, for example studying their structures.