Class 7 || Science || Ch. 6 PHYSICAL AND CHEMICAL CHANGES
Q. What do you mean by the physical properties of the substance?
Ans.
Physical Properties: Appearance, texture, Shape, Size, colour, odor, melting point, boiling point, density, solubility, State (solid, liquid and gas) of substance and many others are known as physical properties.
Physical properties can be observed or measured without changing the composition of matter. Physical properties are used to observe and describe matter.
Some other physical properties :– brittleness, opacity, luster, boiling point, melting point, color, concentration, density, ductility, hardness, elasticity, plasticity, electric charge, electrical conductivity, flow rate, fluidity, intensity, luminance, malleability, permeability, resistivity, solubility, specific heat, temperature, tension, pressure
Q. Give some examples of the change took place around us.
Ans. Growth of tree, rising of sun, setting of sun, different shape and size of moon, burning of coal, paper, wood, etc. are the examples of changes around us.
Q. What do you mean by the physical change, explain with example?
Ans. A change in which a substance undergoes a change in its physical properties is called a physical change. A physical change is generally reversible. In such a change no new substance is formed.
For example; converting of water into the ice and water vapour are physical change.
Q. What do you mean by the Chemical properties of the substance?
Ans. Chemical Properties: The internal properties like heat of combustion, reactivity with water, PH, and electromotive force, Toxicity of a substance are known as chemical properties.
Q. How can you say that a change took place?
Or
How do we come to know that a change has taken place?
Ans. By examining the change of colour, state and temperature of substance (or product), or excretion of gases, smoke, vapour or bubbles or forming of any type of precipitate shows, then we can say the chemical change took place.
Q. What are main types of changes?
Ans.
Change: Which process alters the specific properties of a mater is called change.
There are two types of change:
01. Chemical Change
02. Physical Change
Q. What do you mean by Chemical change?
Ans. Chemical Change: In which reaction a new substance is formed is known as chemical change. This change altars the chemical property of a substance.
OR
Any change in the chemical composition and its constituent or molecules of the substances is called chemical change.
2H2 + O2 → 2H2O
In the chemical change or reaction reactants converted into the products. We cannot get the reactant from product. Like burning of candle, turning the milk into the curd.
Q. What are Reactant and Product in a chemical reaction?
Ans. Look at the following equation where A and B are Reactants and C and D are Products.
Reactant: The substances took place in a reaction is known as reactant. In the given equation (2H2 + O2 → H2O) H2 and O2 are the reactant.
Product: - The substances formed into the chemical reaction is known as product. In equation (2H2 + O2 → 2H2O) H2O is the product.
We see different types of changes; everyday in our surrounding.
Reversible: Things or processes which can be reversed are called: Things or processes which can be reversed are called reversible. For example, a folded sheet of paper can be unfolded and hence folding a sheet of paper is reversible.
Irreversible: Things or processes which cannot be reversed are called irreversible. For example, when milk turns into curd; it cannot be changed back to milk and hence is an irreversible change.
Types of Changes: - Physical Change and Chemical Change.
Physical Change:
Change in which only physical properties of a substance change and no new substance is formed is called Physical Change.
Most of the physical changes are reversible, this means we can get the substance back even after the change.
Examples:
Folding of a paper sheet: A paper can be folded or unfolded and hence it is an example of physical change. Moreover, no new substance is formed in this process, so it is a physical change.
Tearing of paper sheet: Even after being torn in very small pieces; each piece is a piece of paper. Since no new substance is formed in this process, so it is a physical change.
Melting of wax: In melting, only the state of the wax changes; from solid to liquid. Solid wax can be obtained from molten wax. This i: In melting, only the state of the wax changes; from solid to liquid. Solid wax can be obtained from molten wax. This is an example of physical and reversible change.
Melting of ice into water: In melting, only the state of water changes. Water can be changed to ice and vice-versa. This is an example of physical and reversible change.
Freezing of water:After freezing, water changes into ice. In this change; there is only the change in the state of water. Water can be obtained back from ice; by melting.
Change of water into vapour (Vaporisation): Vapour is another state of water. Water can be obtained after condensation of vapour. This is a physical and reversible change.
Change of vapour into water (Condensation): This is also a physical and reversible change as water can be changed into vapour again by the process of vaporization.
Stretching of a rubber band: In this change, only the size of the rubber band changes. The rubber band comes back in its original shape and size, once it is released. This is a physical and reversible change.
Chemical Changes:
Changes in which chemical properties of substance change and a new substance is formed are called chemical changes. Some of the chemical changes are irreversible. Even in case of reversible chemicalChanges in which chemical properties of substance change and a new substance is formed are called chemical changes. Some of the chemical changes are irreversible. Even in case of reversible chemical change, the change cannot be reversed by simple physical processes.
Examples:
Burning of paper, wood, fuel or anything:
When something is burnt, many new substances are formed; especially carbon dioxide is formed in most of the cases. Once something is burnt, the ash or carbon dioxide cannot be turned into the original substance. Hence, burning oWhen something is burnt, many new substances are formed; especially carbon dioxide is formed in most of the cases. Once something is burnt, the ash or carbon dioxide cannot be turned into the original substance. Hence, burning of anything is chemical and irreversible change.
Melting of wax and burning of wax are different kinds of change. Burning of wax is chemical change while melting of wax is a physical change. Burning of wax is irreversible while melting of wax is reversible.sed back.
Reaction between vinegar and Baking Soda:
Vinegar is an acid (Acetic acid). The chemical name of baking soda is sodium hydrogen carbonate or sodium bicarbonate.
When vinegar reacts with acetic acid, it gives carbon dioxide. A hissing sound is produced when baking soda is added to vinegar. This happens because of production of carbon dioxide. When carbon dioxide is passed through lime water (calcium hydroxide), it turns the lime water milky. This happens because of formation of calcium carbonate. Water is also formed; along with calcium carbonate. The reaction involved in this can be written as follows:
Class seven science Chemical and Physical Changes Reaction between acetic acid and calcium hydrogen carbonate
These are the examples of chemical changes. In the first case, a new substance; carbon dioxide is formed. In the second case, a new substance; calcium carbonate is formed. The lime water turns milky because of formation of calcium carbonate.
These reactions are irreversible, as original substance cannot be retrieved from these by simple physical processes.
Burning of magnesium ribbon:
Magnesium ribbon burns with dazzling light in air and forms magnesium oxide. The ash of magnesium oxide gives magnesium hydroxide when mixed with water. Solution of magnesium hydroxide turns blue litmus paper red; this shows its basic characteristic. Reaction involved in it can be written as follows:
Class seven science Chemical and Physical Changes Reaction of burning of magnesium ribbon in air
These are chemical changes as new substances are formed after change. Original substances cannot be retrieved by simple physical processes.
Reaction between solution of copper sulphate and iron: Copper sulphate solution is blue in colour. Copper sulphate is also known as blue vitriol. When an iron nail or shaving blade is left in copper sulphate solution; for some time; the colour of solution changes from blue to pale green. This happens because of the formation of iron sulphate. Additionally, a layer of brown copper gets deposited over the iron nail or blade. The reaction can be written as follows:
Class seven science Chemical and Physical Changes Reaction between solution of copper sulphate and iron
Rusting of Iron: Deposition of a brown layer on iron is called rusting. In rusting, a new substance is formed. The chemical structures of iron and rust are completely different. Rust is iron oxide. Iron is a grey-black material while rust is reddish brown
Thus, this is a chemical and irreversible change. Reaction in rusting can be written as follows:
Class seven science Chemical and Physical Changes Reaction of rusting of Iron
When articles made of iron come in contact with moisture present in air, they get rusted. Iron is converted into iron oxide, i.e. rust. The iron article becomes weak in due course as all the iron slowly turns into rust. This is corrosion of iron. Rusting gives a huge monetary loss to the people and nation.
Prevention of rusting: For rusting, both water and oxygen should come in contact with iron. If anyone of these is prevented to come in contact with iron, rusting can be prevented. So, rusting is prevented using following methods:
Painting: articles such as; iron gates, grills, etc. are painted at regular intervals of time.
Applying of layer of grease: Applying a layer of grease prevents the iron articles from coming in contact with moist air. This prevents rusting. That is why grease is applied over the chain of bicycle and also over many machine parts.
Galvanisation: In the process of galvanization; a layer of non-reactive metal, such as zinc is deposited over iron articles. The layer of non-reactive metal prevents the iron articles from coming in contact with moisture. Thus it prevents rusting. Water p
Crystallisation: Common salt is obtained by the vapourisation of sea water, but crystals of common salt are very small. When a small crystal of common salt is left dipped in the saturated solut In the process of galvanization; a layer of non-reactive metal, such as zinc is deposited over iron articles. The layer of non-reactive metal prevents the iron articles from coming in contact with moisture. Thus it prevents rusting. Water pipes, which are made of iron, are galvanized to prevent rusting.
Crystallisation:Common salt is obtained by the vapourisation of sea water, but crystals of common salt are very small. When a small crystal of common salt is left dipped in the saturated solution of common salt for some time, big crystal of common salt is obtained. Formation of big and pure crystal of a substance from the saturated solution is called CRYSTALLISATION.
Characteristics of Physical change:
Only physical properties of substance change.
No new substance is formed in this change.
Most of the Physical changes are reversible.
Characteristics of Chemical change:
Chemical properties of substance change.
New substance is formed.
Some of the Chemical changes are irreversible.
Chemical change cannot be reversed by simple physical means.
Signs of Chemical Change:
Evolution of heat.
Production of sound.
Change in colour.
Change in smell.
Evolution of gas.
Physical And Chemical Changes
NCERT Book Exercise Class VII science Question and Answer
1. Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f ) Digestion of food
Answer:
(a) Chemical change.
(b) Physical change.
(c) Chemical change
(d) Physical change
(e) Physical change
(f) Chemical change
2. State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
(a) Cutting a log of wood into pieces is a chemical change. (True/False)
(b) Formation of manure from leaves is a physical change. (True/False)
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
(d) Iron and rust are the same substances. (True/False)
(e) Condensation of steam is not a chemical change. (True/False)
Answer:
(a) False (b) False (c) True (d) False (e) True
3. Fill in the blanks in the following statements:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of _________.
Answer: Calcium carbonate.
(b) The chemical name of baking soda is _________.
Answer: Sodium hydrogen carbonate.
(c) Two methods by which rusting of iron can be prevented are _________ and _________.
Answer: Polishing, Glavanisation.
(d) Changes in which only _________ properties of a substance change are called physical changes.
Answer: Physical
(e) Changes in which new substances are formed are called _________ changes.
Answer: Chemical
4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Answer: When baking soda is mixed with lemon juice, bubbles formed because of evolution of carbon dioxide gas. This is a chemical change, because in this reaction new substances are formed.
5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.
Answer: In burning of candle some of the wax melts, which is physical change, while most of the wax burnt, which is a chemical change.
In the burning of wood, water present in wood changes into vapour, this is physical change, while burning of wood is a chemical change.
6. How would you show that setting of curd is a chemical change?
Answer: In the setting of curd, milk changes into new substance, curd. This is an irreversible process that means milk cannot get back from curd. Formation of new substance and irreversible process shows that setting of curd is a chemical change.
7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer: In burning of wood, new substances are formed and hence is a chemical change. While cutting the wood into small pieces no new substance is formed, thus it is a physical change.
Hence, burning of wood and cutting it into small pieces are considered as two different types of changes.
8. Describe how crystals of copper sulphate are prepared.
Answer:
Process:
Water is taken in a beaker.
Few drops of sulphuric acid is added to the water.
Water is let to boil.
When water starts boiling, copper sulphate powder is added to the water with continuous stirring the solution.
When copper sulphate starts deposited into bottom of the beaker, then solution is filtered and left for cooling.
After cooling, the crystals of copper sulphate is prepared.
9. Explain how painting of an iron gate prevents it from rusting.
Answer: Iron get rusted because of reaction with oxygen present in moist air. By painting iron gate, iron is prevented to come into contact with oxygen present in moist air, which prevent it be getting rusted.
10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer: Iron get rusted because of reaction with oxygen present in moist air. In coastal areas, air contains more moisture because of sea or ocean, while in deserts air is dry and hot. Thus because of getting more moist air iron objects getting rusted faster in coastal areas than in deserts.
11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer: (ii) Process – B is a chemical change.
12. Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer: (iii) Both processes A and B are chemical changes.
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