Physical and Chemical Changes - Class 7 - Science
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Back Questions - Physical and Chemical Changes | NCERT | Science | Class 7
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
(a) Photosynthesis: This is a chemical change. During photosynthesis, plants convert carbon dioxide and water into glucose and oxygen using sunlight. This process forms new substances, which is characteristic of chemical changes.
(b) Dissolving sugar in water: This is a physical change. When sugar dissolves in water, it simply disperses into the water and can potentially be recovered by evaporating the water. No new substances are formed.
(c) Burning of coal: This is a chemical change. Burning coal results in the production of carbon dioxide, ash, and other gases, indicating the formation of new substances.
(d) Melting of wax: This is a physical change. Melting wax changes its state from solid to liquid but does not alter its chemical composition. It can solidify back into solid wax upon cooling.
(e) Beating aluminium to make aluminium foil: This is a physical change. Beating aluminium changes its shape but not its chemical structure. No new substances are formed.
(f) Digestion of food: This is a chemical change. Digestion involves breaking down food into simpler molecules that the body can absorb and use, resulting in the formation of new substances.
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)
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Sign up nowFill in the blanks in the following statements:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of ___________.
(b) The chemical name of baking soda is ___________.
(c) Two methods by which rusting of iron can be prevented are ___________ and ___________.
(d) Changes in which only ___________ are called physical changes. properties of a substance change
(e) Changes in which new substances are formed are called ___________ changes.
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Sign up nowWhen baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
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Sign up nowWhen 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.
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Sign up nowHow would you show that setting of curd is a chemical change?
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Sign up nowExplain why burning of wood and cutting it into small pieces are considered as two different types of changes.
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Sign up nowDescribe how crystals of copper sulphate are prepared.
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Sign up nowExplain how painting of an iron gate prevents it from rusting.
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Sign up nowExplain why rusting of iron objects is faster in coastal areas than in deserts.
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Sign up nowThe 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 $-\mathrm{A}$ ) then it burns (Change $-\mathrm{B}$ ). The following statements pertain to these changes. Choose the correct one.
(i) Process - A is a chemical change.
(ii) Process - $\mathrm{B}$ is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
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Sign up nowAnaerobic 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 - $\mathrm{B}$ is a chemical change.
(iv) None of these processes is a chemical change.
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Sign up nowDescribe two changes that are harmful. Explain why you consider them harmful. How can you prevent them?
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Sign up nowTake three glass bottles with wide mouths. Label them A, B and C. Fill about half of bottle A with ordinary tap water. Fill bottle B with water which has been boiled for several minutes, to the same level as in A. In bottle $\mathrm{C}$, take the same boiled water and of the same amount as in other bottles. In each bottle put a few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in bottle $\mathrm{C}$ so that it forms a film on its surface. Put the bottles away for a few days. Take out nails from each bottle and observe them. Explain your observations.
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Sign up nowPrepare crystals of alum.
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Sign up nowCollect information about the types of fuels used for cooking in your area. Discuss with your teachers/parents/others which fuels are less polluting and why.
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Sign up nowNear the Qutub Minar in Delhi stands an iron pillar (Fig. 5.7) which is more than 7 metres high. It weighs more than $6000 \mathrm{~kg}$. It was built more than 1600 years ago. After such a long period it has not rusted. For its quality of rust resistance it has been examined by scientists from all parts of the world. It tells something about the advances India had made in metal technology as back as 1600 years ago.
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Sign up nowExtra Questions - Physical and Chemical Changes | NCERT | Science | Class 7
Convert the temperature of $276 \mathrm{~K}$ to the Celsius scale.
A) $3^{\circ} \mathrm{C}$
B) $4^{\circ} \mathrm{C}$
C) $2^{\circ} \mathrm{C}$
(D) $5^{\circ} \mathrm{C}$
To convert the given temperature from Kelvin to Celsius, we start with the fundamental relationship between the Kelvin (K) and Celsius (C) temperature scales:
$$ K = C + 273 $$
Given: Temperature in Kelvin $= 276 \text{ K}$.
By substituting the value into the equation:
$$ 276 = C + 273 $$
Solving for $C$:
$$ C = 276 - 273 = 3^{\circ} \mathrm{C} $$
Hence, the temperature $276 \mathrm{~K}$ is equivalent to $\mathbf{3^{\circ} \mathrm{C}}$.
The correct answer is (A) $3^{\circ} \mathrm{C}$.
A sheet of paper is used to make a paper boat. This is a:
A) Reversible change
B) Irreversible change
C) Reversible change and irreversible change
D) It is not a change
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Sign up nowIron and phosphorus powder are mixed together to form a magnetic black and red mixture. This is a:
A) Chemical change B) Physical change C) Irreversible change D) No change
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