GSEB Class 7 Science Solutions Chapter 6 ભૌતિક અને રાસાયણિક ફેરફારો

GSEB Class 7 Science Physical and Chemical Changes Textbook Questions and Answers

Textbook Exercise Questions

Question 1. નીચે આપેલી પ્રક્રિયાઓમાં થતાં ફેરફારોનું ભૌતિક ફેરફાર તથા રાસાયણિક ફેરફારમાં વર્ગીકરણ કરો:
1. પ્રકાશસંશ્વેષણ
2. પાણીમાં સાકર કે ખાંડનું ઓગળવું
3. કોલસાનું દહન
4. મીણનું પીગળવું
5. ઍલ્યુમિનિયમના ટુકડાને ટીપીને તેમાંથી ઍલ્યુમિનિયમ ફૉઈલ બનાવવી
6. ખોરાકનું પાચન
Answer:
ભૌતિક ફેરફાર (Physical Changes): In this type, a substance changes its form but not its chemical identity. The items that are physical changes are: melting of wax, dissolving sugar or jaggery in water, and flattening aluminum pieces to make aluminum foil.
રાસાયણિક ફેરફાર (Chemical Changes): In this type, new substances with different properties are formed. The items that are chemical changes are: photosynthesis, burning of coal, and digestion of food.
In simple words: Physical changes alter how something looks but not what it is. Chemical changes create something entirely new.

Exam Tip: Remember, a physical change does not result in a new substance, while a chemical change always forms one or more new substances.

Question 2. સાચા વિધાન સામે ‘T’ કરો અને ખોટા વિધાન સામે 'F' કરો:
(i) લાકડાને કાપીને તેના ટુકડા કરવા એ રાસાયણિક ફેરફાર છે. (F)
(ii) પાંદડાંમાંથી ખાતર બનવું એ રાસાયણિક ફેરફાર છે. (T)
(iii) લોખંડની પાઈપ પર જસતનો ઢોળ ચડાવતાં તેને જલદી કાટ લાગતો નથી. (T)
(iv) લોખંડ અને તેના કાટ બંને એક જ પદાર્થ છે. (F)
(v) વરાળનું કારણ એ રાસાયણિક ફેરફાર નથી. (T)
Answer:
(i) The cutting of wood into pieces is a physical change because the wood's basic composition remains unchanged.
(ii) Leaves changing into compost is a chemical change, as new substances are formed through decomposition.
(iii) Applying a zinc coating (galvanization) on iron pipes prevents rust effectively.
(iv) Iron and rust are different substances, as rust is iron oxide, a new compound formed from iron and oxygen.
(v) Vaporization is a physical change, as water simply changes its state from liquid to gas without forming new substances.
In simple words: Physical changes like cutting wood or making steam don't change what something is. Chemical changes like composting leaves or rusting iron make entirely new things. Galvanizing helps stop rust.

Exam Tip: Understand the core difference: physical changes alter form, chemical changes alter composition. This helps correctly identify T/F statements.

Question 3. નીચે આપેલાં વિધાનોમાં ખાલી જગ્યા પૂરોઃ
(i) જ્યારે ચૂનાના નીતર્યા પાણીમાં કાર્બન ડાયૉક્સાઈડ પસાર કરવામાં આવે છે, ત્યારે તે ............ને કારણે દૂધિયું બની જાય છે.
(ii) બેકિંગ સોડાનું રાસાયણિક નામ છે.
(iii) લોખંડને કાટ લાગતા બચાવવાની બે રીતો .......... અને .......... છે.
(iv) પદાર્થના માત્ર .......... ગુણધર્મમાં થતા ફેરફારને જ ભૌતિક ફેરફાર કહે છે.
(v) એવો ફેરફાર જેમાં નવો પદાર્થ બને છે, તેને .......... ફેરફાર કહે છે.
Answer:
(i) જ્યારે ચૂનાના નીતર્યા પાણીમાં કાર્બન ડાયૉક્સાઈડ પસાર કરવામાં આવે છે, ત્યારે તે કેલ્શિયમ કાર્બોનેટને કારણે દૂધિયું બની જાય છે.
(ii) બેકિંગ સોડાનું રાસાયણિક નામ સોડિયમ હાઇડ્રોજન કાર્બોનેટ છે.
(iii) લોખંડને કાટ લાગતા બચાવવાની બે રીતો રંગ કરવો અને ગેલ્વેનાઇઝેશન છે.
(iv) પદાર્થના માત્ર ભૌતિક ગુણધર્મમાં થતા ફેરફારને જ ભૌતિક ફેરફાર કહે છે.
(v) એવો ફેરફાર જેમાં નવો પદાર્થ બને છે, તેને રાસાયણિક ફેરફાર કહે છે.
In simple words: When carbon dioxide passes through limewater, it turns cloudy because of calcium carbonate. Baking soda's chemical name is sodium hydrogen carbonate. Painting and galvanizing stop iron from rusting. A physical change only affects physical traits, while a chemical change makes a brand new substance.

Exam Tip: For fill-in-the-blanks, focus on keywords and scientific terms. Understand the underlying concepts for each blank.

Question 4. જ્યારે લીંબુના રસની સાથે બેકિંગ સોડાને ભેળવવામાં આવે છે, ત્યારે પરપોટા થઈને વાયુ મુક્ત થાય છે. આ કયા પ્રકારનો ફેરફાર છે, તે સમજાવો.
Answer: When lemon juice is mixed with baking soda, carbon dioxide gas and other new substances are produced. This process forms new chemicals, so it is a chemical change. The bubbles indicate gas formation, a common sign of a chemical reaction occurring.
In simple words: Mixing lemon juice and baking soda creates gas bubbles and new things. This is a chemical change because the original stuff changes into new stuff.

Exam Tip: Gas production, color change, heat change, or formation of a precipitate are strong indicators of a chemical change. Mentioning the formation of new substances is key to a complete answer.

Question 5. જ્યારે મીણબત્તી સળગે છે, ત્યારે ભૌતિક અને રાસાયણિક બંને ફેરફારો થાય છે. આ ફેરફારોને ઓળખો તથા એક બીજું એવું ઉદાહરણ જણાવો કે જેમાં ભૌતિક અને રાસાયણિક બંને પ્રકારના ફેરફારો થતાં હોય.
Answer: When a candle burns, both physical and chemical changes occur. First, the wax melts, which is a physical change because it simply changes state from solid to liquid. After that, the melted wax burns with oxygen, performing a chemical reaction that produces carbon dioxide gas and water vapor. This burning is a chemical change because new substances are created. Therefore, both types of changes happen when a candle burns. Another example of both physical and chemical changes occurring together is heating sulfur. When sulfur is heated, it first melts into liquid sulfur, which is a physical change. If heating continues, it transforms into a gas and then burns, forming sulfur dioxide gas, which is a chemical change.
In simple words: When a candle burns, the wax melts (physical change), and the melted wax burns (chemical change), making new things. Another example is heating sulfur: it melts (physical) and then burns (chemical).

Exam Tip: For processes involving multiple stages, identify each stage and categorize the change. Provide a clear, distinct second example to demonstrate understanding.

Question 6. તમે કેવી રીતે બતાવી શકશો કે દહીંનું જામવું તે રાસાયણિક ફેરફાર છે?
Answer: The original substance for making curd is milk. Through the action of bacteria, milk converts into curd, which is a new substance with different properties. Milk and curd possess distinct chemical characteristics. Since a new substance (curd) with different chemical properties is formed, the setting of curd is considered a chemical change. This change cannot be easily reversed to get milk back.
In simple words: Curd is made from milk, but it's a new substance with different qualities because of bacteria. You can't turn curd back into milk. So, making curd is a chemical change.

Exam Tip: Emphasize the formation of a *new substance* with *different properties* and the *irreversible* nature of the change to correctly classify it as chemical.

Question 7. સમજાવો કે લાકડાનું દહન થવું તથા લાકડાને તેના ટુકડામાં કાપવું બંને જુદા જુદા પ્રકારના ફેરફાર ગણવામાં આવે છે.
Answer: The burning of wood is a chemical change because, during combustion, chemical reactions occur, forming new substances like ash and carbon dioxide. These new products have completely different properties from the original wood. In contrast, cutting wood into small pieces is a physical change. Although the shape and size of the wood change, the small pieces are still wood; no new substance is formed. Therefore, burning wood and cutting wood into pieces are considered different types of changes.
In simple words: Burning wood is a chemical change because it makes new things like ash. Cutting wood is a physical change because it's still wood, just smaller pieces. They are different kinds of changes.

Exam Tip: Clearly distinguish between changes in physical appearance (size, shape) versus changes in chemical composition (new substances formed).

Question 8. કૉપર સલ્ફટના સ્ફટિક કેવી રીતે બનાવવામાં આવે છે તેનું વર્ણન કરો.
Answer: The process of obtaining copper sulfate crystals is called crystallization. The method to get copper sulfate crystals is as follows:
1. Take a cup of water in a beaker. Add two to three drops of dilute sulfuric acid to it.
2. Heat the water. Keep heating until the water boils.
3. Now, add copper sulfate powder while stirring the mixture continuously.
4. When no more copper sulfate dissolves in the solution, filter the solution. (This solution is called a saturated solution.)
5. Allow this solution to stand undisturbed for a few hours.
6. After that, check if crystals appear in the solution. You will observe large blue copper sulfate crystals in the copper sulfate solution.
In simple words: To make copper sulfate crystals, first heat water with a little sulfuric acid. Then, slowly add copper sulfate powder until no more dissolves. Filter this mix, let it sit still for hours, and you'll see blue crystals form.

Exam Tip: When describing a procedure, use clear, numbered steps. Ensure to mention the role of sulfuric acid, the process of saturation, and the final appearance of the crystals.

Question 9. સમજાવો – લોખંડના દરવાજાને રંગવાથી તેને કેવી રીતે કાટ લાગવામાંથી બચાવી શકાય છે.
Answer: Painting an iron door creates a thin layer of paint on the iron surface. This paint layer prevents the iron from coming into direct contact with air and moisture. Since rusting occurs when iron is exposed to both air (oxygen) and moisture (water), preventing this contact stops the rusting process. Therefore, painting an iron door helps to protect it from rusting.
In simple words: Painting an iron door puts a thin layer on it. This layer stops air and water from touching the iron, which prevents it from rusting.

Exam Tip: When explaining rust prevention, always mention the exclusion of air and moisture as the primary mechanism. Other methods like galvanization or oiling work on similar principles.

Question 10. સમુદ્રકિનારાના પ્રદેશોમાં રણવિસ્તારના પ્રદેશો કરતાં લોખંડને કાટ ઝડપથી લાગતો હોય છે તે વાત સમજાવો.
Answer: Iron rusts in the presence of air and moisture. Coastal regions have humid air, while desert regions have dry air (without much moisture). Moreover, coastal areas often have salty water, which speeds up the rusting process of iron. Due to these factors, iron rusts more quickly in coastal areas compared to desert regions.
In simple words: Iron rusts faster near the sea than in deserts because coastal air is wet, and salty air makes rust happen even quicker. Deserts have dry air, which slows rust down.

Exam Tip: When comparing rusting rates, remember to discuss both humidity/moisture levels and the presence of dissolved salts, as both accelerate corrosion.

Question 11. રસોડામાં રસોઈના કામમાં વપરાતો ગૅસ એ લિક્વિફાઈડ પેટ્રોલિયમ ગૅસ (LPG)' છે. સિલિન્ડરમાં તે પ્રવાહી સ્વરૂપે હોય છે. તે જ્યારે સિલિન્ડરમાંથી બહાર આવે ત્યારે તે વાયુમાં રૂપાંતરણ થાય છે. (ફેરફાર –A) ત્યારબાદ તેનું દહન થાય છે. (ફેરફાર -B) આ ફેરફારો સાથે સંબંધિત નીચેના વિધાન સંબંધ ધરાવે છે. સાચા વિધાનની પસંદગી કરોઃ
A. ફેરફાર – A રાસાયણિક ફેરફાર છે.
B. ફેરફાર – B રાસાયણિક ફેરફાર છે.
C. ફેરફાર – A અને B બંને રાસાયણિક ફેરફાર છે.
D. ઉપરોક્ત એક પણ ફેરફાર રાસાયણિક ફેરફાર નથી.
Answer: (B) ફેરફાર– B રાસાયણિક ફેરફાર છે.
In simple words: When LPG leaves the cylinder, it changes from liquid to gas (Change A), which is a physical change. When it burns (Change B), it reacts with oxygen to make new substances, which is a chemical change.

Exam Tip: Changing state (liquid to gas) is always a physical change. Burning (combustion) is always a chemical change, as new substances are formed and energy is released.

Question 12. પ્રાણીજ કચરાને પચાવીને અજારક બૅક્ટરિયા બાયોગૅસ બનાવે છે. (ફેરફાર –A) ત્યારબાદ બાયોગૅસનું બળતણ તરીકે દહન થાય છે. (ફેરફાર –B) તો તેની સાથે સંબંધિત નીચે આપેલાં વિધાનોમાંથી સાચું વિધાન પસંદ કરોઃ
A. માત્ર ફેરફાર – A – રાસાયણિક ફેરફાર છે.
B. માત્ર ફેરફાર – B – રાસાયણિક ફેરફાર છે.
C. ઉપરોક્ત બંને ફેરફાર રાસાયણિક ફેરફાર છે.
D. ઉપરનામાંથી એક પણ ફેરફાર રાસાયણિક ફેરફાર નથી.
Answer: (C) ઉપરોક્ત બંને ફેરફાર રાસાયણિક ફેરફાર છે.
In simple words: When bacteria turn animal waste into biogas (Change A), it's a chemical change because new substances are formed. When biogas burns as fuel (Change B), it's also a chemical change because it reacts with oxygen to make other new substances. Both processes create new materials.

Exam Tip: Decomposition and burning are both processes that involve the formation of new chemical substances, making them chemical changes.

GSEB Class 7 Science Physical and Chemical Changes Textbook Activities

Understanding Textbook Activities

Activity 1: કાગળને કાપીને ટુકડા કરવા એ ભૌતિક ફેરફાર છે તે દર્શાવવું.
Method:
1. Take a piece of paper. Cut it into four equal parts.
2. Cut each of these pieces into four equal parts again.
3. Arrange these pieces on a table to resemble the original shape of the paper.
Observation: Cutting the paper makes its size smaller, but both the original paper and the cut pieces are still the same substance, which is paper. No new material is formed during this process.
Conclusion: Cutting paper into pieces is a physical change.
In simple words: Cutting paper into smaller pieces changes its size and shape, but it's still paper. This shows it's a physical change, not a chemical one.

Exam Tip: A physical change alters the form or appearance of a substance but not its chemical composition. Illustrating this with simple, observable actions helps understand the concept.

Activity 2: ભૌતિક ફેરફાર ઊલટાવી શકાય તેવા ફેરફાર હોય છે તે દર્શાવવું.
Method:
1. Take a piece of chalk. Break it into powder.
2. Add a little water to the chalk powder and make a paste.
3. Roll the chalk paste to form it back into the shape of a chalk stick.
4. Let it dry. It will be similar to the original chalk.
Observation: It is possible to get the chalk back from its powder form.
Conclusion: Physical changes are reversible changes.
In simple words: Breaking chalk into powder and then turning it back into a chalk stick shows that physical changes can be undone.

Exam Tip: Reversibility is a key characteristic often associated with physical changes. Examples like melting ice or dissolving sugar are also good illustrations.

Activity 3: પદાર્થની સ્થિતિમાં થતો ફેરફાર એ ઊલટાવી શકાય તેવો ફેરફાર હોય છે તે દર્શાવવું.
Method:
1. Take some ice in a glass.
2. Place it in the sunlight. Observe it after a while.
Observation: Some ice in the glass melts and becomes water.
3. Now, place the glass with the mixture of ice and water in the freezer. Observe it after some time.
Observation: The water turns back into ice.
Conclusion: A change in the state of a substance is a reversible change.
In simple words: Ice melts into water in the sun, and water freezes back into ice in the freezer. This shows that changing a substance's state is a reversible process.

Exam Tip: Changes of state (solid to liquid, liquid to gas, etc.) are classic examples of physical and reversible changes because the chemical identity of the substance remains constant.

Activity 4: પદાર્થની સ્થિતિમાં થતો ફેરફાર એ ભૌતિક ફેરફાર છે તે દર્શાવવું.
Method:
1. Take some water in a container and boil it.
2. Observe the water vapor rising from the surface of the water.
3. Hold a vessel with a handle upside down slightly above the boiling water's steam for some time. Observe the inner surface of the vessel.
Observation: Water droplets are seen condensed on the inner surface.
Conclusion: A change in the state of a substance is a physical change.
In simple words: Boiling water makes steam (a gas), and when that steam cools on a surface, it turns back into water droplets. This shows that changing a substance's state is a physical change.

Exam Tip: Condensation (gas to liquid) and evaporation (liquid to gas) are physical changes that demonstrate reversibility and no change in chemical composition.

Activity 5: ધાતુને ગરમ કરવાથી રંગમાં થતો ફેરફાર એ ભૌતિક ફેરફાર છે તે દર્શાવવું.
Method:
1. Take a used hacksaw blade.
2. Hold it with tongs.
3. Heat the other end of the blade over a gas stove flame.
4. Observe its color when the blade is heated properly.
5. Remove the blade from the flame and observe its color after some time. Note the observation.
Observation:
1. When the hacksaw blade is heated red-hot, it appears red.
2. Upon cooling, its red color disappears, and it returns to its original color.
Conclusion: The change in color of a metal upon heating is a physical change.
In simple words: When you heat a metal blade, it turns red. When it cools, it goes back to its normal color. This means heating metal only changes its look, not what it's made of, so it's a physical change.

Exam Tip: Temporary color changes due to heating, where the substance reverts to its original color upon cooling, typically indicate a physical change rather than a chemical reaction.

Activity 6: મૅગ્નેશિયમનું સળગવું એ રાસાયણિક ફેરફાર છે તે દર્શાવવું.
Method:
1. Take a magnesium ribbon. Rub its ends with sandpaper.
2. Hold this end over a candle flame. It will start burning with a brilliant white light.
3. When it burns completely, white ash is obtained.
Magnesium \( (Mg) + \) Oxygen \( (O_2) \implies \) Magnesium Oxide \( (MgO) \)
Does this ash look like magnesium?
[Caution: Directly looking at a burning magnesium ribbon can be harmful.]
Observation: This ash is magnesium oxide, which is a new substance different from magnesium. Therefore, magnesium oxide is formed.
Conclusion: The burning of magnesium is a chemical change.
In simple words: When magnesium burns, it glows brightly and turns into white ash, which is a new substance called magnesium oxide. This shows that burning magnesium is a chemical change.

Exam Tip: The formation of ash, a new substance with different properties, is a clear indication of a chemical change (combustion). Always include the chemical equation when relevant.

Activity 7: કૉપર સલ્ફટના દ્રાવણમાં લોખંડની બ્લેડ મૂકવાથી થતો ફેરફાર રાસાયણિક ફેરફાર છે તે દર્શાવવું.
1. Take one beaker and fill it with half a cup of water.
2. Add a spoonful of copper sulfate (blue vitriol) to it and make a solution.
3. Add a few drops of dilute sulfuric acid to the solution. You will see a blue-colored solution.
4. Keep a small amount of this solution as a sample in a test tube.
5. Put an iron blade (used for shaving) in the remaining solution.
6. Wait for about half an hour.
7. Now check the color of the solution. Compare it with the color of the sample solution. Keep it aside for preservation.

Did you notice any change in the color of the solution? Remove the submerged blade. Did you see any change in it?
Observation:
1. The change in the color of the solution from blue to green is due to the formation of a new substance, iron sulfate.
2. Brown-colored particles are observed on the surface of the blade, which are copper, a new substance.
Chemical Equation:
Copper Sulfate (Blue) \( + \) Iron \( \implies \) Iron Sulfate (Green) \( + \) Copper (Brown Metal)
\( CuSO_4 \) (વાદળી રંગ) \( + Fe \implies FeSO_4 \) (લીલો રંગ) \( + Cu \) (કથ્થાઈ રંગની ધાતુ)
Conclusion: The change that occurs when an iron blade is placed in a copper sulfate solution is a chemical change.
In simple words: When an iron blade sits in blue copper sulfate solution, the solution turns green (iron sulfate forms), and the blade gets covered in brown copper. This shows new substances are made, so it's a chemical change.

Exam Tip: In displacement reactions like this, the color change of the solution and the deposition of a new metal on the object are key indicators of a chemical change. Always write down the chemical equation.

Activity 8: વિનેગર (ઍસિટિક ઍસિડ) અને બેકિંગ સોડા(ખાવાનો સોડા)ને મિશ્ર કરવાથી થતો ફેરફાર રાસાયણિક ફેરફાર છે તે દર્શાવવું.
Method:
1. Take a spoonful of vinegar in a test tube.
2. Add a pinch of baking soda to it.
3. Gas bubbles will be seen emerging.
4. Pass this gas through freshly prepared limewater. Observe what happens to the limewater.

Observation and Explanation: The limewater turns milky.
Reason for this:
1. The chemical reaction between vinegar and baking soda produces carbon dioxide gas along with bubbles.
2. The chemical reaction of carbon dioxide with limewater forms calcium carbonate. Calcium carbonate is a white-colored substance insoluble in water, which causes the solution to become milky.
Conclusion: The change that occurs when vinegar and baking soda are mixed is a chemical change.
In simple words: Mixing vinegar and baking soda creates carbon dioxide bubbles. When this gas goes into limewater, the limewater turns milky because a new white substance called calcium carbonate forms. This means it's a chemical change.

Exam Tip: The reaction of carbon dioxide with limewater to produce calcium carbonate (which causes milkiness) is a classic test for carbon dioxide gas. Understanding this reaction is crucial.

Activity 9: કૉપર સલ્ફટના સ્ફટિક મેળવવા.
The process of obtaining copper sulfate crystals is called crystallization. The method to get copper sulfate crystals is as follows:
1. Take a cup of water in a beaker. Add two to three drops of dilute sulfuric acid to it.
2. Heat the water. Keep heating until the water boils.
3. Now, add copper sulfate powder while stirring the mixture continuously.
4. When no more copper sulfate dissolves in the solution, filter the solution. (This solution is called a saturated solution.)
5. Allow this solution to stand undisturbed for a few hours.
6. After that, check if crystals appear in the solution. You will observe large blue copper sulfate crystals in the copper sulfate solution.
In simple words: To make copper sulfate crystals, first heat water with a little sulfuric acid. Then, slowly add copper sulfate powder until no more dissolves. Filter this mix, let it sit still for hours, and you'll see blue crystals form.

Exam Tip: Crystallization is an important purification technique. Ensure you remember the steps: dissolving, heating, saturating, filtering, and slow cooling for crystal formation.