PHYSICAL CHANGES
Q. What are the physical properties of a substance?
Properties such as shape, size, colour and state of a substance are called its physical properties.
Q. What is a physical change?
A physical change is a change in which a substance undergoes a change in its physical properties like shape, size, colour or state without forming any new substance. For example: cutting paper, folding of paper, making furniture from wood, and the change of ice to water.
CHEMICAL CHANGE
Q. What is a chemical change?
A chemical change is a change in which one or more new substances with different properties are formed. A chemical change is also called a chemical reaction. For example, digestion of food in our body, ripening of fruits, fermentation of grapes, and burning of paper.
Q. What changes may accompany a chemical change besides the formation of new products?
In addition to new products, the following changes may accompany a chemical change:
i. Heat, light or any other radiation may be given off or absorbed
ii. Sound may be produced
iii. A change in smell may take place or a new smell may be given off
iv. A colour change may take place
v. A gas may be formed
i. Heat, light or any other radiation may be given off or absorbed
ii. Sound may be produced
iii. A change in smell may take place or a new smell may be given off
iv. A colour change may take place
v. A gas may be formed
Q. What are the differences between physical and chemical changes?
| Physical Change | Chemical Change |
|---|---|
| No new substance is formed | A new substance is formed |
| The change is usually reversible | The change is usually irreversible |
| Only physical properties like shape, size or state change | Chemical composition and properties change |
| Little or no heat/light is produced | Often heat, light, or gas is produced |
| Examples: Melting of ice, breaking of glass | Examples: Rusting of iron, burning of paper |
Q. What are some examples of chemical changes?
Some examples of chemical changes are:
1. Burning of any substance
2. Rusting of iron
3. Digestion of food
4. Formation of ozone
5. Photosynthesis
1. Burning of any substance
2. Rusting of iron
3. Digestion of food
4. Formation of ozone
5. Photosynthesis
Q. Why is the burning of any substance considered a chemical change?
Burning of any substance is always considered a chemical change because it is accompanied by production of heat and light, and new substances are formed during the process.
BURNING OF MAGNESIUM RIBBON
Q. What happens when magnesium ribbon is burnt in air?
When magnesium ribbon is burnt in air, it gives off a bright white flame and forms a white powder called magnesium oxide. This is a chemical change because a new substance is formed. The reaction can be represented as:
Magnesium + Oxygen → Magnesium Oxide + brilliant white light
Mg (s) + O₂ (g) → MgO (s)
Magnesium + Oxygen → Magnesium Oxide + brilliant white light
Mg (s) + O₂ (g) → MgO (s)
Q. Why is burning of magnesium considered a chemical change?
Burning of magnesium is considered a chemical change because:
i. A new substance (magnesium oxide) is formed
ii. It produces bright white light during the reaction
i. A new substance (magnesium oxide) is formed
ii. It produces bright white light during the reaction
Q. What happens when magnesium oxide is mixed with water?
When magnesium oxide (MgO) is mixed with water (H₂O), it forms magnesium hydroxide [Mg(OH)₂], which is another new substance.
REACTION BETWEEN COPPER SULPHATE AND IRON
Q. What happens when an iron nail is dipped in copper sulphate solution?
When an iron nail is dipped in blue copper sulphate solution, a chemical change occurs where the blue colour gradually fades and a reddish-brown layer of copper forms on the iron nail. This happens because iron reacts with copper sulphate to form iron sulphate (which gives a greenish solution) and copper. The reaction is represented as:
Iron + Copper Sulphate → Iron Sulphate + Copper
Fe + CuSO₄ → FeSO₄ + Cu
Iron + Copper Sulphate → Iron Sulphate + Copper
Fe + CuSO₄ → FeSO₄ + Cu
Q. Why is this reaction considered a chemical change?
This reaction is considered a chemical change because new substances are formed (iron sulphate and copper), there is a colour change from blue to green, and it is an example of a displacement reaction where iron replaces copper.
VINEGAR AND BAKING SODA REACTION
Q. What happens when vinegar is mixed with baking soda?
When vinegar (acetic acid) is mixed with baking soda (sodium bicarbonate), a chemical reaction occurs where carbon dioxide gas is released with bubbles or fizz, along with the formation of other substances. The reaction can be represented as:
Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + other substances
Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + other substances
Q. Why is this reaction considered a chemical change?
This reaction is considered a chemical change because new substances are formed and a gas (carbon dioxide) is released during the reaction.
Q. How can we identify that the gas released in this reaction is carbon dioxide?
The gas released can be identified as carbon dioxide by passing it through lime water, which turns milky due to the formation of calcium carbonate. The reaction is represented as:
Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃) + Water (H₂O)
Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃) + Water (H₂O)
RUSTING OF IRON
Q. What happens when iron objects are left in moist air?
When iron objects are left in moist air (presence of air and water), a brown flaky layer called rust forms on them through a slow chemical change known as rusting of iron. The reaction can be represented as:
Iron (Fe) + Oxygen (O₂) + Water (H₂O) → Rust (Iron oxide Fe₂O₃)
Iron (Fe) + Oxygen (O₂) + Water (H₂O) → Rust (Iron oxide Fe₂O₃)
Q. What are the conditions required for rusting to occur?
Rusting of iron requires two essential conditions:
1. Presence of air (oxygen)
2. Presence of water or moisture
Without either air or water, rusting does not occur.
1. Presence of air (oxygen)
2. Presence of water or moisture
Without either air or water, rusting does not occur.
Q. How can we prevent rusting of iron objects?
We can prevent rusting of iron objects by:
1. Painting the iron surface
2. Applying oil or grease
3. Galvanization - coating iron with a layer of zinc
4. Keeping iron objects away from water and moisture
1. Painting the iron surface
2. Applying oil or grease
3. Galvanization - coating iron with a layer of zinc
4. Keeping iron objects away from water and moisture
Q. Why does rusting occur faster in coastal areas compared to deserts?
Rusting occurs faster in coastal areas because there is more moisture in the air due to proximity to the sea, and salty air accelerates the rusting process. In deserts, the air is dry with low humidity, so rusting happens much more slowly.
Q. Why do ships rust faster in seawater?
Ships rust faster in seawater because they are in constant contact with saltwater which contains electrolytes that accelerate rusting, and they are continuously exposed to moist sea air that provides both oxygen and moisture required for rusting.
Q. What is galvanization and how does it prevent rusting?
Galvanization is the process of coating iron or steel with a thin layer of zinc to prevent rusting. The zinc layer protects the iron by acting as a barrier against air and moisture, and even if the coating is scratched, zinc continues to protect iron through sacrificial protection.
BURNING OF CANDLE
Q. Why is burning of a candle considered both a physical and chemical change?
Burning of a candle involves both physical and chemical changes:
• Physical Change: The melting of wax where solid wax changes to liquid state without forming new substances
• Chemical Change: The burning of wax where it reacts with oxygen to form new substances like carbon dioxide and water vapor
Therefore, the burning of a candle demonstrates both types of changes simultaneously.
• Physical Change: The melting of wax where solid wax changes to liquid state without forming new substances
• Chemical Change: The burning of wax where it reacts with oxygen to form new substances like carbon dioxide and water vapor
Therefore, the burning of a candle demonstrates both types of changes simultaneously.
CRYSTALLISATION
Q. What is crystallization?
Crystallization is a physical process in which a pure solid substance is separated from its solution in the form of crystals with definite geometrical shapes.
Q. What are some examples of crystallization?
Examples of crystallization include:
1. Formation of salt crystals from sea water
2. Crystallization of sugar from sugar solution
3. Crystallization of copper sulphate from its solution
1. Formation of salt crystals from sea water
2. Crystallization of sugar from sugar solution
3. Crystallization of copper sulphate from its solution
Q. What are the characteristics of crystals?
Crystals are solid substances in which particles are arranged in a regular, repeating pattern. They typically have a shiny surface, definite geometrical shapes, and flat faces with sharp edges.
Q. How can we obtain copper sulphate crystals from its solution?
Copper sulphate crystals can be obtained through these steps:
1. Dissolve copper sulphate powder in hot water to make a saturated solution
2. Filter the solution to remove impurities
3. Allow the solution to cool slowly without shaking or disturbance
4. After cooling, blue crystals of pure copper sulphate form and can be collected
1. Dissolve copper sulphate powder in hot water to make a saturated solution
2. Filter the solution to remove impurities
3. Allow the solution to cool slowly without shaking or disturbance
4. After cooling, blue crystals of pure copper sulphate form and can be collected
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