Class 8 Science Chapter 4 Important Questions

We see around us, chair, pen, pencil, fan, washing machine, bullock cart, cycle, cooking utensils, books, clothes, toys, water, stones and many other objects.

Yes, iron piece will change its shape as it is a metal. It increases in size and does not break. If a wood log is beaten it does not change its shape but it breaks into pieces.

Metals can be distinguished from non-metals on the basis of their physical and chemical properties.

Perhaps not, because iron rod, nail and copper wire are good conductors of heat and electricity while plastic, wood, sulphur piece, coal piece are poor conductors.
Therefore, wooden or plastic handle protects us from being hurt while handling hot things.

The objects which are made of metals produce ringing sound when struck hard. Suppose we have two boxes similar in appearance, one made of wood and the other of metal. If we struck hard on wood, no sound will produce. Whereas, metals produce ringing sounds. Hence, they are said to be sonorous. The materials other than metals are not sonorous.

The materials which are hard, lustrous, malleable, ductile, sonorous and good conductors of heat and electricity. The materials which generally possess these properties are called metals. The examples of metals are iron, copper, aluminium, calcium, magnesium, etc.
In contrast, materials like coal and sulphur are soft and dull in appearance. They break down into powdery mass on tapping with hammer. They are not sonorous and are poor conductors of heat and electricity. These materials are called non-metals. The examples of non-metals are sulphur, carbon, oxygen, phosphorus, etc.

Mercury is the only metal which is found in liquid state at room temperature.

To check the nature of rust formed as a result of the reaction between iron, oxygen and water. Collect a spoonful of rust and dissolve it in a very little amount of water. We will find that the rust remains suspended in water. Shake the suspension well. Test the solution with red and blue litmus papers. Our observation is that the red litmus turns blue. That means, oxide of iron is basic in nature.

Take a small amount of powdered sulphur in a deflagrating spoon or metallic cap of any bottle and heat it. As soon as sulphur starts burning, introduce the spoon into a gas jar or glass tumbler. Cover the tumbler with a lid to ensure that the gas produced does not escape. Remove the spoon after some time. Add a small quantity of water into the tumbler and quickly replace the lid. Shake the tumbler well.
The name of the product formed in the reaction of sulphur and oxygen is sulphur dioxide gas. When sulphur dioxide is dissolved in water sulphurous acid is formed. The reaction can be given as follows:
Sulphur dioxide (SO2) + Water (H2O) → Sulphurous acid (H2SO3). Check the solution with red and blue litmus papers. The sulphurous acid turns blue litmus paper red. That means, oxides of non-metals are acidic in nature.

We know that sodium metal is very reactive. It reacts vigorously with oxygen and water. A lot of heat is generated in the reaction. Some other metals do not do so. For example, iron reacts with water slowly. Generally, non-metals do not react with water though they may be very reactive in air. Such non-metals are stored in water. For example, phosphorus is a very reactive non-metal. It catches fire if exposed to air. To prevent the contact of phosphorus with atmospheric oxygen, it is stored in water.

Take samples of metals and non-metals as:
Magnesium(ribbon), Aluminium (foil), Iron (filings), Copper (peeled flexible wire), Charcoal (powder), Sulphur (powder) in separate test tubes and label them as A, B, C, D, E, and F.
With the help of a dropper add 5 mL of dilute hydrochloric acid to each test tube one by one. Observe the reactions carefully. If no reaction occurs in a cold solution, warm the test tube gently. Bring a burning matchstick near the mouth of each test tube.
You can listen the ‘pop’ sound in some cases be due to when a burning match stick is brought near the mouth of the test tubes.
You must have found that non- metals generally do not react with acids but metals react with acids and produce hydrogen gas that burns with a ‘pop’ sound.

Take five 100 mL beakers and label them A, B, C, D and E. Take about 50 mL of water in each beaker. Dissolve in each beaker a teaspoonful of each substances.
Beaker A: Copper sulphate (CuSO4) + Zinc granule (Zn),
Beaker B: Copper sulphate (CuSO4) + Iron nail (Fe),
Beaker C: Zinc sulphate (ZnSO4) + Copper turnings (Cu),
Beaker D: Iron sulphate (FeSO4) + Copper turnings (Cu),
Beaker E: Zinc sulphate (ZnSO4) + Iron nail (Fe)
Keep the beakers undisturbed for some time.
In beaker [A] zinc (Zn) replaces copper (Cu) from copper sulphate (CuSO4). That is why the blue colour of copper sulphate disappears and a powdery red mass of copper is deposited at the bottom of the beaker. Similarly, iron replaces the copper in Beaker [B].
Since we do not see any change in beaker [C], [D], and [E].
Remember that science is not arbitrary. It follows definite rules based on facts. And the rule here is that zinc is more reactive than copper and iron. A more reactive metal can replace a less reactive metal, but a less reactive one cannot replace a more reactive metal.
[Note: The order of activity for metals, from most reactive to least reactive, is: Li, K, Sr, Na, Ca, Mg, Al, Zn, Cr, Fe, Cd, Co, Ni, Sn, Pb, H, Sb, As, Bi, Cu, Hg, Ag, Pd, Pt, and Au.]

After a long standing, the copper of utensils, reacts with oxygen of air and forms its oxides which are greenish in nature. That is why, the copper utensils get green deposition on their surface on long standing.

Zinc metal can displace copper from copper sulphate solution as zinc is more reactive than copper. But copper cannot displace zinc from zinc sulphate solution because copper is less reactive than zinc.

Sodium metal is highly reactive metal, when exposed in air it starts to burn to form oxide. In the same way, it forms hydroxide with water. It does not react with kerosene. So, to prevention against any reaction, it is kept in kerosene not in water.

The most common soluble sources of magnesium to use as fertilizer are magnesium sulphate, sulphate of potash magnesia and magnesium oxide.
Besides these, magnesium is found in plants in the form of chlorophyll. It is present in the chlorophyll of the plant and helps in photosynthesis.

Chlorine which is non-metal, is used in water purification plants because chlorine has the ability to kill germs.

No, we cannot store lemon pickle in aluminium utensils, as metal react with acids to liberate hydrogen gas. The pickle (which is acidic in nature) can be spoiled.

Take about 50 ml of water in a beaker and dissolved 5 gram of copper sulphate in it to obtain copper sulphate solution (which is blue in colour). Put a clean iron nail in the solution and keep the beaker and disturb for some time.
We will find that the colour blue colour of copper sulphate solution starts fading gradually and the iron nail gets covered with the “Red Brown” layer of copper metal. It is because iron is more reactive than copper, so it displaces copper metal from its solution of copper sulphate. It is the copper metal set free from its compound which forms a “Red Brown” layer on the surface of iron nail.

Copper sulphate + Iron → Iron sulphate + Copper
(Blue solution) (Grey) (Greenish Solution) (Red-brown)

To polish a gold ornament, it is dipped in an acid (called Aqua Regia). The outer layer of gold dissolves in the acid and the inner shiny layer is visible. Because of loss of upper layer of jewellery, its weight is reduced.