Class 7 Science Chapter 5 Question Answers - Physical and Chemical Changes

Short Q & A

Q1: Explain physical reaction along with examples.
Ans : A change in which a substance undergoes a change in its physical properties is called a physical change. For example; melting of ice.

Q2: Explain chemical reaction along with examples.
Ans : A change in which one or more new substances are formed is called a chemical reaction. For example; rusting of iron.

Q3: Differentiate between physical and chemical reactions.
Ans : 

Q4: State the condition necessary for the occurrence of any reactant.
Ans : For the occurrence of any reaction, the molecules or atoms of the reactants must collide with one another, in order to break old bonds and form new bonds.

Q5: State rate of chemical reaction.
Ans : The reaction rate or rate of reaction is the speed at which reactants are converted into products.

Q6: Why new products are formed in a chemical reaction?
Ans : In a chemical reaction the old bonds of the reactants are broken down and fresh bonds are formed, resulting in formation of new products with different properties of that of earlier products.

Q7: Explain the process of rusting.
Ans : When an iron object is left in damp air (or water) for a considerable time, it gets covered with a red-brown flaky substance called rust. This is called rusting of iron. During the rusting of iron, iron metal combines with the oxygen (of air) in the presence of water (moisture) to form a compound iron oxide.

Q8: How can we prevent rusting?
Ans : Whenever you get iron, water and oxygen together, you get rust. So the best way to prevent it is to keep them apart; that's what paint does, or the spray-on wax and oil coatings that the car protection companies sell. Keep your tools dry; wipe down your bike after a ride; keep the water away and it can't rust.

Q9: Setting of curd is regarded as a chemical change, explain why?
Ans : Setting of curd is a chemical change because we cannot get the original substance (milk) back. The new substance, i.e. curd is different from the milk in taste, smell and chemical properties.

Q10: What happens when baking soda is treated with vinegar?
Ans : When baking soda is treated with vinegar, we hear a hiss sound and see bubbles of a gas coming out. This gas is carbon dioxide.
Vinegar (Acetic acid) + Baking soda (Sodium hydrogen carbonate) → Carbon dioxide + other substances.

 Q11: Which one is better technique to obtain sugar from sugar solution- crystallization or evaporation to dryness?
Ans : Crystallization is better because,
1. It occurs on the entire surface.
2. we get solid in the pure crystallized form.
3. The soluble impurities get removed in this process.
4. It doesn't require very high temperature conditions.

Q12: What happens when magnesium oxide is dissolved in water?
Ans : Magnesium hydroxide forms in the presence of water (MgO + H₂O → Mg(OH)₂), but it can be reversed by heating it to separate moisture.

Q13: What happens when a piece of iron metal is placed in copper sulphate solution?
Ans : When iron is placed in Copper sulphate solution, the iron is coated with a brown coloured substance "copper" and CuSO₄ solution changes from blue to light green. This is due to iron displacing copper as it is more reactive than copper.

Q14: Explain the changes occuring in burning of candle.
Ans : When a candle burns, both physical and chemical changes take place. On a burning candle, the wax melts but can be solidified again on cooling. This shows that melting of wax is a physical change. Burning of candles also produces light and some gases like carbon dioxide. Hence, burning the wick of the candle is a chemical change.

Q15: Baking soda is mixed with lemon juice, bubbles are formed with the evolution of gas, explain the changes occurred here.
Ans : It's a chemical reaction. Here, a new substance carbon dioxide is formed.

Q16: Name some of the process in which both chemical and physical changes take place.
Ans : Another example of a change that is both chemical and physical is the burning of wood. The moisture present in the wood turns to vapor, when heated, which is a physical change. The actual burning of wood will generate carbon dioxide (among other products) and is a chemical change.

Q17: Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Ans : Burning of wood produces ash and smoke. Hence the properties of wood are changed and new substances are formed. So, it is a chemical reaction. When a log of wood is cut into small pieces,there is no new substance formed. Each small piece bears the properties of wood. So, its a physical change.
Obviously, burning and cutting of wood are two different types of changes.

Q18: Explain the formation of crystal of copper sulphate.
Ans : A cup of water taken in a beaker and a few drops of dilute Sulphuric acid are added into it. The water is hated. When it starts boiling copper sulphate powder is added slowly while stirring continuously till no more powder can be dissolved. The solution is filtered and allowed to cool down. Crystals of copper sulphate slowly form at the bottom of the beaker.

Q19: Explain how painting of an iron rod prevents it from rusting.
Ans : For rusting, iron must be in contact with both air and moisture. When the iron gate is painted the layers of paint cut the contact between air, moisture and iron. Thus, it prevents rusting.

Q20: Rusting of an iron object is faster is coastal areas than in desert area. Explain why?
Ans : In coastal areas there is more moisture in air due to the presence of sea. But, in the desert there is a scarcity of water and hence air is almost dry there. Both air and moisture are necessary conditions for rusting. So, rusting is faster in coastal areas than in desert.

Q21: A part from new products, many other things accompany a chemical change, what are those things?
Ans : Part from new products, many other things accompany a chemical change, those things are:

• Heat, light or any other radiation may be given off or absorbed.
• Change in smell may take place.
• Sound may be produced.
• Change in colour may take place.
• A gas may be formed

Q22: Why chemical changes are very important in our life?
Ans : A change in which the composition of a substance is altered is called a chemical change. As a result, the original properties get changed and one or more new substances are formed. Burning of paper, rusting of iron, cooking of food. All new substances are formed as a result of chemical reactions , a medicine is the end product of the chain of chemical reaction, photosynthesis , digestion is the chemical reaction, useful materials like plastics, detergents and many more products are formed by chemical reactions only, thus we can say chemical changes are very important in our life.

Q23: Burning of any substance is the chemical change. Discuss.
Ans : In burning always heat is produced, the old bonds of the reactant are broken down and fresh bonds are formed, resulting in formation of new products with different properties of that of earlier products.

Q24: Why spoiled food produces foul smell?
Ans : When food goes bad and starts to become pungent, it is most often due to the growth of spoilage microbes such as bacteria, yeasts and mold. Odors can come from two sources: chemicals that are released from the food as the microbes decompose it, or chemicals produced directly by the microbes themselves.

Q25: A slice of apple acquires a brown colour if it is not consumed immediately, explain.

Ans : This is because apples are a rich source of iron and when we cut an apple slice ,the cells ruptures. Oxygen in the air oxidises the iron availability in these cells. Also an enzyme called POLYPHENOL OXIDASE present in these cells catalyse the oxidation of iron in a quick matter of time.

Q26: State Four characteristics which are included in the physical properties of matter.

Ans : Properties such as shape, size colour and state of matter are called its physical properties.

Q27: Define malleability along with an example.

Ans : Malleability is a property by the virtue of which a metal can be beaten into sheets without breaking. Malleability is the ability of a metal to be hammered into sheets. Gold and silverware are highly malleable. When a piece of hot iron is hammered it takes the shape of a sheet.

Q28: Why formation of manure from leaves is a chemical change?

Ans : Formation of manure from leaves is chemical change because the manure formed has different composition from that of the leaves.

Q29: Why cutting of wood is a physical change?

Ans : Cutting of wood is a physical change because the identity and composition of wood does not change.

Long Q & A

Q1: Explain the physical and chemical processes along with examples.

Ans : 

A change in which a substance undergoes a change in its physical properties is called physical change, physical properties involves shape, size, colour and state of a substance. It is generally reversible; in physical change no new substance is formed. For example: evaporation, condensation etc.
A change in which one or more new substances are formed is called chemical change. Chemical change is irreversible. It is also called chemical reaction; in this type of change a new substance is formed. Example—Burning of coal, photosynthesis.
The difference between physical change and chemical change is as follows-Physical change
(1) No new substance is formed. A substance undergoes a change in its physical properties.
(2) Physical change is generally reversible.
Example—Melting of ice, lighting of bulb Chemical Change
(1) One or more new substances are formed by chemical reaction.
(2) Chemical change is irreversible.
Example—burning of coal, photosynthesis

Q2: Explain the process of rusting of iron, and measures to prevent the same.
Ans : Prevention method of rusting
Galvanization is the process of applying a protective layer of zinc on a metal. It is a very common method of preventing the rusting of iron. Providing the metals with an electric charge can help inhibit the electrochemical reactions that lead to rusting.

Q3: Explain the following:
a. Galvanisation
b. Crystallisation
Ans :
a. Depositing a layer of zinc or chromium on iron to prevent it from rusting is known as galvanization. Doing this prevents direct contact of iron with air and oxygen and thus prevents it from rusting.
b. Some substances can be obtained in pure state from their solutions by the process of crystallisation.