NCERT 7th Class (CBSE) Science: Chemical and Chemical Changes

Question: Distinguish between symbols, chemical formulae and chemical equations.

Answer: Symbols: The representation of an element using the short form or abbreviation of its name is known as its chemical symbol. For example – C – Carbon, N-Nitrogen, Cu- Copper(Cuprum)

Chemical Formulae: The symbolic representation of the composition of a chemical compound is known as its chemical formula. A chemical formula shows the different elements present in a molecule of compound and the number of atoms of each combining or constituent element in the compound. For example, the chemical formula of sodium chloride (Common salt) is NaCl, water is H₂O.

Chemical Equation: The representation of a chemical reaction using symbols and formulae of the elements or compounds involved in the reaction is known as a chemical equation. For example, Mg + O₂ → MgO.

Question: When a candle burns both physical and chemical changes takes place. Identify these changes. Give another example of a familiar process in which both the chemical and physical change take place.

Answer: When a candle burns, both physical and chemical changes take place. The melting wax is physical change as there is no formation of new product. The burning of wax is chemical change as light is lit up by consuming the energy from wax. Cooking of food is both physical and chemical because raw vegetables get cooked which is a chemical change and the water changes into steam which is a physical change.

Question: How would you show that setting of curd is a chemical change?

Answer: The curd is formed from milk. Both curd and milk have different properties. Also, once the curd is formed it cannot be reversed back into milk. So, there is formation of new substance with different properties and also an irreversible process, setting of curd is a chemical change.

Question: Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Answer: When we burn wood, it turns into ashes which is a new substance and process is irreversible one, hence is a chemical change. While cutting the wood into small pieces no new substance is formed, hence is a physical change.

Question: Describe how crystals of copper sulphate are prepared.

Answer: Crystals of copper sulphate are prepared by the method of crystallization.The process is as followed:

1. A cupful of water in a beaker is taken.
2. Few drops of dilute sulphuric acid is added to it.
3. Water is heated and when it starts boiling copper sulphate powder is added slowly while stirring till no more copper sulphate powder dissolved in it.
4. Solution is filtered and let it cool without disturbance. After some time the crystals of copper can be observed in it.

Question: Explain how painting of an iron gate prevents it from rusting.

Answer: Painting of an iron gate prevents it from rusting because it cut the direct contact of iron from the environment and therefore there is no further exposure of iron to oxygen in moisture which is the causes for rusting.

Question: Explain why rusting of iron objects is faster in coastal areas than in deserts

Answer: Iron objects get rusted because of the reaction with oxygen present in moist air. In coastal areas, the presence of moisture is more because of sea or ocean. while in deserts the air is dry and hot. Therefore, rusting of iron objects is faster in coastal areas than in deserts due to presence of moisture.

Question: Formation of clouds is a physical change. Explain.

Answer: Formation of clouds is a physical change. Clouds are formed by condensation of water vapours present in the atmosphere. When rainwater goes back on the earth no new product is formed. Therefore, it is a physical change.

Question: The glowing of an electric bulb is a physical change. Give reason.

Answer: When an electric bulb is switched on an electric current passes through its filament. The filament of bulb becomes white hot and glows to give light. When the current is switched off, the filament returns to its normal condition and the bulb stops glowing. No new substance is formed in the bulb during this process. So, the glowing of an electric bulb is a physical change.

Question: Most physical changes are reversible. Give reason with two examples.

Answer: All Physical changes are reversible because in physical change, no new product is formed. They can be reversed easily. For example,

1. Dissolving of sugar in water is a physical change and we get back sugar and water easily.

Formation of ice from water. In melting of ice, we get water back.