Changes Around Us Physical and Chemical — Question 12

In Figure (a), when vinegar reacts with baking soda, carbon dioxide gas is released. This carbon dioxide gas travels through the straw into the test tube with lime water, where it reacts to form calcium carbonate, a white solid substance that makes the lime water appear milky.
Class 7 Changes Around Us Physical and Chemical Question Answer (Activities)
Activity 5.1 Let Us Think and Reflect (Page 58)
Change
Observation(s)
Melting ice cubes
Ice melts to water, state changes
Chopping vegetables
Size and shape changes
Boiling water
Water changes to water vapour, state changes
Making popcorn from corn
Corns get roasted into bigger size popcorns (new substance formed), shape and size changes
Cutting a piece of paper
Shape and size changes
Adding beetroot extract to water
Colour of water changes to pink
Burning wood
Wood changes to ash (new substance formed), heat and light produced
Drying wet clothes
Water from wet clothes gets evaporated, state of water changes from liquid to gas
Making small balls of dough
Shape and size changes
Rolling small balls of dough into chapatis
Shape and size changes
Cooking of food
Composition, smell and taste changes
Making pot from mud
Shape and size changes
Conclusion: We can categorise these changes in two types of changes, one in which only shape, size or state of a substance changes or other in which new substance(s) is/ are formed.
Activity 5.2: Let Us Create and Discuss (Page 59)
Changes listed in A, B and C are as follows:
A. Creating some objects with paper: When we create different objects like aeroplane, boat, bird, etc. by folding a sheet of paper, only shape and size changes. We can get back the original sheet by unfolding it again.
B. Playing with a balloon: When we loosen the grip of the inflated balloon we get the original shape of balloon back, and we can inflate it again but when we prick the inflated balloon with a pin, it gets deflated with a hole in it, so we cannot able to inflate it again.
C. Crushing a piece of chalk: When we crushed a piece of chalk to powder form, we cannot can get back the original piece of a chalk.
Observation and conclusion: In all these types of changes, material (piece of paper, balloon, chalk) remains the same but their size and shape changes only. Thus, these are examples of physical changes.
Activity 5.3: Let Us Create and Discuss (Page 60)
Observation and conclusion: When we blow air (carbon dioxide) from mouth to tumbler A containing water, we do not observe any change in the colour of water. But when we blow air into tumbler B containing lime water (calcium hydroxide), lime water turns milky and white-coloured insoluble substance (calcium carbonate) gets settle down. Thus, this is classified as chemical change as new substance is being formed.
Activity 5.4: Let Us Experiment (Page 61)
Observation and conclusion: When we add a pinch of baking soda (sodium hydrogen carbonate) to vinegar or lemon juice, bubbles of carbon dioxide gas are produced with a fizzing sound. When we pass this gas to lime water, it turns milky. Thus, in this process, new substances are formed, it is a chemical change.
Activity 5.5: Let Us Investigate (Pages 62-63)
Observation and conclusion: In figure (a), candle keeps on burning as it is not covered and supply of air (oxygen) is available. But in figure (b), as candle is covered with a glass tumbler, does not get a continuous supply of air, so, it gets extinguished. This concludes oxygen is required for combustion.
Activity 5.6: Let Us Investigate (Page 64)
Observation and conclusion: The substance catches fire only when its ignition temperature is reached. Ignition temperature is the minimum temperature at which the substance catches fire. When a lighted matchstick is brought near to the paper it catches fire immediately because the temperature of the matchstick is already higher that the ignition temperature of the paper. In another case, when sunrays are focused using a magnifying glass on a piece of paper, it gets heated up and the temperature of paper increases and it attains its ignition temperature and starts burning and emit smoke.
Requirements for combustion process to occur:
A combustible substance (fuel)
Oxygen
Attainment of ignition temperature.
Activity 5.7: Think, Pair, and Share (Page 65)
Conclusion: Burning of a candle involves both physical and chemical change. Melting of wax, evaporation of wax to wax vapour and solidification of melted wax to solid wax are all physical changes. While burning of wax to produce new substance, carbon dioxide is a chemical change.
Activity 5.8: Let Us Think (Page 66)
Table 5.2; Can changes be reversed?
Change
The original state can be brought back (Yes/ No)
Melting ice cubes
Yes
Chopping vegetables
No
Boiling water
Yes
Making popcorn from corn
No
Cutting a piece of paper
No
Adding beetroot extract to water
No
Burning wood
No
Diying wet clothes
Yes
Making small balls of dough
Yes
Rolling small balls of dough into chapatis
Yes
Cooking of food
No
Making pot from mud
No
Conclusion: Changes around us can be grouped into reversible (original substance can be brought back) and irreversible (original substance cannot be brought back) changes.