Physical changes and chemical changes 0.2 Flashcards
The setting of curd is a chemical change. Explain.
The setting of curd is a chemical change because curd cannot be converted into milk again. Moreover, a new substance, the curd is formed which is completely different in taste and properties from the milk.
Which of the following are chemical changes? Give reasons for your answers.
Cutting grass Burning toast Making tea with a tea bag and water Frying an egg Combustion of petrol in a car Ironing of clothes Preparing custard from milk Drying wet hair Ans. The chemical changes are
Burning toast- We cannot get the bread back as a new substance is formed while toasting.
Making tea with a tea bag and water- Here the new substance ( tea) has properties different from water and tea bags. Here color and taste are mainly changed.
Frying an egg- We cannot get the egg back.
Combustion of petrol in a car- Here petrol is burned to produce new substance and we cannot get the petrol back
Preparing custard from milk- Custard is a new substance formed and we cannot get the milk back.
Give two characteristics that determine the occurrence of a chemical reaction.
Two characteristics that determine the occurrence of a chemical reaction are
Change in color: In some reactions, the chemical change is indicated by a change in color. The cut surface of an apple, if kept for a while, turns brown. This is because a chemical reaction takes place between air and chemicals on the cut surface of the apple.
Evolution of gas: Sometimes gas is released during a chemical reaction.Example: baking soda mixed with lemon juice, bubbles are formed due to the evolution of carbon dioxide gas.
How rusting can be prevented by oiling?
When metal is coated in oil it forms a protective barrier that stops any external elements from seeping through. Thus water and air are unable to penetrate the metal and the rusting process doesn’t start.
Why do cut apples take up a brown coloration when exposed to the air?
When cut apples are exposed to air it gets a brown coloration because of the oxidation reaction taking place on the surface of it. The skin of the apple acts as a barrier from the surrounding air, when the apple is cut, the protective layer is removed.
Why does lime water turn milky on passing carbon dioxide gas into it?
When carbon dioxide is passed through lime water it turns milky due to the formation of calcium carbonate.
What do you observe when you mix vinegar and baking soda? How do you perform a test for the evolving gas?
When we mix vinegar and baking soda, carbon dioxide is produced. the rising bubbles confirm the release of the gas.
To identify the evolving gas, pass it through lime water if it turns milky due to the formation of calcium carbonate. Then the gas that evolved is carbon dioxide.
Describe how crystals of copper sulfate are prepared?
The crystals of copper sulfate are prepared as follows:
A cup of water is taken in a beaker.
Add some drops of dilute sulphuric acid to it.
Heat it and when it starts boiling add copper sulfate powder slowly with continuous stirring.
The copper sulfate powder is added to the saturation level.
Filter the solution and cool it.
Leave it undisturbed and we will observe copper sulfate crystals after some hours.
An iron nail is taken to the moon. Another nail is kept near the Gateway of India, Mumbai, while the third one is kept near the Red Fort, Delhi. Compare the degree of rust formation on the three nails.
Ans. Rust formation will be fastest in the case of iron nail present near Gateway of India, Mumbai because the water to oxygen content is higher in Mumbai, and rust formation requires both oxygen and water. In Delhi, it will be at a slower rate due to low water to oxygen content and in Moon rusting will not take place as there is no oxygen.
Liquified petroleum gas (LPG) is the gas that is used for cooking at home. In the cylinder, it comes as a liquid. When it comes out from the cylinder it becomes a gas(change A). Then it burns ( Change B). What are the differences between change A and B?
The gas we use in the ‘kitchen’ is called ‘Liquefied petroleum gas’ (LPG). In the ‘cylinder’, it exists as a ‘liquid’. When it ‘comes out’ from the ‘cylinder’, it becomes a ‘gas’ (Change – A) then it ‘burns’ (Change – B).
Thus Process A is a ‘physical change’ since Process A involves the change of state of matter from liquid to gas. This is a reversible process, as the gaseous LPG can again be converted to the liquid stage.
However, Process B involves the combustion of LPG, which is a chemical change, which ‘alters’ the properties of the LPG after combustion and is irreversible.
The decomposition of animal waste by anaerobic bacteria produces biogas (change A). The biogas is then burnt as fuel ( change B ). What are the similarities between the two changes?
Bacteria acts on waste and converts it to biogas (change A). Hence, it is a chemical change during biogas production (change B), it works as fuel and produces heat. Hence, A and B both are chemical changes.
