Important Notes for CBSE Class 7 Science Chapter 6 – Physical and Chemical Changes. Physical and Chemical Changes Class 7 Notes and Study Material Download in PDF form.
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Physical and Chemical Changes Class 7 Notes
- Changes are constantly occurring in the environment.
- These changes in the surrounding may produce new substances or it may just change the state of a substance from solid to liquid.
- Changes can be of two types:
PHYSICAL CHANGES:
- Some examples of physical bring about a change in size are piece of paper cut into smaller pieces or chalk broken in smaller pieces.
- State of matter is also physical change for example water can changeits state from solid to liquid, or from gas to liquid
- Physical changes also involve change of color, like when a hack-saw blade is heated, it changes the color.
- So, to summarize physical change involves change in properties such as shape, size, color and state of a substance.
- Physical change is a reversible process.
- In physical process does not involve formation of any new substance.
- Physical change and Chemical change.
CHEMICAL CHANGES:
- A common example of chemical reaction is rusting of iron.
- Rusting: The development of a brownish film on iron is called rusting.
- When iron objects are continuously exposed to the atmosphere contentiously, it will get rusted after some time.
EXAMPLES OF CHEMICAL CHANGES:
Burning magnesium ribbon:
- When magnesium is burnt in air (oxygen), it produces a powdery ash like substance which looks like magnesium oxide (MgO).
- When magnesium oxide (MgO) also know ash is mixed with a small amount of water it forms a new substance known as magnesium hydroxide [ Mg(OH)2 ]
- [ Mg(OH)2 ] produced in chemical reaction is basic in nature.
Reaction of water with copper sulphate:
- Blue vitriol or Neela are the other names of copper sulphate.
- When copper sulphate reacts with water, in presence of dilute sulphuric acid a blue colored solution is formed.
- To this solution if a piece of iron for example a nail is added, the color of the solution changes to green after some time.
- The green color is a result of chemical reaction which takes place in between copper sulphate and iron.
- The product formed is iron sulphate (green) which is a new chemical product. Also, in the same reaction, copper gets deposited on the surface of the iron nail. The iron nail gets a brown color due the deposition of copper on its surface.
Reaction of Vinegar with baking soda:
- Vinegar also known as acetic acidreacts with baking soda. it produces Sodium hydrogen carbonate and carbon dioxide which is released in form of bubbles and makes a whistling sound and other substances.
- Carbon dioxide reacts with lime water or calcium hydroxide to generates Calcium Carbonate(CaCO3) and Water (H2O).
- A standard reaction to test carbon dioxide is present or not is mixing the gas with lime water and checking if it gets milky.
- Changes which generate one or more new substances are formed is called a chemical change.
- These changes are essential for generation of new substances. The other byproducts formed include Heat, light or any other radiation like U.V. rays.
- After from these gases, sounds, different smells or colors may be produced.
- Other examples of chemical change include Burning of coal, wood or leaves etc., Explosion of a firework, food spoilage.
- The process of neutralization is a chemical change.
- Ozone layer protects the environment from the harmful ultraviolet radiation which come from the sun.
RUSTING OF IRON:
- Rusting is the common cause of slow destruction of iron articles like bridges, ships, cars, truck bodies etc.
- This is not a beneficial process as it causes economic losses.
- The process of rusting involves reaction of iron with oxygen in air and water and results in formation of rust or iron oxide (Fe2O3).
- Rusting is faster if climate is high in humidity.
- To prevent rusting of iron articles, they can be coated with paint, grease or layerof a metal like chromium or zinc on iron.
- Galvanisation: The process of preventing rusting by depositing a layer of metal like zincon surface of iron is called galvanization.
- Galvanized iron pipes are used in household water caring pipes, outer surface of ships etc.
- The outer surface of ships can be painted to prevent its rusting by coming in contact with salty water.
- An alternative is to use stainless steel instead of iron. Stainless steel is composed of iron mixed with carbon, chromium, nickel and manganese which does not rust.
CRYSTALLISATION:
- This is a type of physical change.
- Common example of naturally occurring crystallization is formation of salt crystals by evaporation of seawater.
- Salt crystal obtained is impure and irregular crystals are formed.
- Crystallization is a process by which crystals are formed due to the arrangement of atoms or molecules in a particular manner.