Science [ 2 Marks Questions ]

When hot glass is cooled fast, the glass cools down unevenly and therefore cause the glass to crack because inside contracts while the outside remains expanded. Glass expands when hot and contracts when cold. It’s a physical but irreversible change.

A change can occurs in a substance by an alteration in the properties such as colour, texture and the state of the substance since there is formation of a new substance.

1. Physical changes:

1. No new substance is formed
2. Reversible
3. Change in physical properties like size, shape, state
4. Melting of ice, tearing of paper freezing, evaporation vaporization

2. Chemical changes:

1. New substance is formed
2. Irreversible
3. Change in properties of reactants and products
4. Burning of paper, photosynthesis, digestion of food, rusting of iron

The ice kept in the beaker receives heat from the surrounding air, to melt and form water.

The process of evaporation is not a good technique because the soluble impurites do not get removed in the process of evaporation.

Explosion of a firework is a chemical change. This explosion produces heat,Tight, sound andampleasant gases that pollute the atmosphere.

Catalysts are substances that speed up the process of a chemical change and it will not undergo any change during the course of the reaction. For example, yeast acts as the catalyst in the fermentation of sugar.

Iron articles can be prevented from making contact with oxygen, water/water vapour. A simple way is to apply a coat of paint or grease. These coats should be applied regularly to prevent rusting.

Heat a piece of iron wire on fire. After some time, it turns red in colour. On cooling it is again converted into its original colour.

Burning of a fire cracker is accompanied by evolution of heat, light and sound.

When an iron surface is painted, iron does not come in contact with air or moisture. Paint acts as a protective layer and prevents exposure of iron to the atmosphere. Since air and water are necessary for rusting, iron does not get rusted, if painted.

When milk is set to curd, the properties of milk are completely changed and a new product is formed. Curd cannot be converted back to milk, hence it is a chemical change.