Science 2 Marks Questions

When we burn wood, a new substance, coal, is formed. Therefore, it is a chemical change. However, when we cut wood, only the shape and size of the wood are changed. No new substance is formed. Therefore, it is a physical change.

Once the curd is formed, milk cannot be re-obtained from it. Also, both milk and curd have different properties. Since these are the properties of a chemical change, setting of curd is a chemical change.

Both air and moisture are required for rusting to take place. In coastal areas, the quantity of moisture present in air is more than that in deserts. In desert areas, the amount of moisture in air is even lower. Therefore, rusting of iron objects is faster in coastal areas than in deserts.

Physical changes are beating of aluminium metal to make aluminium foil and cutting of a log of wood into pieces. Chemical changes are digestion of food and burning of crackers.

When a candle burns, both physical and chemical changes take place. On burning candle, the wax melts but can be solidified again on cooling. This shows that melting of wax is a physical change. Burning of candle also produces light and some gases like carbon dioxide. Hence, burning of wick of the candle is a chemical change.

Curd is formed by adding some sour substance to milk and keeping it undisturbed for some hours. Some useful bacteria help in the setting of curd. Curd cannot be converted into milk. It is a different substance than milk. Hence, formation of curd is a chemical change.

Rusting is aided by both moisture (water) and air (oxygen). By painting an iron gate, we prevent its contact from the air and moisture present in the atmosphere. Hence, rusting is prevented.

A more reactive element displaces a less reactive element from its compound. For example when a piece of iron metal is placed in copper sulphate solution, then iron sulphate solution and copper metal is formed. Iron acquire reddish brown colour that deposit is copper.

Spoiled food produces foul smell because of the chemical reaction occurred in it which alter the composition of substance, the old bonds of the reactants are break down and fresh bonds are formed, resulting in formation of new product with different properties of that of earlier products.

Malleability is a property by virtue of which a metal can be beaten into thin sheets without breaking. Malleability is the ability of a metal to be hammered into thin sheets. Gold and silver are highly malleable. When a piece of hot iron is hammered it takes the shape of a sheet.

Crystallization is a better technique than evaporation because it has no dissolve impurities that may left in evaporation. So Salt obtain from evaporation is mixed in boiled water. Then solution is filtered to remove dissolved impurities. The filtrate is left undisturbed for few hours. Sodium chloride aggregate and forms pure crystal of salt.

Wood burns and reduces to form a new substance like ash, (carbon), Carbon Dioxide gas, water vapours, heat and light; this kind of change is irreversible. But the cutting of wood into smaller pieces is a physical change becausethe original composition of the wood does not change and no new substance is formed here.

We can prevent rusting by following methods:

1. By painting or greasing: By applying a coat of paint or grease regularly on iron articles.
2. By galvanizing: Depositing a layer of zinc or chromium on iron. The process is known as galvanization.

When an iron blade of a knife is dipped in a copper sulphate solution, then iron blade is coated with reddish brown deposits of copper. And the blue colour of copper sulphate solution changes to light green due to the formation of iron sulphate. So, it is a chemical change.

A change in which one or more new substances are formed is called chemical change. Chemical change is irreversible. It is also called chemical reaction; in this type of change a new substance is formed. Example- Burning of coal, photosynthesis.

The brown colour is because your apple has rusted! That's because apples are rich in iron, which is present in all their cells. When you cut an apple, the knife damages the cells. Oxygen from the air reacts with the iron in the apple cells, forming iron oxides. This is just like rust that forms on the surface of iron objects.

1. Iron + air + water → iron oxide.
2. Copper sulphate + iron → iron sulphate + copper.

Two other examples are:

1. Setting of curd from milk.
2. Burning of magnesium ribbon to form magnesium oxide.

No, rusting of iron is a chemical change because in this reaction, a new substance, rust (iron oxide) is formed. It cannot be reversed by any simple method.

The two methods to prevent rusting are:

• Painting the iron articles.
• Greasing or oiling the iron articles.

1. On heating, wax melts (physical change) but on burning it forms carbon dioxide (chemical change).
2. On heating, water converts to water vapours (physical change) but on passing electricity through it, it splits into hydrogen and oxygen (chemical change).

A change in which a substance undergoes a change in its physical properties is called physical change, physical properties involves shape, size, colour and state of a substance. It is generally reversible; in physical change no new substance is formed. For example: evaporation, condensation etc.