Is heating zinc oxide a physical or chemical change?
Zinc oxide is a white powder. When heated strongly (above 800°C) it turns yellow but becomes white again when cold.
What happens is that the zinc oxide loses some of its oxygen when it is hot but this reaction is reversed on cooling. The hot zinc oxide absorbs some of the blue wavelengths of light making the reflected light look more yellow. The loss of oxygen makes it a reversible chemical change.
Note: For the Common Entrance exam you need to know about the colour change when zinc oxide is heated but not why it happens.
You will not be asked if it is a physical or chemical change.

What is the meaning of accuracy?
The accuracy of a instrument shows us how near it reads to the correct answer.
For example a balance might read to the nearest gram.
Accuracy can be expressed as a number
eg an instrument for measuring distance might have an accuracy of +/- 1cm.
Accuracy is not to be confused with sensitivity.
A sensitive balance will weigh very light objects but may not be accurate.

What is the scientific meaning of ‘trend’?
More results pointing in one direction than another.

In other words the results are inclining towards one particular conclusion.
Example: The weather pattern getting warmer over time is a trend.

When iodine is heated what gas is evolved?
When solid iodine is heated it sublimes and becomes a purple vapour.  (See heating chemicals).

The iodine is turning into  a gas, not evolving (giving off) a gas.
Subliming is a physical change that describes what happens when a solid turns into a gas when heated (similar to evaporation, when a liquid changes to a gas).
No chemical change has taken place so  we would not normally use the word evolved.

What will leave black residue after being heated strongly?
The answer you are looking for is probably copper carbonate.
Copper carbonate is a green powder. When heated it decomposes evolving carbon dioxide gas and leaving a residue of a black powder which is copper oxide.
Word equation: Copper carbonate → copper oxide + carbon dioxide↑

Which white solid when heated produces co2?
Most compounds which are a ‘carbonate’ will decompose on heading producing carbon dioxide gas.
A good one to know  is calcium carbonate (found in chalk).

If calcium carbonate is heated it decomposes into calcium oxide (lime) evolving carbon dioxide gas
Word equation: Calcium carbonate → calcium oxide + carbon dioxide↑


What green solid on heating gives carbon dioxide?
Malachite is an ore of copper and is rich in copper carbonate, a green solid.  (See heating chemicals).
If copper carbonate is heated it decomposes, giving off carbon dioxide gas and leaves a residue of a black powder which is copper oxide.
Word equation: Copper carbonate → copper oxide + carbon dioxide↑

Examples of acids and alkalis?
(see acids and alkalis)
Strong acids: Hydrochloric acid, sulfuric acid, nitric acid

Weak acids: Citric acid, found in lemon and lime juice; Ethanoic (acetic)  acid, found in vinegar.
Strong alkalis: Sodium hydroxide (caustic soda), found in oven cleaner.
Weak alkalis: magnesium hydroxide solution (milk of magnesia), found in indigestion mixture; ammonia, found in window cleaner.

Does copper carbonate sublime?
No. Copper carbonate will decompose (split apart) when heated releasing carbon dioxide gas and leaving copper oxide (a black powder).  Colour change: Green to black.  (See heating chemicals).
Chemicals that do sublime include iodine and dry ice (frozen carbon dioxide)