Which reactions performed in the experiment involved chemical changes? crushing calcium carbonate mixing calcium carbonate and hcl boiling water heating copper sulfate pentahydrate separating iron filing and sulfur mixing potassium iodide and lead nitrate combining magnesium and hcl burning the candle?

Mixing calcium carbonate and HCl. 
Heating copper sulfate pentahydrate. 
Mixing potassium iodide and lead nitrate. 
Combining magnesium and hcl. 
Burning the candle. 
 Let's look at the available options and see what are chemical changes and what are physical changes. As a side note, line breaks, capitalization, and punctuation are VERY important. I spent almost as much time attempting to actually figure out what your options were as I spent in distinguishing between what were chemical reactions and physical changes. 
 Crushing calcium carbonate.
 * This is a mere physical change. You start with large pieces of calcium carbonate and end up with smaller pieces. So this is a bad choice. 
 Mixing calcium carbonate and HCl.
 * This is a chemical reaction where the calcium carbonate and hydrochloric acid react with each other and produce carbon dioxide plus other compounds. So this is a good choice. 
 Boiling water.
 * Another physical change from liquid to vapor. You can cool down the resulting vapor and end up with the original water. So this is a bad choice. 
 Heating copper sulfate pentahydrate.
 * This is a chemical change in that you're converting CuSO4 . 5H2O(s) into CuSO4 and H2O. So this too is a good choice. 
 Separating iron filing and sulfur.
 * You start out with iron filings and sulfur and end up with iron filings and sulfur. Things are a bit more orderly, but no chemical reactions have occurred. So this is a bad choice. 
 Mixing potassium iodide and lead nitrate. * You start with 2 white solids and upon mixing them, you get a yellow solid.
A chemical reaction has occurred. So this is a good choice. 

 Combining magnesium and hcl.
 * When you combine these two items, you get hydrogen gas as a product. So you have a chemical reaction. And this is a good choice. 
 Burning the candle.
 * Another chemical reaction. The wax combines with the oxygen in the air and produces water vapor and carbon dioxide. So this is a good choice.

The chemical changes are observed in experiments of mixing calcium carbonate and HCl, heating copper sulfate pentahydrate, mixing potassium iodide and lead nitrate, combining magnesium and HCl and burning the candle.

What occurs during the chemical changes?

When calcium carbonate and hydrochloric acids are mixed together they react to produce carbon dioxide and other substances and hence are chemical changes. This can be shown as,

When copper sulfate pentahydrate is heated then the water bond gets broken from the salt due to the chemical change. The reaction can be shown as,

When potassium iodide and lead nitrate reacts to produce a yellow coloured compound that indicates a chemical reaction in the reactants and hence depicts the chemical change. The reaction can be shown as,

When magnesium and hydrochloric acids are reacted together then hydrogen gas is released due to the chemical reactions and changes and hence indicates a chemical change. The reaction can be shown as,

When a candle is burned then the wax and the oxygen from the surroundings react to produce water vapour indicating the chemical change.

Therefore,  chemical changes alter the chemical and physical properties of the substances.

Learn more about chemical changes here:

link

Cr(oh)3 -> cr(3+) + 3oh-

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