Grammar, Usage / The usage of "this" and "it" in a given example [2]

I came across these sentences in Cambridge, and I do not understand which words "this " and "it" refer to. I have three questions that need your help as answers.

Here are they.
The example: "Put the butter, chocolate and sugar in a saucepan. Heat this over a low flame until it melts."
Question:
(1) "this" here refers to which word?
(2) "it" in that text is a dummy subject or a real subject?
(3) If both "this" and "it" imply a group of words before, why "this" is used first and" it" is used after that?

Writing Feedback / IELTS WRITING TASK 1- Line Graph- shop closures and openings [3]

shop closures and openings

Question:
The graph below shows the number of shops that closed and the number of new shops that opened in one country between 2011 and 2018.
Summarise the information by selecting and reporting the main features, and make comparisons where relevant.
Picture:
My essay:
The line graph compares the number of shop closures and openings in one nation from 2011 to 2018.

Overall, it is clear that the number of shop closures changed considerably and tended to increase, whereas that of openings declined over the whole period.

First of all, in 2011, the figure for shops that closed was about 6000. However, in 2012, the chart witnessed a decline to exactly 6000 shops closing. In 2015, it decreased to less than 1000, the lowest one in the period of seven years. Also, approximately 5000 shops closed in 2018.

Moreover, at the beginning of the period, the number of shop openings stood at more or less 9000 while that in 2012 was 4000. Next, there was a dramatic rise of 2000 shops between 2012 and 2014, and then went down sharply to 4000 again in 2015. After that, this figure had been remaining until 2016. Finally, at the end of the period, the number of new shops that opened in that country was precisely 3000.

(167 words)

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Student Talk / I need help to improve IELTS, especially Writing and Speaking Skills. [8]

I have just taken an IELTS test. I received a disappointing score of 5.5 overall. I am bad at 4 skills. Especially, the speaking and writing skills are really poor. How do I enhance my skills?

This is crucial for me because I am going to take another test with the aim of 7.0 overall to submit to my higher education institution within this year.

Feel free to discuss and give me advice and tips.