Elements
Elements have one type of atom. They are unable to be broken down to any simpler form.

Examples of elements : Sulphur, gold, platinum, lead etc.

Compounds
Compounds contain two or more elements that are chemically combined. They are only able to be separated through chemical methods(electrolysis). Heat is released or absorbed in the forming of compounds. They are combined in a fixed proportion.

Examples of compounds : Water, salt, carbon dioxide, methane etc.

Mixtures
Mixtures contain two or more elements/compounds that are not chemically combined. They can be separated through physical methods(distillation, evaporation, magnetic attraction, sieving etc.) Heat is not involved in the forming of mixtures. They are not mixed in a fixed proportion.

Examples of mixtures : Sea water, salt solution, blood, air etc.

For KH, have a thorough understanding of all the chapters in Teras as the first 50(or was it 40?) questions of the KH paper are based on Teras. Buy those Bank Soalan books, do the practices in them and you'll be fine. The questions in KH are often recycled ones. They don't differ much from year to year.

For the Akaun part, I would advise you to do plenty of exercises on that topic too. However, make sure that you fully understand Simpan Kira before attempting it or you'll just be wasting your time.

Median is basically the middle number of a given set of numbers. For example, {2,3,4,5,6} Therefore, the median of the set of numbers will be the number 4. Keep in mind that the numbers are not always in order. They can give you {8,3,2,6,7} and the first thing you need to do is to rearrange the numbers to make {2,3,6,7,8} Then you'll see that the median is 6 instead of 2.

*Disclaimer: From this part onwards I'm not quite sure if it's accurate or not. Please refer to a reference book/textbook to double-check.

Sometimes, they can also present the question in the form of a table in Paper 2 of Maths. You will have to calculate the total in the table. If it's an odd number(let it be x), the formula to calculate the median is (x+1)/2. Then with the obtained answer, compare with the subject given on the left side of the table and the subject that includes the answer in its range is the median.

For even numbers, (let it be n), the formula is (n/2)+1 AND also (n+1)/2. Do the same thing for even numbers.

You can look in any Science reference book or even search it up online and they will mention heat being involved in the formation of compounds. Really? Most of the examples I gave are pretty standard.

It's kind of hard to explain here without any diagrams/pictures. You can try borrowing any friend's Maths reference book to look up the method. Pretty sure it's not mentioned in the textbook.